JXB Advance Access originally published online on October 3, 2006
Journal of Experimental Botany 2006 57(13):3531-3542; doi:10.1093/jxb/erl158
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Evolution of Flowers and Inflorescences |
Catching a ‘hopeful monster’: shepherd's purse (Capsella bursa-pastoris) as a model system to study the evolution of flower development
1Friedrich-Schiller-Universität Jena, Lehrstuhl für Genetik, Philosophenweg 12, D-07743 Jena, Germany
2Universität Osnabrück, Fachbereich Biologie/Chemie, Spezielle Botanik, Barbarastrasse 11, D-49076 Osnabrück, Germany
* To whom correspondence should be addressed. E-mail: guenter.theissen{at}uni-jena.de
Received 28 June 2006; Accepted 8 August 2006
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Abstract |
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Top
Abstract
Introduction: macroevolution,...Capsella is a small genus within the mustard family (Brassicaceae). Its three species, however, show many evolutionary trends also observed in other Brassicaceae (including Arabidopsis) and far beyond, including transitions from a diploid, self-incompatible, obligatory outcrossing species with comparatively large and attractive flowers but a restricted distribution to a polyploid, self-compatible, predominantly selfing, invasive species with floral reductions. All these evolutionary transitions may have contributed to the fact that Capsella bursa-pastoris (shepherd's purse) has become one of the most widely distributed flowering plants on our planet. In addition, Capsella bursa-pastoris shows a phenomenon that, although rare, could be of great evolutionary importance, specifically the occurrence of a homeotic variety found in relatively stable populations in the wild. Several lines of evidence suggest that homeotic changes played a considerable role in floral evolution, but how floral homeotic varieties are established in natural populations has remained a highly controversial topic among evolutionary biologists. Due to its close relationship with the model plant Arabidopsis thaliana, numerous experimental tools are available for studying the genus Capsella, and further tools are currently being developed. Hence, Capsella provides great opportunities to investigate the evolution of flower development from molecular developmental genetics to field ecology and biogeography, and from morphological refinements to major structural transitions.
Key words: ABC model, flower development, homeosis, macroevolution, MADS-box gene, saltation, stamenoid petals
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Introduction: macroevolution, evo-devo, homeosis, and all that |
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As encompassed by the ‘Synthetic Theory’ (or ‘Modern Synthesis’) of evolutionary biology, the 20th century has provided a thorough understanding of the mechanisms of microevolution (Dobzhansky, 1937; Mayr, 1942; Simpson, 1944; Mayr and Provine, 1980). It is relatively well known how organisms adapt to their environment and, arguably, even how new species originate. However, whether this knowledge suffices to explain macroevolution, narrowly defined here to describe evolutionary processes that bring about fundamental novelties or changes in body plans (Theissen, 2006), has remained highly controversial.
How fundamental innovations (or novelties) originate in evolution remains one of the most enigmatic questions of biology. According to the proponents of the Synthetic Theory, the gradual process of evolution by natural selection that operates within populations and species also creates the unique traits recognizable at higher taxonomic levels, meaning that macroevolution is just microevolution extended over relatively long periods of time.
However, it has been repeatedly pointed out that innovation is different from adaptation, and that the Synthetic Theory, which is largely based on population genetics, falls short of explaining innovations, novelties, and the evolution of body plans (Riedl, 1977; Gilbert et al., 1996; Bateman et al., 1998; Erwin, 2000; Wagner, 2000; Haag and True, 2001; Wagner and Müller, 2002; Wagner and Laubichler, 2004; Müller and Newman, 2005; Theissen, 2006). These are not the only shortcomings of the Synthetic Theory. It considers evolution as the result of changes in allele frequency due to natural selection that engender subtle modifications of phenotype. According to the Synthetic Theory evolution always occurs gradually, in a countless number of almost infinitesimally small steps. Given sufficient time, these gradual changes accumulate and result in the larger differences that typically seperate higher taxa. The fossil record, however, with its often abrupt transitions, provides limited evidence for the gradual evolution of new forms (Gould and Eldredge, 1993). In addition, the branching patterns of higher taxa in both animals and plants, as revealed by cladistics, do not support the view that the major features of body plans and their constituent parts arose in a gradual way (Vergara-Silva, 2003).
These shortcomings of the Synthetic Theory in explaining evolutionary novelties led to the reintegration of developmental biology into evolutionary biology, giving rise to the discipline of ‘evolutionary developmental biology’ (‘evo-devo’). The rationale of evo-devo takes into account that multicellular organisms usually develop from single cells (zygotes) in each generation anew. Thus, all evolutionary changes in the morphology of an organism occur by changes in developmental processes. Since development is largely under genetic ‘control’, novel morphological forms in evolution frequently result from changes in so-called ‘developmental control genes’, many of which encode transcription factors. Consequently, evo-devo projects typically study the phylogeny of genes encoding certain classes of transcription factors (such as homeodomain or MADS-domain proteins) and their role in the evolution of morphological features (for further details and discussions of the evo-devo rationale see Gould, 1977; Gilbert et al., 1996; Theissen et al., 2000; Carroll, 2001; Arthur, 2002; Müller and Newman, 2005).
