Data from: Evolution of haploid–diploid life cycles when haploid and diploid fitnesses are not equal

Many organisms spend a significant portion of their life cycle as haploids and as diploids (a haploid–diploid life cycle). However, the evolutionary processes that could maintain this sort of life cycle are unclear. Most previous models of ploidy evolution have assumed that the fitness effects of new mutations are equal in haploids and homozygous diploids, however, this equivalency is not supported by empirical data. With different mutational effects, the overall (intrinsic) fitness of a haploid would not be equal to that of a diploid after a series of substitution events. Intrinsic fitness differences between haploids and diploids can also arise directly, for example because diploids tend to have larger cell sizes than haploids. Here, we incorporate intrinsic fitness differences into genetic models for the evolution of time spent in the haploid versus diploid phases, in which ploidy affects whether new mutations are masked. Life-cycle evolution can be affected by intrinsic fitness differences between phases, the masking of mutations, or a combination of both. We find parameter ranges where these two selective forces act and show that the balance between them can favor convergence on a haploid–diploid life cycle, which is not observed in the absence of intrinsic fitness differences.

众多生物在生命周期的大部分阶段均以单倍体（haploid）与二倍体（diploid）形式存在，即具备单倍体-二倍体生命周期。然而，能够维持这类生命周期的演化机制目前仍未阐明。此前多数倍性演化模型均假设，新突变在单倍体与纯合二倍体中的适合度效应相等，但这一等价性并未得到实证数据的支持。若突变效应存在差异，经过一系列替换事件后，单倍体的整体（内在）适合度将与二倍体不再相等。单倍体与二倍体之间的内在适合度差异也可直接产生，例如二倍体的细胞尺寸通常大于单倍体。本研究将内在适合度差异纳入遗传模型，用以探究单倍体阶段与二倍体阶段所占时长的演化规律，该模型中倍性会影响新突变是否得以掩盖。生命周期的演化可受到阶段间内在适合度差异、突变掩盖效应，或二者共同作用的调控。我们找到了这两种选择压力发挥作用的参数区间，并证明二者间的平衡可促使系统趋于形成单倍体-二倍体生命周期，而在缺乏内在适合度差异的情况下，这一现象并不会出现。