Occasional sex in an ‘asexual’ polyploid hermaphrodite

Abstract

Asexual populations are usually considered evolutionary dead-ends because they lack the mechanisms to generate and maintain sufficient genetic diversity. Yet, some asexual forms are remarkably widespread and genetically diverse. This raises the question whether asexual systems are always truly clonal or whether they have cryptic forms of sexuality that enhance their viability. In the planarian flatworm Schmidtea polychroa parthenogens are functional hermaphrodites (as are their sexual conspecifics), copulate and exchange sperm. Sperm is required for initiation of embryogenesis but usually does not contribute genetically to the offspring (sperm-dependent parthenogenesis). Using karyology and genotyping of parents and offspring, we show that in a purely parthenogenetic population an estimated 12% of all offspring are the result of partial genetic exchange. Several processes of chromosome addition and loss are involved. Some of these result in an alternation between a common triploid and a rare tetraploid state. We conclude that genetic recombination does not necessarily require segregation and fusion within the same generation, as is the case in most sexual species. These occasional sexual processes help to explain the geographical dominance of parthenogens in our study species.

Differential effects of polyploidy and diploidy on fitness of apomictic Boechera

“…Diploid apomictic genotypes that show low levels of functional meiosis (reduced gamete formation, facultative apomeiosis) have been identified, in addition to sexual diploids having low levels of apomeiosis (unreduced gamete formation; Aliyu et al 2010). We propose that this flexibility found in diploids increases the opportunities for apomicts to purge mutations and refresh their genomes (Lynch and Gabriel 1983;Wagner and Gabriel 1990;D'Souza et al 2004). Diploid and triploid apomicts respond differently to mutation accumulation, and the flexibility within diploids could be one of the drivers for the establishment of new apomictic lines via backcrossing with sexual genotypes, thus providing an explanation for the success of apomixis within the genus Boechera.…”

Production of diploid and triploid offspring by inbreeding of the triploid planarian Dugesia ryukyuensis

“…In S. polychroa, it has been observed that reciprocal transitions between triploid and tetraploid pseudogamic populations were occasionally caused by genome gain of paternal chromosomes derived from haploid sperm (D'Souza et al 2004(D'Souza et al , 2005(D'Souza et al , 2006. This phenomenon was referred to as "occasional sex".…”

Population genetic structure of parthenogenetic flatworm populations with occasional sex

“…Although individuals of this species are simultaneous hermaphrodites, S. polychroa reproduces parthenogenetically and is not capable of selffertilisation (Benazzi Lentati, 1970). In fact, tetraploids in this species are derived from triploids through such rare sexual events (D'Souza et al, 2004). However, the sperm does not contribute genetically to offspring: Paternal alleles are expelled with one of the polar bodies or degenerate within the zygote, leading to clonal offspring (Benazzi Lentati, 1970).…”

“…Different sample sizes across locations are due to sampling or amplification failure. 1) (D'Souza et al, 2004. Throughout this manuscript, the term 'clone' is defined as a group of individuals with the same ploidy and genotype irrespective of their descent.…”

“…Parthenogens, however, are always polyploid (mostly triploid, sometimes tetraploid) and reproduce via sperm-dependent parthenogenesis (Benazzi Lentati, 1970;Benazzi, 1982). Because parthenogens maintain a small, but fertile male function with haploid sperm, such sexual processes do not require the presence of sexual conspecifics, but also occur in populations with parthenogens only (D'Souza et al, 2004. The resulting offspring is, as a rule, clonal: sperm donors generally do not contribute their genetic material to their offspring.…”