A model for the evolution of translocation heterozygosity

Abstract

This paper uses algebraic analysis and computer simulation to examine the contribution of homozygote disadvantage created by mutational load to selection for translocation heterozygosity in selfing populations. It is shown first that in structurally homozygous populations mutation pressure can lead to the accumulation of deleterious mutations, as opposed to the establishment of a stable equilibrium between mutational input and selective elimination. The speed of accumulation depends on the mutation rate and inversely on the selection coefficients against deleterious alleles, the population size and the amount of recombination. It is also shown that translocations can be selected, given a sufficiently high rate of mutation per chromosome and provided that crossing-over is suppressed in structural heterozygotes. Incomplete dominance of deleterious mutations lowers the strength of selection for translocations, compared with the case of complete recessivity. In all cases when translocations are selected there is accumulation of deleterious genes in structural heterozygotes, so that the final population consists entirely of structural heterozygotes, the homozygotes behaving effectively as recessive lethals. The model is discussed in relation to what is known about translocation heterozygosity in natural populations, and about mutation rates and selection coefficients for deleterious genes. It is concluded that an unrealistically high mutation rate is probably needed for this mechanism to be the sole factor involved in initiating the evolution of complex heterozygosity in largely self-fertilising plants. It may, however, be an important contributary factor, and we show that it is likely to be more important, the larger the number of interchanges already established in the population.