Danaus erippus, ISSUE

Smith, David A. S., Lushai, Gugs & Allen, John A., 2005, A classification of Danaus butterflies (Lepidoptera: Nymphalidae) based upon data from morphology and DNA, Zoological Journal of the Linnean Society 144 (2), pp. 191-212 : 198

THE ERIPPUS ISSUE

From its first description as Papilio erippus Cramer, 1775 , erippus has been conventionally separated from plexippus , which it replaces south of the Amazon. However, Forbes (1939), Clark (1941) and Urquhart (1960: 179) treated erippus as one of three subspecies of plexippus (the other two being the migratory plexippus s.s. and the nonmigratory megalippe). On the other hand, by the time he reached p. 182, Urquhart had changed his mind and D. erippus was once more a species! Whereas A & V-W grudgingly award erippus specific status, they say it is ‘hardly distinct’ and ‘may be better regarded as conspecific [with plexippus ]’.

The only distinguishing character observed in adult butterflies is the paler hindwing margin of erippus compared to plexippus and megalippe (character 60, Appendix 1A). However, Kitching (1985) found two pupal characters to distinguish erippus from plexippus and subsequently ( Kitching, 1986) identified three diagnostic allozyme differences, though his sample of erippus was small (N = 9, plus 9 progeny). We have sequenced the mitochondrial 12S (344 bp) and nuclear 18S rRNA (525 bp) genes of both plexippus from Australia (N = 8) and north-eastern America (N = 4) and erippus (N = 1) and found no differences. Whereas both these genes are relatively conserved, the 12S locus is, nevertheless, otherwise 100% diagnostic for all the Danaus species , and even some subspecies of D. chrysippus s.l. ( Lushai et al., 2003a).

The erippus issue has, however, recently been addressed by Brower & Jeansonne (2004). Using mtDNA sequence data from the COI, tRNAleu and COII loci, they find an average genetic divergence (GD) of 4.8% between erippus (represented by only one specimen) and plexippus + megalippe, compared to a pairwise mean GD of only 0.3% that separates geographically diverse samples among the latter group. Thus, they claim that D. erippus is a distinct species, as originally described by Cramer, and not merely a southern race of D. plexippus , as suggested by Kitching, Ackery & Vane-Wright (1993). We concur with this decision.

Applying the 1.2% per million years mtDNA divergence rate hypothesized for closely related insect taxa ( Brower, 1994), the separation of D. erippus and D. plexippus probably occurred some 2 million years ago (late Pliocene-early Pleistocene). We suggest the possibility that an ancestral erippus-plexippus population was initially divided during an interglacial period around this time, when successive raised sealevels of + 100 m and + 60 m (compared to the present day) occurred. At such times Amazonia was inundated and became a series of huge brackish and freshwater lakes that extended west from the Atlantic to the foot of the Andes in eastern Peru ( Haffer, 1987). A succession of Amazon embayments formed a geographical barrier that could have isolated erippus to the south from megalippe to the north for several thousand years at a stretch, ample time to trigger speciation.