Genetic architecture of inbreeding depression and the maintenance of gametophytic self-incompatibility

Gametophytic self-incompatibility is a widespread genetic system, which enables hermaphroditic plants to avoid self-fertilization and mating with close relatives. Inbreeding depression is thought to be the major force maintaining self-incompatibility; however, inbreeding depression is a dynamical variable that depends in particular on the mating system. In this paper we use multilocus, individual based simulations to examine the co-evolution of self-incompatibility and inbreeding depression within finite populations. We focus on the conditions for the maintenance of self-incompatibility when self-compatible mutants are introduced in the population by recurrent mutation, and compare simulation results with predictions from an analytical model treating inbreeding depression as a fixed parameter (thereby neglecting effects of purging within the self-compatible sub-population). In agreement with previous models, we observe that the maintenance of self-incompatibility is associated with high inbreeding depression and is facilitated by high rates of self-pollination. Purging of deleterious mutations by self-compatible mutants has little effect on the spread of those mutants as long as most deleterious alleles have weak fitness effects: in this case, the genetic architecture of inbreeding depression has little effect on the maintenance of self-incompatibility. By contrast, purging may greatly enhance the spread of self-compatible mutants when deleterious alleles have strong fitness effects.

配子体自交不亲和性（gametophytic self-incompatibility）是一类广泛分布的遗传系统，可使雌雄同株植物避免自花受精以及与近亲交配。近交衰退（inbreeding depression）被认为是维持自交不亲和性的主要驱动力；然而，近交衰退是一类尤其依赖于交配系统的动态变量。本文采用基于个体的多位点模拟（multilocus individual-based simulations），探究有限种群内自交不亲和性与近交衰退的协同演化过程。本研究重点关注当经反复突变（recurrent mutation）产生自交亲和突变体（self-compatible mutants）并侵入种群时，维持自交不亲和性的条件，并将模拟结果与以近交衰退为固定参数的解析模型（analytical model）的预测结果进行对比——该解析模型忽略了自交亲和亚种群内的有害突变净化（purging）效应。与已有模型的结论一致，本研究发现自交不亲和性的维持与高水平近交衰退相关，且高自花授粉（self-pollination）率可促进其维持。当多数有害等位基因（deleterious alleles）的适合度效应（fitness effects）较弱时，自交亲和突变体介导的有害突变净化对这类突变体的扩散影响极小：此时，近交衰退的遗传架构（genetic architecture）对自交不亲和性的维持几乎无影响。与之相反，当有害等位基因的适合度效应较强时，净化作用可大幅促进自交亲和突变体的扩散。