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Comment: grape phylloxera and the grapevine: searching for the science
- Douglas A Downie (9 August 2006)
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Douglas A Downie, lecturer Rhodes University, Grahamstown 6140, South Africa
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Comment: grape phylloxera and the grapevine: searching for the science Douglas A. Downie Department of Zoology and Entomology, Rhodes University, Grahamstown 6140, South Africa, phone: +27 046 603-8086, fax: +27 046 622-8959, d.downie@ru.ac.za Abstract A recent article in Journal of Experimental Botany (Battey and Simmonds 2005) discussed the biology and life cycle of the phylloxerid Daktulosphaira vitifoliae (Fitch), a plant galling parasite of North American grapevines and pest of cultivated grapes, in near mystic terms. Comment is here made on the accuracy of this piece and its appropriateness in a scientific journal. Keywords: coevolution, grape phylloxera, grapevines, plant-insect interactions Grape phylloxera, Daktulosphaira vitifoliae (Fitch), a notorious pest of the cultivated grapevine, has captivated the imaginations of a relatively small subset of people both in and out of science (Ordish 1972, Campbell 2005). The reason for this is not entirely clear for insects are generally a fascinating group of organisms and often have severe economic impacts, but likely it has something to do with the crop it is so notorious for damaging (the wine grape, Vitis vinifera (L.)) and the myths and rituals that have built up around it, as well as the complex and fascinating life cycle of the insect. This basic life history is however quite common in the group of insects to which grape phylloxera belongs, as some 4,400 species (Stern 1995) in the superfamily Aphidoidea share very similar life history patterns, not to mention the even more bizarre and fascinating life histories in the scale insects (superfamily Coccoidea). This somehow suggests that the cult of grape phylloxera is intimately associated with the cult of the grape and wine, perhaps more than the biology itself. Be that as it may, a recent article in Journal of Experimental Botany (Battey & Simmonds 2005) brings to stark and quite astounding juxtaposition how far and absurd this can all get. Three issues provoke comment after reading this metaphysical little ditty: 1) the antiquated and inaccurate depiction of the grape phylloxera-grape interaction, 2) the complete absence of and ignorance of an evolutionary perspective in a piece that uses “Evolution” as a keyword, and, 3) how such non-scientific ramblings come to be published in a peer-reviewed scientific journal. 150 years of research and still the same old story. To begin, I note the unfortunate and persistent practice of perpetuating the common name, phylloxera, for D. vitifoliae. Grape phylloxera has had a checkered nomenclatural history, and the genus names Pemphigus, Rhizaphis, Peritymbia, Viteus, and Daktulosphaira as well as Phylloxera have been applied to it (Russell 1974). For many years Phylloxera was the most commonly applied name so the common name and Latin genus name were one and the same. This name was subsiding from use after 1952 however and Russell (1974) cleared the nomenclatural mess up more than thirty years ago. There are some 41 described species in the genus Phylloxera (Pergande 1903)(grape phylloxera is of course not one of them, under our current but under researched understanding), all of which are equally as interesting as D. vitifoliae, and it is a violation of common sense as well as biological sense and taxonomic practice to give a common name to a species in one genus the Latin name of a diverse group of species in another. It would be comforting to think that the practice has perhaps been maintained by people who haven’t known any better (journalists, grape growers, wine aficionados, etc) but a substantial number of biologists who should know better have contributed to this misnomerization. At the very least D. vitifoliae should be more precisely described as “grape phylloxera” to distinguish it from and do justice to all the true Phylloxera species (most of which feed on hickories in North America, creating even more elaborate galls than D. vitifoliae in the process). I follow this practice here, as I do elsewhere. Needless to say, Battey and Simmonds (2005) come nowhere near to bringing this tiny advance into the now nearly purple grape phylloxera literature. In the early days of grape phylloxera deprivations it was thought a disease afflicted the vines. This idea was dispelled when the insect was correlated with the symptoms. It has long been known that pathogens are associated with grape phylloxera damage (Millardet 1892) but grape phylloxera is not a disease in the common usage, it is a parasitic insect. Nowhere in the