Abstract
BackgroundAs 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.ResultsWe 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.ConclusionOur 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.
Highlights
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
Panagrolaimus strains and reproductive mode analysis We initiated our study by requesting available Panagrolaimus strains from the Caenorhabditis Genetics Center and other helpful colleagues
In the summer of 2007 we sampled soils from diverse locations around Oregon and discovered seven additional Panagrolaimus isolate strains from which laboratory cultures were generated and frozen stocks were made
Summary
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. The nematode Caenorhabditis elegans has long served as an exceptional model system for understanding diverse biological phenomena such as development, neurobiology and genome structure [1,2,3]. Over the last decade nematodes in the Caenorhabditis genus have come to serve as increasingly powerful evolutionary models for studying diverse processes ranging from the mutation rate [4,5,6] to the evolution of reproductive mode transitions [7,8] to speciation mechanisms [9,10,11]. Multiple independent evolutionary transitions from gonochoristic to hermaphroditic reproduction have been observed in the latter two nematode genera [17,20] and a broad review of numerous rhabditid nematodes showed up to ten independent transitions to hermaphroditism [21]
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