Michod R E
Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona 85721, USA.
Theor Popul Biol. 1998 Feb;53(1):60-74. doi: 10.1006/tpbi.1997.1341.
According to the repair hypothesis, sex originated as a cooperative interaction-the benefit being damage repair. As with all cooperative strategies, cooperative sex may be vulnerable to selfish mutants. The purpose of the present paper is to understand what implications such selfish mutants may have both for the origin of sex, especially in competition with asexual diploidy, and for the elaboration of the sexual cycle, especially in facultatively sexual organisms. Asexual diploids are assumed to effectively and instantaneously repair all damages without expression of deleterious recessive mutations. Costs to asexual diploidy are considered in terms of its birth rate and mortality rate. The main results of the present paper are as follows. (i) Asexual diploidy wins when the costs of diploidy are small, mortality rates low, and damage rates high. (ii) Beginning with an ancestral state in which cells are asexual haploids, the sexual life cycle would emerge before asexual diploidy as a response to increasing DNA damage. (iii) Selfish sex is a far more robust repair strategy than cooperative sex, especially in competition with asexual diploidy. (iv) Although cooperative sex is more adaptive in extreme environments characterized by high damage and high mortality, selfish sex can still invade in these regions and take the entire system to extinction. (v) Once it is present, selfish sex is stable to asexual diploidy over a wide range of parameter values and can persist in regions of parameter space forbidded to the asexual diploid. These results help to address a concern of the gene repair theory of sex, which is that efficient repair in an asexual diploid is a better strategy than sex. Data from microbes bearing on the results are discussed as is the relationship between facultative sex in multicellular organisms and selfish sex in microbes.
根据修复假说，有性生殖起源于一种合作互动——其益处在于损伤修复。与所有合作策略一样，合作性生殖可能容易受到自私突变体的影响。本文的目的是了解这些自私突变体对有性生殖起源可能产生的影响，特别是在与无性二倍体的竞争中，以及对有性生殖周期的演化可能产生的影响，特别是在兼性有性生物中。无性二倍体被假定能够有效且即时地修复所有损伤，而不会表达有害的隐性突变。无性二倍体的代价是根据其出生率和死亡率来考虑的。本文的主要结果如下：（i）当二倍体的代价较小、死亡率较低且损伤率较高时，无性二倍体占优势。（ii）从细胞为无性单倍体的祖先状态开始，作为对不断增加的DNA损伤的一种反应，有性生殖周期会在无性二倍体出现之前出现。（iii）自私的有性生殖是一种比合作性生殖更稳健的修复策略，特别是在与无性二倍体的竞争中。（iv）尽管合作性生殖在以高损伤和高死亡率为特征的极端环境中更具适应性，但自私的有性生殖仍能在这些区域入侵并导致整个系统灭绝。（v）一旦出现，自私的有性生殖在广泛的参数值范围内对无性二倍体是稳定的，并且能够在无性二倍体被禁止的参数空间区域中持续存在。这些结果有助于解决有性生殖的基因修复理论中的一个问题，即无性二倍体中的高效修复是比有性生殖更好的策略。文中讨论了与这些结果相关的微生物数据，以及多细胞生物中的兼性有性生殖与微生物中的自私有性生殖之间的关系。
