Popad'in K Iu
Department of General Ecology, Biological Faculty of M.V. Lomonosov Moscow State University, Moscow 119992.
Zh Obshch Biol. 2003 Nov-Dec;64(6):463-78.
Prevalence of sexual reproduction is still enigma. The main character of sex is alleles mixing that could be advantageous either in unstable environment (in this case sex provides high temp of evolution) or in unstable genotype (in this case sex provides purge of genome from deleterious mutations). As long as not all species inhabit highly changeable environments, variation of genotypes is more important factor. As the majority of new mutations is deleterious, effective mechanism of genome purging is needed. Maintenance of "purging mechanism" may be a single role of sex. Two promising mutational hypotheses--clade selection (Muller's ratchet and Nunney's hypothesis) and mutational deterministic hypothesis of Kondrashov claim that more effective elimination of slightly-deleterious mutations provides main advantage to sexual population in comparison with asexual. Despite prima facie similarity, these hypotheses differ in mechanisms, work at different temporal scales and have different consequences. Kondrashov's hypothesis reveals short-term advantage of sexual reproduction, and thus, based on the individual selection. Clade selection displays long-term advantage of sexual reproduction that could be realized only by group selection. The role of mobile elements in evolution of sexual reproduction is also discussed. Firstly, mobile elements ("sexual molecular parasites") can complicate the problem: having been domesticated in asexual genomes and remaining active in sexual genomes they lead to higher mutational rate in sexual organisms and so violate assumption critical for both mutational hypotheses of "other things being equal". Secondly, mobile elements could be leader factor of origin of sex (hypothesis proposed by Hickey). Because theory of group selection could explain maintenance of sex, but not its origin, mobile elements could induce the origin of sex but were not able to maintain it, so the next scenario of evolution of sex is proposed: mobile elements induced origin of sex, which was established later by group selection because provided long term benefit (Muller's ratchet and Nunney's hypothesis). So, on all stages of evolution, sex was not advantageous for the organism per se.
有性生殖的普遍性仍是个谜。性的主要特征是等位基因混合，这在不稳定环境中（在这种情况下，性提供了高进化速度）或不稳定基因型中（在这种情况下，性有助于从基因组中清除有害突变）可能是有利的。只要并非所有物种都栖息在高度多变的环境中，基因型的变异就是更重要的因素。由于大多数新突变是有害的，因此需要有效的基因组清除机制。维持“清除机制”可能是性的唯一作用。两个有前景的突变假说——进化枝选择（穆勒棘轮和南尼假说）以及康德拉肖夫的突变决定论假说——声称，与无性种群相比，更有效地消除轻微有害突变是有性种群的主要优势。尽管表面上相似，但这些假说在机制上有所不同，在不同的时间尺度上起作用，并且有不同的后果。康德拉肖夫假说揭示了有性生殖的短期优势，因此是基于个体选择。进化枝选择显示了有性生殖的长期优势，而这只有通过群体选择才能实现。还讨论了移动元件在有性生殖进化中的作用。首先，移动元件（“性分子寄生虫”）会使问题复杂化：它们在无性基因组中被驯化并在有性基因组中保持活跃，导致有性生物体的突变率更高，从而违反了对两个突变假说都至关重要的“其他条件相同”的假设。其次，移动元件可能是性起源的主导因素（希基提出的假说）。因为群体选择理论可以解释性的维持，但不能解释其起源，移动元件可能诱导了性的起源，但无法维持它，因此提出了性进化的下一个场景：移动元件诱导了性的起源，随后性通过群体选择得以确立，因为它提供了长期利益（穆勒棘轮和南尼假说）。所以，在进化的所有阶段，性本身对生物体并不有利。
