SIB COMPETITION AND SPERM COMPETITIVENESS - AN ANSWER TO WHY SO MANY SPERMS AND THE RECOMBINATION SPERM NUMBER CORRELATION/

Sperm, or inter-ejaculate, competition leads to the evolution of many sperm per ejaculate. However, sperm competition theory does not predic t the correlation between sperm number and chiasma, or that between sp erm number and haploid chromosome number. Firstly, we show that phylog enetic inertia cannot account for at least the latter relation, and se condly, a model is presented which incorporates sib competition (intra -ejaculate competition) and sperm competitiveness to explain the relat ion between sperm numbers and recombination and the question 'Why so m any sperms?'. It is argued that if there are deleterious mutations whi ch affect sperm competitiveness this will lead to sib or intra-ejacula te competition. If inter-ejaculate competition also exists, then there will be selection for increased recombination. A chiasmate male heter ozygous for n mutations which reduce sperm competitiveness can produce gametes with 0...n mutations. The proportion of gametes with 0 mutati ons per ejaculate is s = 0.5(n), 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-ejaculat e 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 h aploid state; (ii) the mutation rate; and (iii) the recombination rate . A correlation between recombination rates and sperm numbers is there fore to be expected.