A combinational theory for maintenance of sex

Abstract

Sexual reproduction implies high costs, but it is difficult to give evidence for evolutionary advantages that would explain the predominance of meiotic sex in eukaryotes. A combinational theory discussing evolution, maintenance and loss of sex may resolve the problem. The main function of sex is the restoration of DNA and consequently a higher quality of offspring. Recombination at meiosis evolved, perhaps, as a repair mechanism of DNA strand damages. This mechanism is most efficient for DNA restoration in multicellular eukaryotes, because the initial cell starts with a re-optimized genome, which is passed to all the daughter cells. Meiosis acts also as creator of variation in haploid stages, in which selection can purge most efficiently deleterious mutations. A prolonged diploid phase buffers the effects of deleterious recessive alleles as well as epigenetic defects and is thus optimal for prolonged growth periods. For complex multicellular organisms, the main advantage of sexuality is thus the alternation of diploid and haploid stages, combining advantages of both. A loss of sex is constrained by several, partly group-specific, developmental features. Hybridization may trigger shifts from sexual to asexual reproduction, but crossing barriers of the parental sexual species limit this process. For the concerted break-up of meiosis-outcrossing cycles plus silencing of secondary features, various group-specific changes in the regulatory system may be required. An establishment of asexuals requires special functional modifications and environmental opportunities. Costs for maintenance of meiotic sex are consequently lower than a shift to asexual reproduction.

Long telomeres are associated with clonality in wild populations of the fissiparous starfish Coscinasterias tenuispina

“…However, older clones of aspen species has been found to exhibit a significant reduction in reproductive performance associated with male sexual fitness decline, suggesting that at least some long-lived clonal organisms may be vulnerable to senescence over long periods of time (Ally et al, 2008). A hypothesis to explain the excess of heterozygosity found in the studied populations of C. tenuispina may be a positive selection of heterozygotes to keep genetic diversity, besides having greater individual adaptability by high phenotype plasticity (Hörandl, 2009;Goudie et al, 2012). Nevertheless, our current results cannot actually test this hypothesis, and the heterozygotes excess found in all populations may be results from other stochastic processes that have not been controlled in this study.…”

Sex is a ubiquitous, ancient, and inherent attribute of eukaryotic life

“…That sex creates genetic variation, repairs DNA breaks, restores methylation patterns, and prevents accumulation of disadvantageous mutations is well accepted (72,73). These features have varying importance in different eukaryotic lineages, yet it is likely that the advantages of sex, elaborated in detail for multicellular organisms (71,74), either apply also for unicells or that there are some other advantages.…”

“…In general, sex seems to allow superior responses to rapid challenges, both internal and external. Indeed, there is no consensus on a single major advantage for sex, and prevailing opinion is shifting toward a combination of advantages (71). That sex creates genetic variation, repairs DNA breaks, restores methylation patterns, and prevents accumulation of disadvantageous mutations is well accepted (72,73).…”

Geographic parthenogenesis in a consumer-resource model for sexual reproduction

“…34 Theories for geographic parthenogenesis propose that parthenogenesis is a side effect of hybridization in boundary regions [26] or of selection for polyploidy [37]. Other authors emphasize 36 that asexuals are capable of colonizing new habitats faster [9,30] or that asexual reproduction arises whenever environmental changes may have provided opportunities for shifts to asexuality 38 [21]. In the only spatially extended mathematical model for geographic parthenogenesis that exists so far, Peck(1998) [40], showed explicitly that a sufficiently strong source-sink effect [10] 40 can lead to a dominance of parthenogenetic reproduction in boundary regions, because sexuals cannot establish the phenotype that is optimal for this region.…”