Isopod crustaceans of the family Asellidae are one of the most widespread freshwater crustacean groups. Members of the family occur throughout North America, and occupy a wide range of habitats (wetlands, streams, surface springs, caves). However, they are not well studied, with very little known about their evolutionary history and biogeography. Here we aim to shed light on this family by examining the evolutionary and biogeographic history of selected Nearctic Asellids, with a focus on generic and geographical diversity. Phylogenetic analyses were conducted using a segment of the nuclear 28S rDNA and mitochondrial Cytochrome c oxidase subunit I (COI), with both novel sequences generated here, and data obtained from GenBank. From concatenated data Maximum-Likelihood phylogenies were reconstructed using IQTREE, and a time-calibrated Bayesian phylogeny was reconstructed using BEAST, based on a fossil calibration scheme. Results generated here reveal intriguing patterns not yet observed within the family. Several genera such as Caecidotea and Lirceolus appear to not be monophyletic; in addition, groundwater-adapted taxa appear in numerous places across the phylogeny, suggesting multiple colonizations of this habitat. Asellids occurring along the west coast of North America appear to be distinct when compared to others, with taxa such as Calasellus californicus showing affinity to Eurasian taxa, and Caecidotea occidentalis appearing basally-derived when compared to more diverse eastern taxa. These data might suggest that asellid taxa reached the Nearctic through a western route. Time-calibrated phylogenies place such an event during the Cretaceous, approximately 135 MYA. Given this time frame, asellid colonization of North America may have been influenced by the Hauterivian Cold Snap.
Author(s): Kye Tomoyasu, Andrew G. Cannizzaro, Thomas R. Sawicki & David J. Berg
Advisor(s): David Berg, Biology
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