Considerable progress has recently been made in understanding the genetic mechanisms that bring about drastic, yet co-ordinated, changes in the adult phenotype by modification of developmental processes. Changes in both the timing (heterochrony) and the position (heterotopy) of developmental events can occur, although, in the case of plants, they are often difficult to distinguish (Kellogg, 2000). A considerable number of key innovations and characteristics of major clades, most of which have been used as taxonomic characters for decades, can be explained as the result of heterotopy or heterochrony, corroborating the importance of developmental shifts for macroevolution (Kellogg, 2000).
An important subset of heterotopic changes are homeotic transitions (Baum and Donoghue, 2002), which describe a type of variation in which ‘something has been changed into the likeness of something else’ (Lewis, 1994). Well-known examples include the Ultrabithorax mutant of the fruit fly, Drosophila melanogaster, wherein halteres (rudimentary wings) are replaced by a second pair of ‘true’ wings. Homeotic mutants also occur frequently in plants, affecting both vegetative and reproductive organs (Sattler, 1988; Meyerowitz et al., 1989).
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Developmental genetics of floral homeotic mutants |
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Particularly well-known examples of homeotic mutants in plants are floral homeotic mutants: mutant plants with flowers that have more-or-less normal floral organs in places where organs of another type are typically found. Many flowers consist of four different types of organs, which are arranged in four (or more) whorls, with sepals in the first, outermost whorl, followed by petals, then stamens, and finally carpels in the innermost whorl (an example is given in Fig. 1a). In the model plant Arabidopsis thaliana (thale cress), homeotic mutants can be categorized into three classes, termed A, B, and C. Ideal Class A mutants have carpels instead of sepals in the first whorl and stamens rather than petals in the second whorl. Class B mutants have sepals rather than petals in the second whorl and carpels rather than stamens in the third whorl. Class C mutants have flowers in which reproductive organs (stamens and carpels) are replaced by perianth organs (petals and sepals, respectively), and in which the determinacy of floral growth is lost, resulting in an increased number of floral organs (Meyerowitz et al., 1989). Such ‘filled flowers’ are well known from many wild and ornamental plants.
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The defined classes of floral homeotic mutants are usually explained by simple combinatorial genetic models such as the ABC model (Fig. 1a; reviewed by Theissen, 2001a, b; Ferrario et al., 2004). This proposes three different floral homeotic functions to explain how the different floral organs adopt their unique identities during development. Corresponding to the aforementioned mutant classes these functions are termed A, B, and C, with A specifying sepals in the first floral whorl, A+B petals in the second whorl, B+C stamens in the third whorl, and C carpels in the fourth whorl.
In Arabidopsis the Class A genes are represented by APETALA1 (AP1) and APETALA2 (AP2), the Class B genes by APETALA3 (AP3) and PISTILLATA (PI), and the (single) Class C gene by AGAMOUS (AG), all encoding putative transcription factors (reviewed by Theissen, 2001a; Krizek and Fletcher, 2005). Thus, the products of the ABC genes probably all control, and hence co-ordinate, the transcription of other genes (termed target genes), whose products are directly or indirectly involved in the formation or function of the different floral organs. Except for AP2, all ABC genes are MADS-box genes encoding MIKC-type MADS-domain transcription factors (reviewed by Riechmann and Meyerowitz, 1997; Theissen et al., 2000; Becker and Theissen, 2003; De Bodt et al., 2003).
Despite the fact that the ABC genes are required for the specification of floral organ identity, it became clear that they alone are not sufficient to generate a typical flower. Moreover, the ABC model did not provide a molecular mechanism for the interaction of floral homeotic genes during the development of floral organ identity (Theissen, 2001b). These shortcomings of the ABC model have meanwhile been overcome by the identification of Class D genes involved in ovule development, and Class E genes required for development of all four kinds of floral organs. This led, step by step, to the extension of the ABC model to the ABCDE model (Angenent and Colombo, 1996; Pelaz et al., 2000; Theissen, 2001a; Ditta et al., 2004). Transgenic studies in Arabidopsis revealed that the ABCDE genes are not only required, but are even sufficient, to at least specify petal and stamen identity (Honma and Goto, 2001). Moreover, the observation that floral homeotic proteins form multimeric complexes provided an explanation for the interaction of floral homeotic genes at the molecular level (Egea-Cortines et al., 1999; Honma and Goto, 2001), as described by the ‘floral quartet model’ (Theissen, 2001a; Theissen and Saedler, 2001). However, all considerations that follow use the simpler ABC model.
