Michod R E
Department of Ecology and Evolutionary Biology, University of Arizona, Tucson 85721.
J Hered. 1993 Sep-Oct;84(5):360-71. doi: 10.1093/oxfordjournals.jhered.a111357.
Mathematical models and experiments on transformation are reported testing the hypothesis that sex and diploidy evolved as a DNA repair system. The models focus on the origin of diploidy and sex by studying selection between asexual haploids, sexual haploids, and diploids. Haploid cells are efficient replicators, while diploid cells are resistance to damage. A sexual haploid may combine the advantages of both: spending much of its life cycle in the haploid state, then temporarily fusing to become diploid, followed by splitting to the haploid state. During the diploid state DNA damage can be repaired, since there are two copies of the gene in the cell and one copy is presumed to be undamaged. Five basic rate parameters are employed: birth and death; genomic damage (for the haploids alone); and, for the sexual cell, fusion and splitting. Parameter space bifurcation diagrams for the equilibria are drawn, and solutions of the equations are described in terms of these diagrams. Each type of cell has a region of the parameter space that it occupies exclusively (given its initial presence in the competition). The haploid wins in environments characterized by low damage. The diploid wins in environments characterized by high damage, low mortality, and abundant resources. In general, only a single type of cell occupies a given portion of the space. We find, however, that competitive coexistence of an asexual diploid and sexual haploid is possible in spite of the fact that they are competing for a single resource (nucleotide building blocks). Sex can increase from rarity if matings occur with asexual cells. Only sex can cope with both high mortality and high damage. We then turn to natural bacterial transformation as a model system for the experimental study of sex. Natural transformation in distributed widely, but apparently sparsely, in all bacterial groups. A very preliminary phylogenetic analysis of the bacilli and related species indicates that transformation is probably not a diversifying force in bacterial evolution. However, it is difficult to be sure because of the ambiguity surrounding negative data. Experiments with the bacterium Bacillus subtilis indicate that transformation frequencies respond adaptively to DNA damage if homologous donor DNA is used. Several specific hypotheses for this response are considered. Recent work in other labs on the evolution of transformation is discussed from the point of view of the hypothesis that transformation functions in DNA repair.
报告了关于转化的数学模型和实验，以检验性别和二倍体作为一种DNA修复系统而进化的假说。这些模型通过研究无性单倍体、有性单倍体和二倍体之间的选择，聚焦于二倍体和性别的起源。单倍体细胞是高效的复制者，而二倍体细胞对损伤具有抗性。有性单倍体可能兼具两者的优点：在其生命周期的大部分时间处于单倍体状态，然后暂时融合成为二倍体，随后再分裂回到单倍体状态。在二倍体状态下，DNA损伤可以得到修复，因为细胞中有该基因的两个拷贝，且假定其中一个拷贝未受损。使用了五个基本速率参数：出生和死亡；基因组损伤（仅针对单倍体）；以及对于有性细胞而言的融合和分裂。绘制了平衡点的参数空间分岔图，并根据这些图描述了方程的解。每种类型的细胞在参数空间中都有一个它独占的区域（给定其在竞争中的初始存在情况）。单倍体在损伤低的环境中占优势。二倍体在损伤高、死亡率低且资源丰富的环境中占优势。一般来说，只有一种类型的细胞占据空间的给定部分。然而，我们发现，尽管无性二倍体和有性单倍体在争夺单一资源（核苷酸构件），但它们有可能竞争共存。如果与无性细胞发生交配，有性生殖可以从稀有状态增加。只有有性生殖能够应对高死亡率和高损伤。然后我们转向自然细菌转化，将其作为性别实验研究的模型系统。自然转化在所有细菌类群中分布广泛，但显然很稀少。对芽孢杆菌及相关物种的非常初步的系统发育分析表明，转化在细菌进化中可能不是一种多样化力量。然而，由于围绕阴性数据的不确定性，很难确定。对枯草芽孢杆菌的实验表明，如果使用同源供体DNA，转化频率会对DNA损伤做出适应性反应。考虑了对此反应的几个具体假说。从转化在DNA修复中起作用这一假说的角度，讨论了其他实验室最近关于转化进化的工作。
