Charlesworth B, Charlesworth D
Department of Ecology and Evolution, University of Chicago, IL 60637-1573, USA.
Genet Res. 1997 Aug;70(1):63-73. doi: 10.1017/s0016672397002899.
Theoretical arguments are presented which suggest that each advance of Muller's ratchet in a haploid asexual population causes the fixation of a deleterious mutation at a single locus. A similar process operates in a diploid, fully asexual population under a wide range of parameter values, with respect to fixation within one of the two haploid genomes. Fixations of deleterious mutations in asexual species can thus be greatly accelerated in comparison with a freely recombining genome, if the ratchet is operating. In a diploid with segregation of a single chromosome, but no crossing over within the chromosome, the advance of the ratchet can be decoupled from fixation if mutations are sufficiently close to recessivity. A new analytical approximation for the rate of advance of the ratchet is proposed. Simulation results are presented that validate the assertions about fixation. The simulations show that none of the analytical approximations for the rate of advance of the ratchet are satisfactory when population size is large. The relevance of these results for evolutionary processes such as Y chromosome degeneration is discussed.
文中提出了理论观点，表明在单倍体无性繁殖群体中，穆勒棘轮的每一次推进都会导致单个基因座上有害突变的固定。在广泛的参数值范围内，对于二倍体完全无性繁殖群体中两个单倍体基因组之一内的固定而言，类似的过程也会发生。因此，如果棘轮在起作用，与自由重组的基因组相比，无性繁殖物种中有害突变的固定会大大加速。在具有单个染色体分离但染色体内无交叉的二倍体中，如果突变足够接近隐性，棘轮的推进可以与固定脱钩。提出了一种关于棘轮推进速率的新的解析近似方法。给出的模拟结果验证了关于固定的论断。模拟表明，当群体规模较大时，对于棘轮推进速率的解析近似方法都不尽人意。讨论了这些结果与Y染色体退化等进化过程的相关性。