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Flowers on the evo-devo agenda: variations on a theme |
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Studies of homeotic genes reveal that major developmental events such as the specification of organ identity are often controlled by a fairly small number of key developmental control genes. The analyses of both conventional mutants and transgenic organisms, revealed that changes in such ‘toolkit genes’ (Carroll, 2005) can cause profound, yet co-ordinated, morphological changes. Specifically, changes in the expression domains of floral homeotic genes in mutant or transgenic plants cause homeotic transformations of floral organs. For example, ectopic expression of Class C genes in the perianth leads to a transformation of sepals into carpeloid organs and of petals into stamenoid organs (Bradley et al., 1993). Similarly, the expression of Class B genes in the first and fourth floral whorls of A. thaliana leads to a transformation of sepals into petaloid organs and of carpels into stamenoid organs (Krizek and Meyerowitz, 1996).
Some homeotic phenotypes in both animals and plants resemble differences in character states between major lineages of living organisms (e.g. consider the dragonfly-like appearance of the Ultrabithorax mutant of the Dipteran fruit fly). However, whether the genes underlying such ‘phylo-mimicking mutants’ define loci that play an important role in character changes during macroevolution has proven controversial (Haag and True, 2001). Nevertheless, much of the structural diversity of flowers can be explained by modifications of the ABC system of floral organ identity specification, especially by changes in the spatiotemporal expression domains of the ABC genes. For example, in the flowers of tulips (Tulipa gesneriana) and other lily-like plants, first-whorl organs are typically petaloid (Fig. 1c), quite similar to the petaloid sepals of eudicotyledonous flowering plants such as Arabidopsis or tobacco that arise when Class B genes are ectopically expressed in the first floral whorl (Davies et al., 1996; Krizek and Meyerowitz, 1996). This suggests that a homeotic transition from sepaloid to petaloid organ identity, or vice versa, occurred in the first floral whorl during the evolution of flowering plants. Petaloid organ identity requires the function of Class B floral homeotic genes. Indeed, when Class B genes were investigated in tulip, they were found to be expressed not only in the petaloid tepals of the second floral whorl, but also in the organs of similar identity in the first whorl (Kanno et al., 2003). This observation supports a ‘modified ABC model’ for tulips and related plants, in which Class B gene expression extends from the second and third into the first floral whorl (Fig. 1c; Kanno et al., 2003). Heterotopic expression of Class B genes in the petaloid organs of the first floral whorl was also found in most, but not all, other monocots with petaloid tepals investigated thus far (Park et al., 2003; Nakamura et al., 2005). Similarly, the petaloidy of first-whorl organs in many flowers of the basal eudicot family Ranunculaceae appears to be due to a shift of Class B gene expression towards the first floral whorl (Kramer et al., 2003). In a somewhat complementary way, reduction of Class B gene expression in petaloid organs of the floral perianth may have contributed to the sepaloid appearance of the perianth organs of some wind-pollinated flowers, such as those of sorrel (Rumex acetosa) (Ainsworth et al., 1995).
These inferences support the view that evolutionary ‘tinkering’ with the boundaries of Class B floral homeotic gene expression prompted floral homeotic changes that contributed to the diversity of floral architecture. Similar arguments can be made for changes in the (mutually exclusive) expression domains of Class A and Class C genes (Fig. 1b; see below for details). Theissen et al. (2000) and Kramer et al. (2003) developed models of how evolutionary variation of the ABC system of floral organ identity specification could explain floral diversification during evolution.
These considerations add to an increasing amount of evidence provided by evolutionary analyses of morphological characters, indicating that homeosis played a significant role in plant evolution (Sattler, 1988; Iltis 2000; Kellogg, 2000; Baum and Donoghue, 2002; Rudall and Bateman, 2002, 2003; Ronse De Craene, 2003; Rutishauser and Moline, 2005; Theissen, 2006). In principle, homeotic changes could occur in a gradual mode of evolution, involving thousands or millions of generations of organisms (Sattler, 1988). However, given that complete transitions in organ identity can occur in an individual carrying a mutation in a single homeotic gene, a saltational mode of character change appears more plausible from a developmental genetic viewpoint. Moreover, a saltational character change would help to overcome a notorious problem of many gradualistic scenarios of evolutionary change, posed by the presumed low fitness of intermediate forms. Saltational changes, however, would be contradictory to the assumption of the Synthetic Theory that all kinds of evolution are gradual and based on changes in allele frequency at many loci. They would have a quite remarkable implication: homeotic mutants should represent critical steps during a macroevolutionary transition. Since homeotic mutants can be considered as profound variants of organismic design, they might reasonably be called ‘hopeful monsters’ (Bateman and DiMichele, 2002; Theissen, 2006), reintroducing the provocative (and widely, if often thoughtlessly, rejected) term of Richard Goldschmidt (1940) (reviewed by Dietrich, 2003).
