Two bees or not two bees, that is the question

DNA barcodes associate male and female of extremely sexually dimorphic wild bee

I have been, and remain, critical of DNA barcoding. Not because it does not have important, even exciting, applications, as this story will illustrate, but because it is foolishly promoted as a substitute for serious taxonomic practices at a time when deep knowledge of species is urgently needed. So, I thought it was time to throw DNA barcoders a bone. One area where DNA barcoding is uniquely useful is in efficiently associating disparate life stages, such as larvae and adults of holometabolous insects, without the need for labor intensive rearing programs. Another one is associating males and females in species with extreme sexual dimorphism.

A recent example involves a wild bee from Australia, Xanthesma brachycera (Cockerell). These bees are 4.5 to 7.8mm in length, patterned in black and yellow. This is one among 1,661 named species of bees native to Australia, although hundreds of additional bee species await discovery or description. For the vast majority of these down-under natives, little is known of their natural history. The most species-rich taxon is the subfamily Euryglossinae of the family Colletidae. These small-bodied bees include the genus Xanthesma. This genus is so poorly known that even associating males with females for many species is, at present, nearly impossible. Because identification schemes have relied heavily on males, identifying females is especially challenging. The genus includes 48 species in 4 subgenera. Xanthesma brachycera illustrates the problem: males and females are so different looking that they were not only named by Cockerell as two separate species, but they were then assigned to two different subgenera! I leave aside the obvious question of the reliability of characters currently used to discriminate among subfamilies.

Figure 1. Female (left) and male (right) Xanthesma (Xenohesma) brachycera. Images are anterior (a and b), lateral (c and d), and dorsal (e and f). Scale bars are 0.5 mm. From Prendergast & Dorey, 2023. Photos: Remko Leijs, South Australian Museum. CC BY-NC-SA 3.0 Deed

It has long been known that, in this group, males have larger eyes than females, something I call the Little Red Riding Hood theory: “It has been suggested that the large eyes, which are an exclusively male feature in some bee groups, have evolved via intrasexual selection (sexual selection driven by competition between members of the same sex) as an adaptation to better spot females in mating swarms (Michener 2007; Somanathanet et al. 2017)” (Prendergast & Dorey, 2023). Or, stated another way: “What big eyes you have!”

Xanthesma brachycera was described by Theodore Dru Alison Cockerell in 1914, based on the female; and, unbeknownst to him, once again, two years later, as Xanthesma perpulchra, based on the male.

As an aside, Cockerell, who lived from 1866 to 1948, emigrated to the U.S. and became a specialist on bees and wasps. He published more than 2,000 scientific papers in which he named about 9,000 species, of which about 6,400 were bees or wasps. In an obituary by R.B. Benson, published in Nature in 1948, it was noted that Cockerell had developed the habit “of hurrying his ideas and observations into print as soon as he could. The habit persisted through his long life, so that almost all his work appeared in the form of short papers.” Given that comprehensive taxonomic revisions and monographs are the gold standard of excellence in systematics, this may not have been entirely complimentary.

Back to our story. Prendergast and Dorey (2023) have conclusively demonstrated that these two nominal Xanthesma are, in reality, one and the same species, based on specimens collected at the same time and place, morphology, and DNA barcoding. This situation is not unique to this species, thus we readily agree with the authors that “Together, DNA and morphological data are needed to reappraise the taxonomy of the euryglossines.”

It is past time that DNA data be accepted for what it is: just one source of data for taxonomy, useful in some circumstances, not so much, or not at all, in others. Morphology, fossils, DNA and developing embryos each give us unique insights. All have strengths, applications and limitations. One strength of DNA barcoding is to expedite the association of life stages and dimorphic sexes in species for which morphology is so divergent as to make such connections nearly impossible without rearing the species or carefully associating them under field conditions. By associating male with female, we not only improve our ability to accurately identify species, we instantly open an additional set of morphological characters for detailed study. We should embrace the synergistic and complementary possibilities for morphological and DNA data, not undermine the information content of taxonomy, or degrade the quality of descriptive morphology, by accepting DNA barcodes as a replacement for taxonomy done well.

Two bees or not two bees? Thanks to Prendergast and Dorey, and DNA barcodes, we now know that the answer is ‘not!’

References

Prendergast, K.S. and J.B. Dorey. 2023. Xanthesma (Xenohesma) perpulchra and Xanthesma (Xanthesma) brachycera are conspecific based on DNA barcodes.  Australian Journal of Taxonomy 39: 1-10.

Michener, C.D. 2007. The Bees of the World, 2^nd ed. Baltimore: Hopkins University Press.

Somanathan, H., Borges, R.M., Warrant, E.J., and A. Kelber. 2017. Visual adaptations for mate detection in the male carpenter bee Xylocopa tenuiscapa.  PLoS ONE 12: e0168452.