Up high and down low: Molecular systematics and insight into the diversification of the ground beetle genus Rhadine LeConte

Abstract

Rhadine LeConte is a Nearctic genus of flightless ground beetles that is poorly studied despite its relevance to evolutionary studies of subterranean fauna. Adults are notable for their slender and leggy habitus and the wide variety of habitat preferences among species, with several known only from mountaintops while others are restricted to caves or more general subterranean habitats. In central Texas, USA there are several cave endemics relevant to conservation. Here we present the first phylogenetic hypothesis for the overall structure of the genus with an emphasis on the troglobites in central Texas. We infer the phylogeny of Rhadine from ∼2.4-kb of aligned nucleotide sites from the nuclear genes, 28S rDNA and CAD, and the mitochondrial gene COI. These data were obtained for 30 species of Rhadine as well as from members of their putative sister group, Tanystoma Motschulsky. Results reveal that Rhadine is polyphyletic, and morphological characters that have been traditionally used to classify the genus into species groups are shown to be convergent in many cases. Rhadine aside from two species of uncertain placement is composed of two major clades, Clades I and II that both include epigean and subterranean species in very unequal proportions. Clade I is primarily composed of subterranean species, and Clade II includes many epigean species and high altitude montane endemics.A clade of troglobitic, cave-restricted species in Texas includes several species of large-eyed cave Rhadine. The slender habitus typical of some species [e.g., R. exilis (Barr and Lawrence), R. subterranea (Van Dyke), R. austinica Barr] evolved independently at least three times. Major biogeographic and evolutionary patterns based on these results include: troglobitic species north of the Colorado River in Texas (that also lack lateral pronotal setae) are found to comprise a monophyletic group, beetles in caves south of the Colorado River likely form another monophyletic group, and the “species pairs” of troglobitic Rhadine known to occur in the same caves are not resolved as each other’s sister group, suggesting that these caves were colonized independently by more than one lineage of Rhadine. Our divergence time estimates support a Miocene age for the split between Clade I and II Rhadine and indicate that all subterranean Clade I Rhadine began diversifying in the late Miocene–early Pliocene, contemporary with cave formation in the Balcones Escarpment.