However, there cannot be much doubt that homeotic mutants often originate as rare individuals in populations of wild-type organisms and hence, like all other mutants, are subject to the rules of population genetics. To qualify as hopeful monsters and to establish new evolutionary lineages, homeotic mutants must survive many years under conditions of natural selection until the mutant homeotic locus achieves fixation and modifying mutations have optimized the new ‘body design’. How does this work? Unfortunately, evo-devo tells us little about the performance of homeotic mutants in natural environments (Theissen, 2000). In addition to theoretical discussions of this issue, the question should be answered experimentally, since saltational modes of evolution involving homeotic mutants may not be generally accepted unless a sufficient fitness of such organisms has been documented in natural habitats. Therefore, the population dynamics of homeotic mutants has to be studied through extensive field work (Theissen, 2000, 2006; Bateman and DiMichele, 2002; Vergara-Silva, 2003; Dietrich, 2003). However, probably only a small fraction of homeotic mutants will qualify as hopeful monsters. Most floral homeotic mutants, including the ‘classical’ mutants that gave rise to the ABC model, have a strongly reduced fitness compared with the corresponding wild type; this almost certainly hampers their long-term survival in nature (Nutt et al., 2006). So, how can a hopeful monster be distinguished from a hopeless mutant?
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Floral homeotic mutants in the wild, more than hopeless cases? |
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One good way to identify floral homeotic mutants with evolutionary potential would be to seek populations of such plants in the wild. If these populations prove more-or-less stable for many years, the respective variants will have demonstrated at least a minimum level of competitiveness under natural growth conditions. However, the literature on such populations is disappointingly thin. One case is bicalyx, a recessive variety of Clarkia concinna (Onagraceae) in which the usually pink and showy trilobed petals are transformed into sepaloid organs due to a mutation at a single genetic locus (Ford and Gottlieb, 1992). The bicalyx variety occurs only in a small population at Point Reyes (north of San Francisco, California, USA), where it is accompanied by a majority (70–80%) of wild-type plants. Ford and Gottlieb (1992) described the population as being stable, probably due to the predominantly selfing mode of propagation. However, the population was observed over a period of just four years, so that the long-term stability of the bicalyx variant remained unknown (Ford and Gottlieb, 1992). Moreover, the bicalyx gene has not been molecularly characterized so far.
Another case of a naturally occurring floral homeotic mutant is a peloric variety of common toadflax (Linaria vulgaris); it has actinomorphic rather than zygomorphic flowers and exists on a small island near Stockholm/Sweden (Cubas et al., 1999). Like the homeotic Clarkia, this Linaria is mutated at a single, recessive locus. It turned out that the toadflax variety is affected in a CYCLOIDEA-like gene, but by methylation of DNA that leads to transcriptionally silencing (epimutation) rather than a change in the DNA sequence (Cubas et al., 1999).
Both the Clarkia and Linaria varieties have a very limited range of distribution, and their fitness in the wild is probably significantly lower than that of the wild-type. The Linaria epimutant, for example, may only propagate vegetatively (Theissen, 2000). Even though it has been known on the island for more than 200 years, it remains unclear whether its long persistence is due to true longevity of an ancestral mutant or to frequent reappearance after repeated rapid extirpation (Theissen, 2000). Taken together, the evolutionary potential of the wild floral homeotic varieties reported thus far is doubtful (Theissen, 2006). Better candidates for floral homeotic mutants that might qualify as hopeful monsters are thus welcome, but finding them is not trivial. For a few promising cases in orchids see Bateman and Rudall (2006).
There could be two reasons for why descriptions of wild populations of floral homeotic mutants are rare. Such populations could indeed be needles in haystacks (which would not, however, necessarily imply that they are of low evolutionary importance; Theissen, 2006). They also could have been neglected by mainstream research, not least because they were not considered crucial (or even were considered detrimental) to predominant scientific theories such as the Modern Synthesis. As will be seen below, these ostensibly competing scenarios are not mutually exclusive.
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Shepherd's purse: from weed to model system |
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In biology, scientific progress often depends primarily on the choice of a suitable model system. Imagine genetics without pea, maize, the fruit fly and E. coli! So, assuming interest in the macroevolution of flowers, which species provides us with a suitable hopeful monster?
Due to several well-known practical reasons (e.g. diploidy, short-life cycle, limited space requirements, and efficient transformability) a small member of the mustard family (Brassicaceae), the thale cress Arabidopsis thaliana, became the most favoured model plant. Its small genome of just about 130 million base pairs has (almost) been fully sequenced (Arabidopsis Genome Initiative, 2000), and a relatively large number of genetic and genomic tools, such as T-DNA-, transposon insertion- and EMS-mutagenized populations facilitating the investigation of gene function, have been developed. With A. thaliana it is possible to investigate traits from an ecological and evolutionary point of view by studying them under different defined conditions in relation to their genetic background. Using A. thaliana to understand the evolution of floral novelties therefore seems an obvious idea. However, even though A. thaliana has a global distribution and gave rise to a considerable number of so-called ‘ecotypes’, its floral architecture is remarkably standardized. Consequently, the natural occurrence of floral homeotic varieties has not, to the best of our knowledge, been reported so far. Arabidopsis thaliana appears to be a comparatively uncompetitive plant and it is actually relatively rare in the wild. How many readers would find one in the wild within 10 min walk from their home? In fact, one much more frequently stumbles over a close relative, the shepherd's purse, Capsella bursa-pastoris. Indeed, C. bursa-pastoris is one of the five most widespread flowering plants on our planet, the four others being Polygonum aviculare, Stellaria media, Poa annua, and Chenopodium album (Coquillat, 1951; Hurka et al., 2003). But C. bursa-pastoris is not only much more frequent than A. thaliana, it also appears to tinker more extensively with its flower structure. And thus it is C. bursa-pastoris where a floral homeotic variety is found forming quite stable populations in the wild.
