Asexual Daphnia genomes expose something old, new, borrowed, and blue

Abstract

Sex: what is it good for? Anthropocentric answers aside, this remains a central—and largely unanswered—question in evolutionary biology (1). Although asexual reproduction is phylogentically widespread among eukaryotes, none of these asexual lineages [with only a few arguable exceptions (2)] succeed like their sexually reproducing counterparts. In other words, asexual lineages are eventually destined for extinction. These incipient evolutionary failures are invaluable to evolutionary biologists, however, because by understanding the evolution of asexuals, something may be learned about the raison d’etre of sex itself. The “water flea” Daphnia pulex is one of a few model systems used to study the evolutionary maintenance of sex. Most lineages of D. pulex switch between sexual and asexual reproduction, but obligate asexuality has arisen multiple times in natural populations. In PNAS, Tucker et al. (3) describe their analyses of whole-genome sequences of 22 D. pulex lineages (11 sexuals and 11 asexuals) that reveal the molecular underpinnings and genomic consequences of asexuality in this species and illuminate why it is not a sustainable reproductive strategy.