ecological literature will one find the attacks of parasitic herbivores or the herbivores themselves referred to as diseases. All evidence indicates grape phylloxera provides its host with absolutely no benefits but it is more akin to the mites that cause scabies than to disease causing organisms. The role grape phylloxera may sometimes play in facilitating fungal and other pathogens (Omer et al.1995, Omer & Granett 2000) might tempt one to call the phenomenon of this complex interaction a disease, but this should then be given a different name than the common name of the insect itself (which name is of course inappropriate as described above). It has come to be accepted in grape phylloxera lore that vines that had the opportunity to coevolve with the insect have evolved resistance. As logical as this may seem, it should be pointed out that the evolution of resistance of native North American grapevines to grape phylloxera has received absolutely no research attention. There have been attempts to understand the mechanisms and genetics of resistance in cultivars, but none have taken an evolutionary approach. Though the evidence is scant, it suggests that grape phylloxera has not yet colonized many of the native Vitis species (Downie et al. 2000) so that resistance in some cases may derive not from co-evolution but from non-host effects. It is possible that the current host range may be a consequence of a narrowing of a once larger host range but without a phylogenetic analysis of the family Phylloxeridae the plausibility of this hypothesis is difficult to assess. It is also true that there are no data supporting the assumption that wild grapevines infested with grape phylloxera suffer no negative fitness effects (are tolerant). The observation of apparent tolerance led to the fortuitous use of native vines in the development of host plant resistance against grape phylloxera but the ecological and evolutionary basis of this good luck is more complicated than is ever acknowledged. Fitness in native grapevines has never been measured. The reason for this is simple: grapevines may live up to 70 years or more and measuring and comparing fitness among individuals in meaningful ways is problematic; research has understandably tended to focus on the cropping system neglecting the native system and the route to resistance. Further, it is usually neglected that vines under the regime of monoculture, fertilization, pruning, harvest, etc. are not easily comparable to vines in the unmanaged state - susceptibility has more than just genetic or phylogenetic components. Nevertheless, the myth of resistance by coevolution persists. In spite of statements from reputable sources that they do (Granett et al. 2001) grape phylloxera do not cause galls on vine tendrils under natural conditions. In extensive surveys of native North American Vitis species (Downie & Granett 1999, Downie et al. 2000, Downie et al. 2001) during which I have examined hundreds of grape phylloxera infested vines I have never seen a grape phylloxera induced gall on a tendril. I have however seen an unidentified species of Cecidomyidae (gall midge), possibly Lasioptera vitis or a Janetiella species, causing such galls and it may be that some of the illustrations of galls on vine tendrils by Cornu (1878) were of this species. The images have been perpetuated as fact in the grape phylloxera lore however. Cornu did his work on cultivated plants in France and the figured gall in the Battey and Simmonds article appears to be a grape phylloxera gall but rare examples of grape phylloxera induced galls on tendrils and other abnormal locations in vitro, in greenhouses, or other artificial conditions (of which the vineyard situation could be argued to be a rather special example, though galls are not found on tendrils in these situations either) are irrelevant to the native biology of the insect (the topic of Battey & Simmons). Representing the abnormal as the normal is inexplicable. Following many other authors, Battey and Simmonds describe features of the life cycle (“…nymph hatches in response to cooling weather in the autumn.” “...female lays a winter egg in slits of the bark.” p. 3029; “...5000 million descendants...” p.. 3031), and induction of galls as though there were actually hard data supporting these statements. There are not. They derive from casual observations and conjectures that have been repeated so often that they have become ingrained as fact. To add my own casual observations to the din I have observed sexuparous nymphs and adults at all times of the growing season and suggest that they are as likely to be induced by crowding and deterioration of the resource as by cooling weather. Also, experiments that show that exogenously applied auxin can induce gall-like growths on grapevine roots do not demonstrate that this is a mode of gall-induction in