Capsella in brief: it is a small genus of Brassicaceae that may only contain three species; these, however, show remarkable differences in ploidy level, breeding systems, and habitat range (Hurka and Neuffer, 1997; Zunk et al., 1999; Hurka et al., 2005). Two of the species, C. grandiflora and C. rubella, are diploid, whereas the third, C. bursa-pastoris, is tetraploid. Capsella grandiflora is obligately outbreeding due to a sporophytic self-incompatibility (SI) system; it grows only in a limited habitat in Albania, western Greece, and northern Italy. Compared with the other two species, C. grandiflora—nomen est omen—has relatively large, fragrant flowers with showy petals to attract pollinators. By contrast, C. rubella is a completely selfing plant with relatively small flowers that originally grew around the Mediterranean Sea, but it colonized nearly all Mediterranean climatic regions worldwide during historical times (Hurka et al., 1989). The predominantly selfing C. bursa-pastoris generally prefers disturbed, ‘man-made’ habitats, like the margins of agricultural fields (Fig. 2a), and grows all over the world except in the hot and humid tropics (Hurka and Neuffer, 1997).
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How did C. bursa-pastoris originate, and what allowed it to spread over most of the planet? Even though the relationships between the three species in the genus Capsella have been investigated for some time, they remain surprisingly unclear. Based on the results of isozyme electrophoresis and isoelectric focusing experiments of Rubisco, it has been suggested that C. bursa-pastoris is an allopolyploid of C. rubella and C. grandiflora (Mummenhoff and Hurka, 1990). With additional data from restriction enzyme site variation in the chloroplast genome, it was then assumed that C. bursa-pastoris is an autopolyploid of C. grandiflora (Hurka and Neuffer, 1997). More recently, a phylogenetic analysis based on data obtained with chloroplast and nuclear DNA markers suggested a sister relationship between C. bursa-pastoris on the one side and C. rubella plus C. grandiflora on the other. From their data, Slotte et al. (2006) concluded that C. bursa-pastoris is not an autopolyploid of either C. rubella or C. grandiflora, nor did either of these species play the maternal role in an allopolyploidization event (Slotte et al., 2006). Instead, these authors suggested two alternative scenarios. One is that C. bursa-pastoris originated by allopolyploidization, with C. rubella as the paternal ancestor and with an unknown or extinct diploid ancestor as the mother plant. In the alternative scenario, C. bursa-pastoris originated wholly from extinct diploid ancestors, followed by repeated hybridization and backcrossing with C. rubella, leading to introgression of C. rubella alleles into the C. bursa-pastoris genome.
Whatever the origin of C. bursa-pastoris, comparison with outgroup species strongly suggests that C. grandiflora represents the most ancestral character states concerning reproductive biology and ploidy level within Capsella and that C. bursa-pastoris is the most derived species (Hurka et al., 2005). That implies that an obligate outbreeding diploid Capsella has evolved into the self-compatible tetraploid C. bursa-pastoris. But what makes this character combination a ‘formula for success’ at least in terms of expanding its geographical range? A crucial event for the successful distribution of C. bursa-pastoris was probably the breakdown of the sporophytic self-incompatibility (SI) system that is active in C. grandiflora (Paetsch et al., 2006). According to the ‘reproductive assurance hypothesis’, the capability to self-fertilize enables the plant to reproduce even under poor outcrossing conditions, for example, in the absence of either conspecific neighbours or suitable pollinators, and thereby allows colonization of unoccupied niches (Shimizu et al., 2004; Shimizu and Purugganan, 2005; and references cited therein). In addition, flowering and reproduction become less season-dependent, because the plant no longer depends on the timing of other plants or pollinator activity. Several ecotypes of C. bursa-pastoris are facultative winter annuals that are able to survive as over-wintering leaf rosettes (Neuffer and Hurka, 1986, 1999; Neuffer and Bartelheim, 1989; Neuffer, 1990; Neuffer and Hurka; Linde et al., 2001). This rapid reproductive cycle enables C. bursa-pastoris plants that germinate in early spring to produce up to three generations of progenitors in just one year (Neuffer and Hurka, 1988; Neuffer and Bartelheim, 1989; Hurka and Neuffer, 1991; Aksoy et al., 1998).
Most annual selfers show rapid floral maturation facilitated by reproducing at a small overall size and by developing smaller flowers and seeds compared with their outcrossing annual relatives (Snell and Aarssen, 2005). This generalization applies to both Capsella (see above) and members of the genus Arabidopsis. The combination of self-compatibility (SC) and flower-size reduction seems to be a general trend; the latter promotes selfing by reducing the distance between anthers and stigma in smaller flowers. Since self-fertilizing plants do not need to attract pollinators an energy cost that is required for developing large flowers could be saved (Snell and Aarssen, 2005).
