Phylogenetic study of the order Polydesmida (Myriapoda: Diplopoda)

Academic abstract The millipede order Polydesmida, or flat-backed millipedes, is the most species rich among the class Diplopoda. At present, the order does not have a phylogenetic systematic classification, and the relationships of the 29 families are unknown. Whole genome DNA sequencing technologies and bioinformatic tools for the analysis of systematic data have improved over the last couple of decades, and have facilitated analysis of phylogenetic relationships. Here I sequenced the whole genomes of 82 species of the order Polydesmida and assembled a set of 281 orthologous genes using aTRAM. These genes were then used to estimate a maximum likelihood phylogeny. The families Paradoxosomatidae, Sphaeriodesmidae, Rhachodesmdidae, Platyrhacidae, Xystodesmidae, Polydesmidae, and Dalodesmidae were recovered as monophyletic groups. Some other families such as Trichopolydesmidae, Chelodesmidae and Macrosternodesmidae now appear to be polyphyletic groups, and a more comprehensive study of their molecular and morphological characters is needed. Phylogenetic trees can be used as maps to trace the evolution of characters in a group. Using the estimated tree for Polydesmida, I mapped morphological and molecular characters to understand their evolution. Traditional taxonomy in millipedes primarily used the morphology of the male gonopods to differentiate species. In my phylogeny, the morphological characters associated with the gonopod were less variable between specimens of the same family than non-gonopodal morphological characters, which showed a great diversity between individuals of the same family. A set of molecular characters that I studied were the gene order of the mitochondrion, as they may be a source of important evolutionary information. I found an inversion of half of the mitochondrial genome, which was consistent for the entire order Polydesmida. The majority of the changes in gene order were observed in the tRNA's, some of which can be considered synapomorphies for some families. Finally, I described three new species of millipedes in the superfamily Platyrhacoidea from southwestern Colombia: Aphelidesmus carcharodus and Pycnotropis svanae in the family Aphelidesmidae, and Barydesmus peineta in the family Platyrhacidae. I present diagnoses, descriptions, color habitus photographs and illustrations for these new species which contribute to the knowledge of millipedes of Colombia.