Kondrashov A S
Section of Ecology and Systematics, Cornell University, Ithaca, New York 14853.
Nature. 1994 Jul 21;370(6486):213-6. doi: 10.1038/370213a0.
Sex involves syngamy (gamete fusion), which doubles the amount of DNA in a cell, and meiosis, which halves it. The result is a 'ploidy cycle' of alternating diploid and haploid phases. Asexual reproduction does not require changes of ploidy, and yet asexual forms may have ploidy cycles. Here I show that such cycles lessen the mutation load, compared with permanent diploidy or polyploidy, and are thus likely to evolve in cases where it is always advantageous to have more than one copy of the genome per cell. The asexual ploidy cycle could have facilitated the origin of sex, by providing a means of orderly genetic reduction available immediately after the origin of syngamy.
有性生殖涉及配子融合（配子结合），这会使细胞中的DNA数量翻倍，还涉及减数分裂，会使DNA数量减半。其结果是形成一个二倍体和单倍体阶段交替的“倍性循环”。无性生殖不需要倍性变化，然而无性生殖形式可能存在倍性循环。在此我表明，与永久二倍体或多倍体相比，这样的循环会减轻突变负荷，因此在每个细胞拥有多于一份基因组拷贝总是有利的情况下可能会进化。无性倍性循环可能通过在配子融合起源后立即提供一种有序基因减数的方式，促进了有性生殖的起源。