However, many flowering plants make ambitious efforts to prevent selfing in bisexual flowers. Such species evolved self-incompatibility systems (like SI in C. grandiflora), time-shifted maturation of pollen and ovules (dichogamy) or structural barriers (herkogamy) to promote outbreeding. Outbreeding leads to allelic variation in the population, which is an important prerequisite for reacting to changes in the environment and to avoid inbreeding depression. That being self-compatible was particularly advantageous for C. bursa-pastoris may be due to the fact that outcrossing persists; rates in the field are up to 20% (Hurka and Neuffer, 1997).
But selfing cannot be the only reason why C. bursa-pastoris is so widely distributed, because it also applies to C. rubella. Perhaps the tetraploidy of C. bursa-pastoris provided another important advantage. Polyploid plants often show broader ecological tolerances and the ability to cope with a wider range of conditions compared with their diploid progenitors. Since, in polyploids, deleterious mutations will be masked by the extra genome, polyploidy is hypothesized to result in reduced inbreeding depression compared with diploid parents (reviewed by Soltis and Soltis, 2000). Therefore, becoming self-fertile is particularly advantageous for polyploid plants such as C. bursa-pastoris, as it results in higher progeny without the negative effects of inbreeding depression.
Thus, within the species-poor genus Capsella, the transition from a diploid, self-incompatible, obligatory outcrossing species with comparatively large and attractive flowers but a restricted distribution (C. grandiflora) to a tetraploid, self-compatible, predominantly selfing species with relatively small flowers but exceptional colonization ability (C. bursa-pastoris) can be inferred. Similar evolutionary scenarios are also inferred in other plant groups, including both close and distant relatives of Capsella (Barrett, 2002; Mable et al., 2005). Capsella might thus serve well in comparative studies aiming to understand parallel and convergent evolution of floral features.
Nevertheless, one might remain reluctant to choose C. bursa-pastoris as a new model plant for studying developmental and evolutionary processes because of the limited number of molecular genetic tools available a priori. However, there is more to this issue than meets the eye. As a result of its close relationship with the model plant A. thaliana, numerous experimental tools are available to study the genus Capsella and more are currently being developed. The order, orientation, and sequence of genes is very similar in the genomes of Arabidopsis and Capsella and their exons show over 90% sequence identity (Acarkan et al., 2000; Koch and Kieffer, 2005). This allows the identification of genes within Capsella with the help of the thoroughly mapped Arabidopsis genome. Experimentation along these lines will be further boosted by the ongoing sequencing of the Arabidopsis lyrata and Capsella rubella genomes (Joint Genome Institute, United States Department of Energy).
Like A. thaliana, C. bursa-pastoris is self-compatible, and easy to cultivate and propagate. Although longer than that of A. thaliana, the life cycle of C. bursa-pastoris is still sufficiently short to allow the production of three to four generations per year (depending on the variety). And although C. bursa-pastoris is a tetraploid, it has already evolved disomic inheritance (Hurka et al., 1989; Hurka and Düring, 1994), which makes crossing experiments easier to interpret. Moreover, C. bursa-pastoris is amenable to powerful techniques such as genetic transformation by the ‘floral dip’ method and gene knockdown via RNA interference (RNAi) (Bartholmes, 2006; C Bartholmes and G Theissen, unpublished data), which greatly facilitates the analysis of gene function.
The authors are thus confident that, due to its interesting biology, close relationship with Arabidopsis, and an increasing number of molecular tools available, the successful globetrotter shepherd's purse will start a second career as a satellite model system of Arabidopsis.
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A hopeful monster in a shepherd's purse |
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In addition to representing a derived state in a typical evolutionary process affecting floral biology, C. bursa-pastoris shows a rare phenomenon which, nevertheless, could be of great evolutionary importance, specifically the occurrence of a homeotic variety in stable populations in the wild (Nutt et al., 2006; Theissen, 2006). This was brought to our attention when we searched for natural homeotic mutants that show competitiveness under natural growth conditions.
The remarkable variety that became our study object has been described for almost two centuries at different places in Europe (Opiz, 1821; Trattinnick, 1821; Schlechtendahl, 1823; Wiegmann, 1823; Murbeck, 1918) and is still found in substantial stable populations. The four petals of its flowers are transformed into stamens, whereas all other floral organs are unaffected, leading to apetalous flowers with ten stamens (Fig. 1b). Therefore, the mutant has been termed ‘dekandrisch’ (‘decandric’) and the phenomenon ‘Staminale Pseudapetalie’ (‘Stamenoid pseudo-apetaly’); the mutant variety has even been considered being a new species, ‘C. apetala’ (Opiz, 1821; Murbeck, 1918). Recently, we termed the locus/loci affected in the mutant varieties ‘Stamenoid petals’ (Spe) (Nutt et al., 2006).
