Sib competition and sperm competitiveness: an answer to ‘Why so many sperms?’ and the recombination/sperm number correlation

Abstract

Sperm, or inter-ejaculate, competition leads to the evolution of many sperm per ejaculate. However, sperm competition theory does not predict the correlation between sperm number and chiasma, or that between sperm number and haploid chromosome number. Firstly, we show that phylogenetic inertia cannot account for at least the latter relation, and secondly, a model is presented which incorporates sib competition (intra-ejaculate competition) and sperm competitiveness to explain the relation between sperm numbers and recombination and the question 'Why so many sperms?'. It is argued that if there are deleterious mutations which affect sperm competitiveness this will lead to sib or intra-ejaculate competition. If inter-ejaculate competition also exists, then there will be selection for increased recombination. A chiasmate male heterozygous for n mutations which reduce sperm competitiveness can produce gametes with 0...n mutations. The proportion of gametes with 0 mutations per ejaculate is s = 0.5n, which is a small fraction. This means that to ensure on average one sperm with 0 mutations per ejaculate, a chiasmate male must produce 1/s sperm. We may therefore expect that 1/s will be positively correlated with sperm numbers. If inter-ejaculate competition leads to an optimum sperm number of x, then the optimum number of sperm per ejaculate is x/s. Sperm numbers will be increased by: (i) the number of loci which affect sperm competitiveness in the haploid state; (ii) the mutation rate; and (iii) the recombination rate. A correlation between recombination rates and sperm numbers is therefore to be expected.

Sperm variation within a single ejaculate affects offspring development in Atlantic salmon

“…Our results suggest that within-ejaculate competition between sperm may contribute not only to the evolution of female 'barriers' to sperm [1][2][3] but also to the evolution of sperm numbers [27,28]. If sperm phenotype affects offspring performance, this may result in indirect selection on ejaculate size favouring males that produce large numbers of sperm even in the absence of between-ejaculate sperm competition (i.e.…”

“…Under this scenario, males that produce a larger number of sperm would father offspring with, on average, a higher fitness. This effect will in theory be stronger, the higher the mutational load is among sperm [27,28]. …”

Sperm competition and sperm cooperation: the potential role of diploid and haploid expression

“…However, recombination processes during meiosis increase the genetic variation among sperm considerably and there is some evidence that recombination rate has an influence on the number of sperm produced (Cohen 1967(Cohen , 1973. This association between recombination rate and sperm number is thought to be based on the fact that sperm haplotypes exhibit varying quality and to make sure that every haplotype is produced at least once, more sperm have to be produced with increasing recombination rate (Manning & Chamberlain 1994). But how important is this variation in haplotypes among sibling sperm for sperm function?…”

The polyandry revolution

“…The intuition of polyandry has had a dramatic impact on our understanding of anisogamy, 'sex roles' and ultimately the operation of sexual selection. Parker [8] had already identified, in sperm competition arising from polyandry, a necessary feature to explain the evolution and maintenance of anisogamy (see also reference [9]). Bateman [10], and subsequently Trivers [11] considered anisogamy the original condition ultimately responsible for the evolution of sex-specific sexual selection, namely a positive gradient of the regression of reproductive success on mating success (Bateman gradient) that is typically steeper in males than in females.…”