Phylogeny and structural evolution of the male genitalia in the genus Limnebius (Coleoptera, Hydraenidae)

Abstract

The genus Limnebius (family Hydraenidae) has approximately 130 described species, distributed in all biogeographical regions. They are small, morphologically uniform water beetles found in most inland aquatic habitats. Traditionally, Limnebius was divided in two subgenera based on the presence (Limnebius sensu stricto) or absence (Bilimneus) of parameres in the male genitalia. These were recently synonymised based on the existence of apparent intermediate groups with unclear affinities. Alternatively, the genus was subdivided in informal ¿species groups¿, mostly based on the secondary sexual characters and the male genitalia. Based on a molecular phylogeny including most of the species groups we reassess the systematics of the group and the evolution of sexual characters. We recover the two traditional subgenera as monophyletic, with the exclusion of the Nearctic species from Bilimneus and the separation of Limnebius sensu stricto in four major clades: 1) the Nearctic species, either without or with a fused paramere; 2) the L. aluta and 3) the L. truncatellus group, both with one paramere; and 4) the L. parvulus group, with two parameres. The L. mundus group, of which we do not have molecular data, would likely be a fifth clade with likely one partially (or completely) fused paramere. Other character complexes of the male genitalia as well as some secondary sexual characters are congruent with this clade separation. We trace the evolution of the complexity of the median lobe with its appendixes and the parameres in the genus, which follows a general pattern from simple to complex, with several independent origins of the most complex forms.