Abstract
Parker, Baker, and Smith provided the first robust theory explaining why anisogamy evolves in parallel in multicellular organisms. Anisogamy sets the stage for the emergence of separate sexes, and for another phenomenon with which Parker is associated: sperm competition. In outcrossing taxa with separate sexes, Fisher proposed that the sex ratio will tend towards unity in large, randomly mating populations due to a fitness advantage that accrues in individuals of the rarer sex. This creates a vast excess of sperm over that required to fertilize all available eggs, and intense competition as a result. However, small, inbred populations can experience selection for skewed sex ratios. This is widely appreciated in haplodiploid organisms, in which females can control the sex ratio behaviorally. In this review, we discuss recent research in nematodes that has characterized the mechanisms underlying highly skewed sex ratios in fully diploid systems. These include self-fertile hermaphroditism and the adaptive elimination of sperm competition factors, facultative parthenogenesis, non-Mendelian meiotic oddities involving the sex chromosomes, and environmental sex determination. By connecting sex ratio evolution and sperm biology in surprising ways, these phenomena link two “seminal” contributions of G. A. Parker.
Highlights
Theodosius Dobzhansky famously stated [1] that “nothing in biology makes sense except in the light of evolution”
The evolution of female-biased sex ratios in response to local mate competition conditions is often facilitated by haplodiploidy, in which unfertilized eggs develop as haploid males, and all fertilized eggs develop into diploid females
We suggest that nematodes offer such a taxon
Summary
Theodosius Dobzhansky famously stated [1] that “nothing in biology makes sense except in the light of evolution”. The evolution of female-biased sex ratios in response to local mate competition conditions is often facilitated by haplodiploidy, in which unfertilized eggs develop as haploid males, and all fertilized eggs develop into diploid females This is typical of wasps (Hymenoptera) [27,28,29] but is found in mites (Acari) [30]. In both plants [39,40] and invertebrates [41], such strategies are often associated with mate scarcity [42] and generally evolve from outcrossing ancestors Both reproductive modes appear to exact costs on lineages utilizing them, such as reduced adaptive potential [43,44], the progressive accumulation of deleterious mutations known as Muller’s ratchet [45]; and, in parthenogenetic species, the prevention of effective meiotic recombination [46,47]. We hope to inspire others to explore that which is still unknown
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