This variety became obscure, receiving only occasional attention in the literature (for details on the history of Spe see Nutt et al., 2006). Fortunately, a new population of Spe plants was discovered in 1991 by Reichert (1998) on field paths in vineyards in Gau-Odernheim (Rheinhessen, Germany). Reichert's report, which covered several years of observations, showed that the Spe plants are stable concerning number and local distribution, even though they are mixed with wild-type plants (Fig. 2a). Already Reichert (1998) considered this wild growing homeotic variety as a very remarkable case due to its ability to establish whole populations rather than merely single plants. The long-lasting existence of the Spe variety in the wild at least indicates that it has a fitness similar to that of the wild-type. Sadly, Reichert's observations attracted little interest, perhaps because floral homeotic mutants in the wild are rare, and rarity is commonly confused with unimportance (Theissen, 2006). Most developmental geneticists might lack interest, because they fail to recognize the difference between Spe plants and their more familiar ‘laboratory artefacts’. Most evolutionary biologists will probably question the scientific relevance of Spe. For us, however, the decandric C. bursa-pastoris variety appears to be an optimal model for studying non-gradualistic changes in floral architecture (Nutt et al., 2006). Hence, it was recently made a research focus in the authors' laboratories. A brief progress report is presented below.
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Studying a hopeful monster in the laboratory and in the field |
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Additional decandric C. bursa-pastoris populations have been identified throughout Europe (P Nutt, B Neuffer, and G Theissen, unpublished data), revealing that Spe plants are more frequent than was initially assumed. Lack of attention evidently contributed to the rarity of descriptions in the literature. Current studies are concentrating on the above-mentioned population in Gau-Odernheim and on one population found on the Desenberg, near Warburg (Westphalia, Germany). We have begun carefully to analyse the phenotype and the mode of inheritance of Spe. The decandric C. bursa-pastoris plants from both Gau-Odernheim and Warburg show almost complete transformation of petals into stamens (Nutt et al., 2006). That applies also to the functional level, because second-whorl stamens of Spe plants produce viable pollen that can successfully pollinate gynoecia of C. bursa-pastoris and thus generate viable seeds (P Nutt, unpublished data).
By crossing true-breeding mutant and wild-type plants, an intermediate phenotype was obtained in the F1 generation; the second-whorl organs were smaller than petals, slightly curled and some showed yellow-coloured edges, thus revealing an organ identity intermediate between petals and stamens (P Nutt, unpublished data). Segregation of the mutant phenotype in the F2 generation varied depending on the parents: stamenoid versus petaloid second-whorl organs were observed in a ratio of about 3:1, or stamenoid versus intermediate versus wild-type second-whorl organs in a ratio of about 1:2:1 (P Nutt, unpublished data). A similar segregation pattern was reported by Dahlgren (1919) for the decandric C. bursa-pastoris variety that he analysed. This suggests that the mutant phenotype of a Spe plant is caused by a co-dominant mutant allele conferring stamen identity at a single locus in one of the two disomically inherited genomes of C. bursa-pastoris. Tests for allelism of the mutant loci in the two investigated populations are under way. Molecular markers, including isozymes and AFLPs, are being used to determine the origin from wild-type and the geographic distribution of the different Spe varieties in detail.
We are interested in both the molecular identity of the Spe locus and the mechanisms by which it generates the mutant phenotype. According to the ABC model, for stamens to develop, expression of both Class B and Class C floral homeotic genes is required (Fig. 1). Therefore, it is hypothesized that in the Spe mutants ectopic expression of a Class C gene, or a closely related gene, is extended from the third and fourth whorl towards the second whorl, thereby suppressing Class A genes in this whorl (Fig. 1b). The most obvious candidate genes for this ectopic expression are an orthologue of the Arabidopsis Class C gene AGAMOUS (AG) or one of its closely related paralogues SHATTERPROOF1 (SHP1), SHP2, and SEEDSTICK (STK), which share high sequence similarity with AG (Becker and Theissen, 2003). For all of these genes, except STK, ectopic expression in A. thaliana leads to the formation of stamenoid organs in the second whorl (Favaro et al., 2003). To test this hypothesis, investigations were made into the mRNA expression patterns of the orthologues of Class A and Class C floral homeotic genes, and closely related paralogues, in wild-type and Spe flowers of C. bursa-pastoris. Moreover, experiments to knock-down AG-like genes in Spe plants employing RNAi technology are underway to determine whether stamenoidy of second-whorl organs depends on the activity of Class C or related genes.
Assuming that the Spe phenotype is brought about by the ectopic expression of a Class C gene (Fig. 1b) does not imply that the Spe locus is an AG-like gene. Ectopic expression of a Class C gene could of course be caused by a change in a cis-regulatory element of an AG-like gene itself, but also by a trans-acting factor functioning upstream of the AG-like gene (and not necessarily directly binding to the AG-like gene itself). Such an upstream-acting factor would probably be a protein, but could also be a regulatory RNA.
