Comments on the proposed conservation of usage of Drosophila Fallén, 1823 (Insecta, Diptera) 1 (Case 3407)

Publication Type:Journal Article
Year of Publication:2008
Authors:S. R. Prigent
Journal:Bulletin of Zoological Nomenclature
Volume:65
Issue:2
Start Page:137
Pagination:137-140
Date Published:06/2008
Type of Article:Comment
ISSN:0007-5167
Full Text

The proposition of Van der Linde et al. (BZN 64: 238–242) to maintain the name of Drosophila melanogaster has two consequences. The first is the loss of the name ‘Sophophora’ through synonymy with Drosophila. The second is the loss of identity of the current genus Drosophila (s.s.). To justify their proposition Van der Linde et al. (BZN 64: 238–242) emphasized the role of D. melanogaster in science and weakened the taxonomical significance of Drosophila (s.l.) and Drosophila (s.s.) that they proposed to split. In my opinion their arguments are oversimplified or not justified. The species of Drosophila (s.s.) have also played a major role in science and the classification is not as messy as it is suggested. I think the proposed nomenclatural change would be more detrimental for science than the simple elevation of Sophophora to the genus rank.

What is the colloquial meaning of Drosophila?
As indicated in the application, the genus Drosophila was established by Fallén (1823) to include twelve species. But the scientific renown of Drosophila was acquired later, at the beginning of the 20th century, when several species became study material for biological research and particularly for genetics. It is worth noting that the Drosophila model has never been restricted to only one species and more than 200 species of Drosophila have been cultured for laboratory research. Today most of the species under study are provided by stock centres, the Tucson and the Ehime centres being the two most important. Despite the name ‘Drosophila stock centres’ both provide species of other genera. The Tucson DrosophilaSpecies Stock Center (http://stockcenter.arl.arizona.edu/) provides subcultures of approximately 240 different Drosophila species. These include species of Chymomyza, Hirtodrosophila, Samoaia, Scaptodrosophila, Scaptomyza, Zaprionus as well as the Hawaiian ‘Drosophila’. The Ehime Drosophila stock centre (http://kyotofly.kit.jp/cgi-bin/ ehime/index.cgi) maintains 400 strains of 50 species and distributes these genetic resources to Drosophila researchers worldwide. These too include Colocasiomyia, Chymomyza, Hirtodrosophila, Scaptodrosophila and Zaprionus. Thus it is clear that, for geneticists, the name ‘Drosophila’ does not mean specifically D. melanogaster but the family DROSOPHILIDAE (and so includes D. melanogaster). Fly geneticists used to refer to the model species as ‘melanogaster’ instead of ‘Drosophila’ because the research community is aware that many species are used as study material. Research is also carried out on albomicans, ananassae, immigrans, indianus, kikkawai, mojavensis, virilis and, whichever genus individual species belong to, all are considered to be ‘Drosophila’. This usage suggests that, even under the name of Sophophora melanogaster, the species will still be considered as a ‘Drosophila’ and the term can be used in the titles and keywords of future publications. We should also note Bulletin of Zoological Nomenclature 65(2) June 2008 137 that ‘drosophila’ (without initial upper case and not italicized) would be more appropriate.

The extent of paraphyly in Drosophila
The paraphyly of the voluminous genus Drosophila has been recognized for decades, and by numerous authors, for example Throckmorton (1975) in his phylogenetic analysis. Nonetheless, the only work to really address the question was the cladistic study by Grimaldi (1990). This contribution was so important that the classification it established for drosophilid species has remained fundamentally unchanged ever since. However, he admitted that a definitive, comprehensive study of relationships between subgenera and species groups in Drosophila remained to be done.
Van der Linde et al. (BZN 64: 238–242) mentioned Hawaiian ‘Drosophila’ as a clade within Drosophila (s.s.), however this situation has not been fully accepted. Grimaldi (1990) grouped those species in the genus Idiomyia Grimshaw, 1901 and he suggested that calling Idiomyia (s.l.) ‘Drosophila’, despite the morphological evidence to the contrary, would be ‘diluting the diagnosis of the genus Drosophila, as a monophyletic group, of biological meaning.’ Moreover the Hawaiian ‘Drosophila’ are generally recognized to be the sister group of the genus Scaptomyza. Therefore it would be illogical to downgrade the Hawaiian ‘Drosophila’ but not Scaptomyza under the genus Drosophila. Despite some unfounded contestation the Hawaiian ‘Drosophila’ were considered to belong to Idiomyia by Grimaldi (1990) and are still classified in this way in Bachli’s database (http://taxodros.unizh.ch/). Therefore, the monophyletic genus Drosophila, as defined by Grimaldi (1990), consists of only three major clades: Sophophora and the immigrans-tripunctata and virilis-repleta radiations of the subgenus Drosophila.
Today there are good arguments to upgrade Sophophora to generic status, particularly thanks to the meticulous work of M.J. Toda and his team (e.g. Hu & Toda, 2001). Moreover, the results of their morphological analyses are also supported by molecular data. Nevertheless, this does not justify disruption of the subgenus Drosophila as there is no morphological analysis indicating its paraphyly and most molecular analyses fail to resolve the branch order in this part of the phylogeny. Van der Linde et al. (BZN 64, pp. 238–242) argue that various genera are positioned within Drosophila (s.l.). In fact these genera are positioned between Sophophora and Drosophila (s.s.), or between the Hawaiian ‘Drosophila’ and the radiations of Drosophila (s.s.), but there is no strong evidence that any such genera intercalate between the immigrans-tripunctata and virilis-repleta radiations. If molecular analysis eventually suggests the paraphyly of Drosophila (s.s.) it would be necessary to study the morphology more deeply. As far as is known, however, the genus Drosophila is monophyletic once the subgenus Sophophora is removed (Grimaldi, 1990; Hu & Toda, 2001).

