Genetic causes of transitions from sexual reproduction to asexuality in plants and animals

Abstract

The persistence of sexual reproduction in the face of competition from asexual invaders is more likely if asexual lineages are produced infrequently or have low fitness. The generation rate and success of new asexual lineages will be influenced by the proximate mechanisms underlying transitions to asexuality. As such, characterization of these mechanisms can help explain the distribution of reproductive modes among natural populations. Here, we synthesize the literature addressing proximate causes of transitions from sexual to asexual reproduction in plants and animals. In cyclical and facultatively asexual taxa, individual mutations can cause obligate asexuality. The evolution of asexuality in obligately sexual groups is more complex, requiring the simultaneous acquisition of two traits generally controlled by different genetic factors: unreduced gamete formation and spontaneous development of unfertilized gametes. At least three 'pre-adaptations' could favour transitions to obligate asexuality in obligate sexuals. First, linkage among loci affecting separate key components of asexuality facilitates its spread, with evidence for these linkage blocks in plants. Second, asexuality should evolve more readily in haplodiploids; support for this hypothesis comes from two examples where a single locus causes transitions to asexuality. Third, standing genetic variation for the production of unreduced gametes could facilitate transitions to asexuality, but whether the ability to produce unreduced gametes contributes to the evolution of obligate asexuality remains unclear. We close by reviewing the associations between asexuality, hybridization and polyploidy, and argue that current data suggest that hybridization is more likely to play a causal role in transitions to asexuality than polyploidy.

Effects of polyploidy and reproductive mode on life history trait expression

“…Like many other polyploid animals (reviewed in Otto and Whitton 2000; Neiman and Schwander 2011; Neiman et al. 2014), polyploid P. antipodarum are obligately asexual, producing nonrecombinant eggs via ameiotic parthenogenesis (Phillips and Lambert 1989). The existence of triploid and tetraploid asexual P. antipodarum means that we can use comparisons between asexuals of different ploidy levels as well as between sexuals and asexuals to simultaneously study the consequences of polyploidy and identify costs and benefits associated with sexual versus asexual reproduction, another key unanswered question in evolutionary biology.…”

Transmission Strategies Used by Gyrodactylus gasterostei (Monogenea) on Its Host, the Three-Spined Stickleback Gasterosteus aculeatus

“…All of the parasites studied lacked excretory bladders, and had ventral bar with short processes characteristic for G. gasterostei. This observation may explain why the sticklebacks differed in the natural infection levels of G. arcuatus [26,31,32].…”

“…The possibility of finding just one species of Gyrodactylus did not raise suspicion; some authors [13,26,31,32,38] have investigated local adaptation and maturity, showing that in the Gasterosteus-Gyrodactylus example, differential host immune resistance may instead explain differential accumulation of parasites. To explain why local adaptation appears lacking, we must understand the strength of selection and the population genetic structure of both parasite and host.…”

Does the avoidance of sexual costs increase fitness in asexual invaders?

“…In some cases, a remedy may consist in using natural genetic variation underlying the different reproductive modes. In several organisms simple genetic mechanisms are responsible for transitions to asex (52). In those cases, it is possible to breed sexual and asexual variants with a similar genetic background (e.g., ref.…”