The systematics of Australian Daphnia and Daphniopsis (Crustacea: Cladocera): A shared phylogenetic history transformed by habitat-specific rates of evolution

Abstract

This study examines the molecular-genetic divergence and evolution of Australian aquatic micro-Crustacea Daphnia and Daphniopsis. The results indicate that species of Daphniopsis are accommodated within the genus Daphnia. Although their phyletic integrity is no longer supported, all Daphniopsis species (save one from North America) form a monophyletic group and may warrant subgeneric recognition pending further systematic investigations. A total of five lineages are shown to occupy Australian inland waters, including an endemic subgenus (Australodaphnia) and representatives of the subgenus Ctenodaphnia. The subgenera (Daphnia and Hyalodaphnia) that dominate the North American fauna are absent in Australia. The large extent of sequence divergence among major groups suggests that continental isolation has helped shape the early evolution of daphniids. More recent speciation is also evident, particularly by the Daphnia carinata species complex, whose numbers have grown to 13 members by the addition of a species previously assigned to the nominal subgenus and species yet to be formally described. The molecular data provide more evidence that the colonization of distinct habitats and ecological settings is a key factor in spurring diversification in the genus, while also modulating the pace of molecular evolution. This study attributes habitat-specific molecular clocks to the intense ultraviolet (UV) exposure in both saline and transparent oligohaline waters. Adaptations to these harsh environments by at least four independent lineages include the convergent acquisition of a melanic carapace. Yet some lineages, clearly under mutational duress, lack this commonly acquired protective trait. There are numerous adaptive lines of defense against UV damage, including the complex regulatory mechanisms required to initiate a cellular response to guard and repair DNA. Functional molecular studies may soon challenge a notion built on morphology that convergence is the general directive to Daphnia's ecological and evolutionary success. © 2006 The Linnean Society of London.