To clone the Spe locus, a combined candidate gene and map-based cloning approach is currently being applied, which is facilitated by the close relationship between Capsella and Arabidopsis. Candidate genes for Spe, currently being tested for co-segregation with the mutant phenotype in F2 populations, include all of the AG-type genes mentioned above (orthologues of AG, SHP1, SHP2, STK), but also orthologues of some trans-acting regulators of AG that show stamenoid petals upon mutation (for details see Nutt et al., 2006). These putatively trans-acting candidates are considered a lower priority, however, because their mutant alleles are usually recessive and mutant plants often display considerable pleiotropic effects beyond stamenoidy of petals, sometimes even outside the flower.
There are many floral homeotic mutants available that can be studied from a developmental genetics point of view. What makes Spe so special, however, is its continuous presence in the wild, and hence the fact that both proximate and ultimate causes of evolution can be studied using the same model system. In addition to trying to understand the molecular mechanism behind Spe we are, therefore, also deeply interested in investigating the performance of Spe plants outside the greenhouse. Hence, the performance of Spe and wild-type plants is being compared, both in the ‘wild’ in Gau-Odernheim and Warburg (where the plants grow, albeit in typical man-made or disturbed habitats) and in ordered field plots in Botanical Gardens (Fig. 2).
As a rough proxy of reproductive fitness, the number of fruits and seeds produced by wild-type and Spe plants is being determined. So far, significant differences have not been observed in our garden experiments (J Ziermann, unpublished data). Although C. bursa-pastoris is predominantly self-pollinating, even very low rates of outcrossing could help to avoid inbreeding depression and hence might be of considerable evolutionary importance. Therefore, outcrossing rates between Spe and wild-type plants are being determined under controlled conditions in the Botanical Garden of Osnabrück using molecular markers. In addition, floral visitation by insects is being investigated as a possible mechanism of outcrossing facilitated by pollinators. Wild-type inflorescences display white petals and hence might be quite appealing to some visually oriented visitors like bees (Fig. 3a); by contrast, inflorescences of Spe plants lack petals, but offer more pollen than wild-type plants (Fig. 3b). Compared with the wild-type, Spe inflorescences might be slightly less attractive to bees but slightly more appealing to beetles. In the case of the similarly predominantly selfing A. thaliana, flower visits by potential pollinators such as solitary bees, dipterans, and thrips have been observed in the field (Mitchell-Olds, 2001; Hoffmann et al., 2003). In field plots in the Botanical Garden of Jena (Fig. 2b) very similar observations were made for C. bursa-pastoris (J Ziermann, unpublished data). A slightly higher preference of bees for wild-type inflorescences and of beetles for Spe inflorescences may have been observed in one year but needs more data for confirmation (J Ziermann, unpublished data). So far a dramatic change has not been observed in the species spectrum or the number of floral visitors when comparing Spe with wild-type plants.
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Overall, evidence that the Spe variety is significantly handicapped in its reproductive fitness has not been found, a conclusion supported by its persistence in wild habitats for many years. To corroborate that view, it is intended to monitor the development of Spe populations in their natural habitats for as long as possible. Taken together, these endeavours will help us to learn more about the evolutionary potential of the Spe variety and, hopefully, of floral homeotic mutants in general.
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Concluding remarks |
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Due to the large time-scales and diverse mechanisms involved, studying macroevolution is hardly a trivial task. Reconciling macroevolution with population genetics appears to be a considerable challenge for the future (Nutt et al., 2006; Theissen, 2006). The Spe variety of C. bursa-pastoris shows a rare, yet potentially evolutionarily important, phenomenon, specifically the occurrence of a homeotic variety apparently forming stable populations in the wild. A detailed study of the Spe variety may not only tell us more about the developmental genetic mechanisms that generate novel structures in the first place, but also indicate whether, and if so how, drastic morphological variants are established in natural populations.
However, in discussions with colleagues we are occasionally told that even clarifying the molecular mechanism that brings about the Spe phenotype, and demonstrating that Spe plants currently have a fitness in the wild that is at least as high as that of wild-type plants of C. bursa-pastoris, would not suffice to make a convincing case for Spe being a hopeful monster. Rather, one would have to demonstrate that Spe is still flourishing in, say, one million years time. However, we do not agree, because it is a characteristic feature of homeosis or other saltational changes in evolution that they readily produce morphological novelties within a short period of time. While these novelties may strongly influence the probability of short-term survival of the affected population, its long-term survival will much more depend on all kinds of contingencies rather than on the newly established morphological feature; hence long-term survival is irrelevant for the plausibility of saltational evolution.
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Acknowledgements |
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We thank Hannelore Simon for providing Fig. 1c. We are grateful to Richard M Bateman and an anonymous reviewer for helpful comments on a previous version of the manuscript. One of us (GT) is grateful to Paula J Rudall and Nick H Battey for their invitation to present our project in Session P5 (Flowering) at the SEB 2006 Meeting in Canterbury (UK) and hence to contribute to this volume. Work on Spe in the authors' laboratories is supported by grants TH 417/4-1 and NE 314/7-1 from the Deutsche Forschungsgemeinschaft (DFG).
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