The Evolution of Uniparental Reproduction in Rhabditina Nematodes: Phylogenetic Patterns, Developmental Causes, and Surprising Consequences

Many groups of eukaryotes with an ancestrally outcrossing sexual system include species that have adopted uniparental reproduction (either parthenogenesis or self-fertility). Nematodes are one such group. Because of recent advances in molecular phylogenetics, the evolutionary patterns of nematode sexual mode are becoming clearer. In this chapter, we first present a molecular phylogeny of over 200 Clade 9 nematodes and map changes in sexual mode upon it. Clade 9 includes both parasites of the order Strongylida and free-living species that include Caenorhabditis elegans, the most studied nematode. Uniparental reproduction has evolved a minimum of 15 times in the clade, but none of these are within the clade of parasitic species. We discern two local phylogenetic patterns after the origin of uniparental reproduction. In Pattern 1, parthenogenic or selfing species are isolated taxa that apparently go extinct before speciation. In Pattern 2, uniparental reproduction is ancestral to a diversified clade and is relatively ancient. We next review the evolutionary developmental biology aspects of self-fertility in Caenorhabditis, the one taxon for which it has been investigated. Finally, we summarize recent work documenting surprising impacts of self-fertility on genome size and content.