Population-genomic insights into the evolutionary origin and fate of obligately asexual Daphnia pulex

Abraham E. Tucker, Matthew S. Ackerman, Brian D. Eads, Sen Xu, Michael Lynch

Research output: Contribution to journalArticlepeer-review

116 Scopus citations


Despite much theoretical work, the molecular-genetic causes and evolutionary consequences of asexuality remain largely undetermined. Asexual animal species are rare, evolutionarily short-lived, and thought to suffer mutational meltdown as a result of lack of recombination. Whole-genome analysis of 11 sexual and 11 asexual genotypes of Daphnia pulex indicates that current asexual lineages are in fact very young, exhibit no signs of purifying selection against accumulating mutations, and have extremely high rates of gene conversion and deletion. The reconstruction of chromosomal haplotypes in regions containing SNP markers associated with asexuality (chromosomes VIII and IX) indicates that introgression from a sister species, Daphnia pulicaria, underlies the origin of the asexual phenotype. Silent-site divergence of the shared chromosomal haplotypes of asexuals indicates that the spread of asexuality is as recent as 1,250 y, although the origin of the meiosissuppressing element or elements could be substantially older. In addition, using previous estimates of the gene conversion rate from Daphnia mutation accumulation lines, we are able to age each asexual lineage. Although asexual lineages originate from wide crosses that introduce elevated individual heterozygosities on clone foundation, they also appear to be constrained by the inbreeding-like effect of loss of heterozygosity that accrues as gene conversion and hemizygous deletion expose preexisting recessive deleterious alleles of asexuals, limiting their evolutionary longevity. Our study implies that the buildup of newly introduced deleterious mutations (i.e., Muller's ratchet) may not be the dominant force imperiling nonrecombining populations of D. pulex, as previously proposed.

Original languageEnglish (US)
Pages (from-to)15740-15745
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number39
StatePublished - Sep 24 2013
Externally publishedYes


  • Evolution of sex
  • Parthenogenesis

ASJC Scopus subject areas

  • General


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