grape phylloxera (Granett 1990) or any other insect. A gall myth: what happened to the evolutionary arms race? One wonders if a temporal or spatial shift has occurred in the firmament upon reading Battey and Simmonds’ “gall myth”, if one has suddenly been transported to a completely different type of publication, in a different time. “gloomy green light” of the gall? “imagine they were once one creature”? “Guttation fluid falling and turning into insect eggs”? The colonization of leaves by first instar crawlers a “return to the insect’s beginning”? The fundatrix “conversing” with the leaf? A “complicity between the insect and plant”? The plant and the insect’s “purpose is to re-unite”? Well, it is a myth, as stated, isn’t it? But what is it doing in a biological, scientific journal? More than a century of research in plant-insect interactions not to mention most other fields of biology have swiftly been discarded, and now the evolutionary interaction between host and parasite has become some kind of mystic mutualism, a romance even. The model of an arms race between plant and insect may be out of date (Stamp 2003) but there is nothing in the current science of plant-insect interactions that could have spawned this! A fascinating evolutionary interaction has been turned into a Reader’s Digest sidebar. Do we need this, on any level whatsoever? What happened to the science? More disturbing than the quasi-religious tone of this piece is its placement. That there is little science in this piece will be obvious to most readers, but why was it not obvious to the reviewers and editors of this journal? Since reading this piece I have learned that this is part of a series of essays of a non-technical nature primarily by Battey. So what? Is a peer-reviewed scientific journal the appropriate venue for such mutterings? Besides the absence of any science the article isn’t even referenced properly. The lapse of editorial standards is inexplicable. There is a way forward for understanding such complex and important biological interactions, and that is by an explicit evolutionary genetic approach informed by both evolutionary and ecological theory rather than by approaches dominated by market or mythic appeals of the vine. Acknowledgements This work was supported by a Joint Research Committee Grant from Rhodes University. References Battey NH, Simmonds PE. 2005. Phylloxera and the grapevine: a sense of common purpose? Journal of Experimental Botany 56: 3029-3031. Campbell C. 2005. The Botanist and the Vintner: How Wine was Saved for the World. Alqonquin Books, Chapel Hill, USA. Cornu M. 1878. Etudes sur la Phylloxera vastatrix. Me´moires pre´sente´s ..a`l’Acade´mie des Sciences de l’Institut de France XXVI, 43–175. Downie DA, Granett J. 1999. Distribution, abundance, and short-term persistence of grape phylloxera in two regions of the native range. Environmental Entomology, 28: 1004 –1013. Downie DA, Granett J, Fisher J. 2000. Distribution and abundance of leaf galling grape phylloxera and Vitis species in the central and eastern United States. Environmental Entomology 29: 979 –986. Downie DA, Fisher JR, Granett J. 2001. Grapes, galls, and geography: the distribution of nuclear and mitochondrial DNA variation across host plant species and regions in a specialist herbivore. Evolution 55: 1345- 1362. Granett J. 1990. Comparison of indolacetic acid and tuberosities induced by grape phylloxera (Homoptera: Phylloxeridae). Journal of Economic Entomology 83:494-499. Granett J, Walker MA, Kocsis L, Omer AD. 2001. Biology and management of grape phylloxera. Annual Review of Entomology 46: 387–412. Millardet A. 1892. New researches upon the resistance of, and exemption from, phylloxera. Ann. Rep. Board State Vitic. Comm. 1891-92, pp. 265–79. Sacramento, CA: State Print. Omer AD, Granett J, De Benedictis JD, Walker MA. 1995. Effects of fungal root infections on the vigor of grapevines infested by root-feeding grape phylloxera. Vitis 34:165–70. Omer AD, Granett J. 2000. Relationship between grape phylloxera and fungal infection of grape vines. J. Plant Dis. Prot. 107:285–94. Ordish G. 1972. The great wine blight. London: JM Dent & Sons. Pergande T. 1903. North American Phylloxerinae affecting hickory and other trees. Proceedings of the Davenport Academy of Sciences 185-273. Russell, L. M. 1974. Daktulosphaira vitifoliae (Fitch), the correct name of the grape phylloxeran. Journal of the Washington Academy of Science 64: 303-308. Stamp N. 2003. Out of the quagmire of plant defense hypotheses. Quat. Rev.Biol. 78: 23-55. Stern DL. 1995. Aphidomorpha. Aphids, green flies, plant lice, adelgids, phylloxerids. Version 01 January 1995 (under construction). http://tolweb.org/Aphidomorpha/10985/1995.01.01 in The Tree of Life Web Project, http://tolweb.org Conflict of Interest:None declared |
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