Background
Aciliini presently includes 69 species of medium-sized water beetles distributed on all continents except Antarctica. The pattern of distribution with several genera confined to different continents of the Southern Hemisphere raises the yet untested hypothesis of a Gondwana vicariance origin. The monophyly of Aciliini has been questioned with regard to Eretini, and there are competing hypotheses about the intergeneric relationship in the tribe. This study is the first comprehensive phylogenetic analysis focused on the tribe Aciliini and it is based on eight gene fragments. The aims of the present study are: 1) to test the monophyly of Aciliini and clarify the position of the tribe Eretini and to resolve the relationship among genera within Aciliini, 2) to calibrate the divergence times within Aciliini and test different biogeographical scenarios, and 3) to evaluate the utility of the gene CAD for phylogenetic analysis in Dytiscidae.
Results
Our analyses confirm monophyly of Aciliini with Eretini as its sister group. Each of six genera which have multiple species are also supported as monophyletic. The origin of the tribe is firmly based in the Southern Hemisphere with the arrangement of Neotropical and Afrotropical taxa as the most basal clades suggesting a Gondwana vicariance origin. However, the uncertainty as to whether a fossil can be used as a stem-or crowngroup calibration point for Acilius influenced the result: as crowngroup calibration, the 95% HPD interval for the basal nodes included the geological age estimate for the Gondwana break-up, but as a stem group calibration the basal nodes were too young. Our study suggests CAD to be the most informative marker between 15 and 50 Ma. Notably, the 2000 bp CAD fragment analyzed alone fully resolved the tree with high support.
Conclusions
1) Molecular data confirmed Aciliini as a monophyletic group. 2) Bayesian optimizations of the biogeographical history are consistent with an influence of Gondwana break-up history, but were dependent on the calibration method. 3) The evaluation using a method of phylogenetic signal per base pair indicated Wnt and CAD as the most informative of our sampled genes.