What would be the impact of the nomenclatural change on the classification?
As mentioned above, paraphyly in Drosophila arises mostly from the inclusion of the subgenus Sophophora. This problem in systematics is easily resolved by upgrading Sophophora to generic rank. This change would affect the names of only the 332 species currently classified within Sophophora, including Drosophila (S.) melanogaster which would then be called Sophophora melanogaster. Stability would be maintained for the remaining Drosophila species, which are considered to be monophyletic. Van der Linde et al. (BZN 64: 238–242) propose retaining the binomen Drosophila melanogaster on the grounds of convenience. In that situation the subgenus Sophophora will become synonymous with Drosophila and the names of all 332 species in Sophophora will consequently become Drosophila. However, the names of all 817 species currently in the subgenus Drosophila will also have to change, as they do not belong to the same clade as Sophophora. Moreover, 78 species that had no subgeneric affiliation in the former Drosophila genus will stay in the new genus. Therefore, the Sophophora subgenus, previously defined as a monophyletic group, will now be mixed together with numerous species with which it probably has no affinity.

What would be the impact on scientific research?
The first research purpose for Drosophila was as a model in evolutionary biology, and this led naturally to the study of numerous species. As the majority of Drosophila species cultured were from the subgenera Sophophora and Drosophila, the literature of the 20th century involves the continuous comparison of the two lineages. This comparison disappears if Sophophora is synonymised with Drosophila (s.s.). Most new students of Drosophila systematics have to learn from the literature and the major changes proposed will render their task particularly difficult. The proposed nomenclatural change would also be more detrimental than its alternative for retrieving information from databases. It is worth noting that 642 taxa are represented under the genus Drosophila in sequence database according to the NCBI Entrez Taxonomy Homepage. With one click in the database, Sophophora could be moved to the genus level beside Drosophila but the proposed changes require Sophophora to be eliminated and reindexing of every one of the 456 taxa included in Drosophila (s.s.). Again reclassifying stocks at the different stock centres will be more difficult under the proposed nomenclatural change than the simple elevation of Sophophora to genus rank.
Changing Drosophila melanogaster to Sophophora melanogaster would have no impact as the model is routinely used and accepted in the scientific community and will never be abandoned. A change to Sophophora melanogaster would not be the end of the world for melanogaster geneticists but the proposed alternative might be so for others. For the stability of numerous branches of science (including systematics) the elevation of Sophophora to generic rank is preferable to the nomenclatural change proposed. A change in the status of Sophophora is likely to be relayed instantaneously through Flybase (http://flybase.bio.indiana.edu/) and major scientific journals. In consequence it would be promptly learnt by all researchers, whereas the alternative would have less media-penetration and the period of misuse and confusion would be consequently longer. It is worth noting that the Blast function of Flybase indicates D. melanogaster as being in the subgenus Sophophora and that most melanogaster geneticists must therefore be aware of its present classification. Geneticists should recognize that science is never static and thus, as an integral part of science, neither is systematics. Furthermore, if D. melanogaster is considered so sacred by geneticists, they should be less reticent to accept S. melanogaster in honour of the outstanding pioneer of genetics, A.H. Sturtevant, who established Sophophora.

Additional references

Grimaldi, D.A. 1990. A phylogenetic, revised classification of the genera in the Drosophilidae (Diptera). Bulletin of the American Museum of Natural History, 197: 1–139.

Hu, Y.G. & Toda, M.J., 2001. Polyphyly of Lordiphosa and its relationships in Drosophilinae (Diptera: Drosophilidae). Systematic Entomology, 26(1): 15–31.

Throckmorton, L.H. 1975. The phylogeny, ecology and geography of Drosophila. Pp. 421–469 in King, R.C. (Ed.), Handbook of genetics. Plenum Press, New York.

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Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith