Phylogenetics and Systematics of the Millipede genus Brachycybe Wood, 1864 (Platydesmida: Andrognathidae)

Abstract

The genus Brachycybe Wood, 1864 (Platydesmida: Andrognathidae) belongs to an ancient millipede lineage that has persisted since the Miocene. The group displays a Holarctic distribution with species in the eastern Nearctic, western Nearctic, and eastern Palaearctic. Like many millipede groups, its taxonomy to date has been based solely on morphology, which may under represent species richness by ignoring cryptic species. Here we present the first phylogenetic analyses for Brachycybe based on molecular data obtained from the mitochondrial genes cytochrome c oxidase 1 and cytochrome b. Standard phylogenetic methods were used to reconstruct the evolutionary relationships of the group. Using the internal phylogenetic framework, we evaluated geographic associations among clades; species boundaries were evaluated using the cohesion species concept. The application of molecular phylogenetics, phylogeographic methods, and the cohesion species concept to the genus Brachycybe is used to evaluate its species diversity. The mitochondrial genome of Brachycybe lecontii Wood, 1864 (Platydesmida: Andrognathidae) is also presented here along with the phylogeny of Myriapoda, produced using the amino acid sequences from the entire mitochondrial genome. The complete mitochondrial genome sequence of Brachycybe lecontii comprises 15,465 base pairs and includes 13 protein-encoding genes, 21 tRNA genes, two rRNA genes, and two large non coding regions that are approximately 436 and 464 bp in length. The genome is uncharacteristic of typical Diplopods and varies dramatically from other sequenced members of the Arthropoda. The unique gene synteny is novel among others because of the rearrangement of two protein coding genes (nad1 and nad5) along with their retained transcriptional polarity. The phylogeny produced from these data recover the monophyly of the Myriapoda classes. However, support values within the Diplopoda are low, possibly due to the inclusion of atp8 in the analysis. This will have important implications for taxonomic research because it will expand the growing bank of complete millipede mitochondrial genomes, allow more robust phylogenetic analyses, and aid in the development of more genetic markers and consequently larger nucleotide datasets.