Extrinsic and intrinsic influences on the phylogeography of the Austrocochlea constricta species group

Abstract

Although extrinsic factors, such as oceanic currents and isolation induced by sea level maxima during Plio-Pleistocene glacial cycles, are often suggested as principal determinants of marine phylogeography, they are not always complete explanations. The counterexamples to predominant phylogeographic patterns in southeastern Australia suggest for example, that intrinsic factors such a habitat preference or reproductive mode can have significant influence. We collected DNA sequences from mitochondrial cytochrome c oxidase I (COI) and two nuclear gene introns from the Austrocochlea constricta species group to determine whether its porcata and constricta phenotypes are genetically distinguishable, to examine the phylogeographic effects of the Bassian Isthmus landbridge formed between Tasmania and Victoria during glacial maxima, and to investigate the importance of intrinsic factors in structuring its genetic variability. No fixed genetic differences between the porcata and constricta phenotypes were identified in any gene so the number of species comprised by the group cannot yet be determined. The two major clades recovered in COI analyses were respectively found principally east or west of the Bassian Isthmus. A. constricta is the first lecithotrophic or estuarine invertebrate known to show this pattern. There were no fixed differences, for any of the three genes, between specimens from estuarine and marine populations within either the eastern or western COI clades. Other intrinsic factors such as breeding period (possibly) and larval type (probably) may play roles in modulating phylogeographic patterns.