Genetic signatures of occasional sex in parthenogenetic subpopulations of the freshwater planarian Schmidtea polychroa

Abstract

1. Occasional sex in organisms that otherwise propagate clonally combines the advantages of both reproductive modes. Although theory predicts that this is a winning combination, evidence for its long-term importance in natural populations is scarce. 2. We studied the sperm-dependent parthenogenetic form of the freshwater planarian Schmidtea (formerly Dugesia) polychroa in which occasional sex is known to occur. By comparing subpopulations within the same lake, we showed that occasional sex does indeed leave telltale signatures in the population genetic structure of this species. 3. Using microsatellites, we found differences in genotypic diversity and evenness between six local subpopulations. Moreover, some locations showed temporal fluctuations in genotype and/or allele frequencies, while stability was observed in others. The results indicate that although some subpopulations show all the traits of clonal reproduction, others clearly have characteristics typical of sexual recombination, including the particular subpopulation for which we previously demonstrated the occurrence of occasional sex. 4. We show that within the same metapopulation, neighbouring localities can differ in the degree of occasional sex, as demonstrated by genetic signatures of clonality versus (occasional) genetic recombination. This makes S. polychroa particularly suitable to test the role of occasional sex to explain the persistence of parthenogenesis.

Correlations between sex rate estimates and fitness across predominantly parthenogenetic flatworm populations

“…Triploid karyotypes represent the original parthenogenetic forms in S. polychroa as they arose from diploid sexuals most probably after fertilization of unreduced eggs (Benazzi & Benazzi Lentati, 1976). Thus, tetraploids are considered a by‐product of this two‐step sexual cycle and indicate the presence of occasional sex in parthenogenetic populations (D’Souza & Michiels, 2006). A third mechanism involves egg and sperm fusion without ploidy change.…”

“…Sampling locations had been analysed previously and found to be genetically isolated as shown by significant differentiation and little overlap in genotype composition (see also this study). Sites differed in genotypic diversity and genotypic evenness, which is likely to be caused by varying degrees of occasional sex (D’Souza & Michiels, 2006).…”

“…In order to avoid that genetic differences may be due to de novo mutations, genotypes were pooled, when differing by fewer than three trinucleotide repeat units across all loci, leading to conservative diversity estimates (Bruvo et al. , 2004; D’Souza & Michiels, 2006).…”

“…Genotypic differentiation among locations was tested by applying log‐likelihood G ‐statistics (Goudet et al. , 1996) as described in D’Souza & Michiels (2006). For this, multilocus genotypes were coded as different alleles of a single locus.…”

Nucleotide Polymorphism and Within-Gene Recombination in Daphnia magna and D. pulex, Two Cyclical Parthenogens

“…Partially asexual life cycles, with only occasional rounds of sexual reproduction, occur in many invertebrates and fungi and may provide insights into the fitness and genomic consequences of occasional sex (Bell 1982). However, few studies of neutral genetic diversity exist for partially asexual organisms, and thus their N e and levels of recombination are largely unknown (e.g., Hughes and Verra 2001;Delmotte et al 2002;D'souza and Michiels 2006).…”