Development of associative overdominance through linkage disequilibrium in finite populations

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Abstract
SUMMARY: Associative overdominance arises at an intrinsically neutral locus through its non-random association with overdominant loci. In finite populations, even if fitness is additive between loci, non-random association will be created by random genetic drift.The magnitude of such associative overdominance is roughly proportional to the sum ofbetween the neutral and the surrounding over-dominant loci, whereis the squared standard linkage deviation, defined between any two loci by the relation in whichpand 1 –pare frequencies of allelesA1andA2in the first locus,qand 1 –qare frequencies of allelesB1andB2in the second locus, andDis the coefficient of linkage disequilibrium. A theory was developed based on diffusion models which enables us to obtain formulae forunder various conditions, and Monte Carlo experiments were performed to check the validity of those formulae.It was shown that ifA1andA2are strongly overdominant whileB1andB2are selectively neutral, we have approximately provided that 4Nec≫ 1, whereNeis the effective population size andcis the recombination fraction between the two loci. This approximation formula is also valid between two strongly overdominant as well as weakly overdominant loci, if 4Nec≫ 1.The significance of associative overdominance for the maintenance of genetic variability in natural populations was discussed, and it was shown thatNes′, that is, the product between effective population size and the coefficient of associative overdominance, remanis constant with varyingNe, if the total segregational (overdominant) load is kept constant.The amount of linkage disequilibrium expected due to random drift in experimental populations was also discussed, and it was shown thatin the first generation, if it is produced by extractingnchromosomes from a large parental population in whichD= 0.