Positive feedback in the transition from sexual reproduction to parthenogenesis

Abstract

Understanding how new phenotypes evolve is challenging because intermediate stages in transitions from ancestral to derived phenotypes often remain elusive. Here we describe and evaluate a new mechanism facilitating the transition from sexual reproduction to parthenogenesis. In many sexually reproducing species, a small proportion of unfertilized eggs can hatch spontaneously ('tychoparthenogenesis') and develop into females. Using an analytical model, we show that if females are mate-limited, tychoparthenogenesis can result in the loss of males through a positive feedback mechanism whereby tychoparthenogenesis generates female-biased sex ratios and increasing mate limitation. As a result, the strength of selection for tychoparthenogenesis increases in concert with the proportion of tychoparthenogenetic offspring in the sexual population. We then tested the hypothesis that mate limitation selects for tychoparthenogenesis and generates female-biased sex ratios, using data from natural populations of sexually reproducing Timema stick insects. Across 41 populations, both the tychoparthenogenesis rates and the proportions of females increased exponentially as the density of individuals decreased, consistent with the idea that low densities of individuals result in mate limitation and selection for reproductive insurance through tychoparthenogenesis. Our model and data from Timema populations provide evidence for a simple mechanism through which parthenogenesis can evolve rapidly in a sexual population.

Sexual conflict and the evolution of asexuality at low population densities

“…This appears possible, e.g. in the form of tychoparthenogenesis, which is the rare hatching of unfertilized eggs in a wide range of normally sexual invertebrates [33].…”

Uncovering Cryptic Asexuality in Daphnia magna by RAD Sequencing

“…Automixis has been hypothesized to represent an intermediate step in the evolutionary pathway from sexual to clonal reproduction (Schwander et al 2010). According to this hypothesis, rare automictic reproduction (tychoparthenogenesis) with mixed fusion may become more frequent, with subsequent selection for increased rates of central fusion and repression of recombination.…”

“…Yet, possible rare automictic reproduction in these cultures would be much more difficult to detect than in NMP clones because MP clones regularly produce males under the conditions needed to stimulate ephippia production. Indeed, rare automictic parthenogenesis occurs in a large number of organisms in the form of rare, spontaneous hatching of unfertilized eggs ("tychoparthenogenesis") with diploidy restored via automixis (Bell 1982;Schwander et al 2010;Neiman et al 2014).…”

Evolutionary mysteries in meiosis

“…Rare automixis (spontaneous development of unfertilized eggs) occurs in many species [1,81]. If this becomes more common, forms of automixis maintaining heterozygosity in centromere regions might be selectively favoured and recombination suppressed, eventually leading to meiosis-derived asexuality with the same genetic consequences as mitosis [87][88][89][90][91]. Indeed, in Arabidopsis, meiosis can be transformed to genetically resemble mitosis, but modification of several genes is needed to achieve this [92][93][94].…”