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Background  As exemplified by the famously successful model organism  Caenorhabditis elegans , nematodes offer outstanding animal systems for investigating diverse biological phenomena due to their small genome sizes, short generation times and ease of laboratory maintenance. Nematodes in the genus  Panagrolaimus  have served in comparative development and anhydrobiosis studies, and the Antarctic species  P. davidi  offers a powerful paradigm for understanding the biological mechanisms of extreme cold tolerance.  Panagrolaimus  nematodes are also unique in that examples of gonochoristic, hermaphroditic and parthenogenetic reproductive modes have been reported for members of this genus. The evolutionary origins of these varying reproductive modes and the Antarctic species  P. davidi , however, remain enigmatic.    Results  We collected nuclear ribosomal RNA gene and mitochondrial protein-coding gene sequences from diverse  Panagrolaimus  species and strains, including newly discovered isolates from Oregon, to investigate phylogenetic relationships in this nematode genus. Nuclear phylogenies showed that the species and strains historically identified as members of  Panagrolaimus  constitute a paraphyletic group, suggesting that taxonomic revision is required for  Panagrolaimus  and related nematode lineages. Strain-specific reproductive modes were mapped onto the molecular phylogeny to show a single origin of parthenogenesis from a presumably gonochoristic ancestor. The hermaphroditic strains were all placed outside a major monophyletic clade that contained the majority of other  Panagrolaimus  nematodes. Phylogenetic analyses of mitochondrial sequences showed that substantial molecular and geographic diversity exists within the clade of parthenogenetic strains. The Antarctic species  P. davidi  was found to be very closely related to two  Panagrolaimus  strains from southern California. Phylogenetic and molecular clock analyses suggested that  P. davidi  and the California strain PS1579 shared a common ancestor in the very recent evolutionary past.    Conclusion  Our study provides a phylogenetic framework for understanding the evolutionary origins and diversification patterns of varying reproductive modes within  Panagrolaimus  and important insights into the origin of the Antarctic species  P. davidi .  Panagrolaimus  offers a powerful nematode model for understanding diverse evolutionary phenomena including the evolution of asexuality and the adaptive evolution of extreme cold tolerance.

Molecular evolution in Panagrolaimus nematodes: origins of parthenogenesis, hermaphroditism and the Antarctic species P. davidi