Paternal inheritance in parthenogenetic forms of the planarian Schmidtea polychroa

Abstract

Parthenogenesis usually includes clonal inheritance, which is thought to increase the risk of the clonal populations' extinction. Yet many parthenogenetic organisms appear to have survived for extended periods. A possible explanation is that parthenogens occasionally reproduce through sex-like processes. Although there is indirect evidence for occasional sex, the underlying mechanisms are currently unknown. In the present study, we examined sex-like processes in the planarian flatworm Schmidtea (Dugesia) polychroa. Parthenogenetic forms of this species are simultaneous hermaphrodites that require sperm to trigger embryogenesis, whereas paternal genetic material is usually excluded from the oocyte (sperm-dependent parthenogenesis). Based on a comparison of parents and offspring, using highly polymorphic microsatellites, we demonstrate the incorporation of paternal alleles in about 5% of the offspring. We detected two distinct processes: chromosome addition and chromosome displacement. Such rare sexual processes may explain the long-term persistence of the many purely parthenogenetic populations of S. polychroa in northern Europe.

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

“…, 2003). This is important as the production of fertile sperm allows for occasional sex in otherwise parthenogenetic populations (D’Souza et al. , 2004, 2006).…”

Expanding the horizon: the Red Queen and potential alternatives

“…If an asexual form is of single origin, the major source of genetic diversity within the asexual population is mutation; however, introgression was reported in some spermdependent asexual species (Schartl et al 1995;Lampert et al 2005;D'Souza et al 2006). As a consequence of a single origin, the genetic diversity within populations of asexuals relative to the sexual progenitors is assumed to be low.…”

Sporadic sexual reproduction: Sex to some degree

“…A longstanding question is why there are so few organisms that solve this paradox with a little bit of sex (Green and Noakes, 1995), by combining the benefits of occasional sexual reproduction with low investment in male function? A recent study (D'Souza et al, 2006) shows that a freshwater flatworm may thrive from predominantly parthenogenetic reproduction mixed with a little bit of sex.…”