James Timothy Y
Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, MI 48109, USA.
Integr Comp Biol. 2023 Oct 10;63(4):922-935. doi: 10.1093/icb/icad037.
Eukaryotes have evolved myriad ways of uniting gametes during sexual reproduction. A repeated pattern is the convergent evolution of a mating system with the fusion of larger gametes with smaller gametes (anisogamy) from that of fusion between morphologically identical gametes (isogamy). In anisogamous species, sexes are defined as individuals that produce only one gamete type. Although sexes abound throughout Eukarya, in fungi there are no biological sexes, because even in anisogamous species, individuals are hermaphroditic and produce both gamete types. For this reason, the term mating types is preferred over sexes, and, thus defined, only individuals of differing mating types can mate (homoallelic incompatibility). In anisogamous fungal species, there is scant evidence that there are more than two mating types, and this may be linked to genetic constraints, such as the use of mating types to determine the inheritance of cytoplasmic genomes. However, the mushroom fungi (Agaricomycetes) stand out as having both large numbers of mating types within a species, which will allow nearly all individuals to be compatible with each other, and reciprocal exchange of nuclei during mating, which will avoid cytoplasmic mixing and cyto-nuclear conflicts. Although the limitation of mating types to two in most fungi is consistent with the cyto-nuclear conflicts model, there are many facets of the Agaricomycete life cycle that also suggest they will demand a high outbreeding efficiency. Specifically, they are mostly obligately sexual and outcrossing, inhabit complex competitive niches, and display broadcast spore dispersal. Subsequently, the Agaricomycete individual pays a high cost to being choosy when encountering a mate. Here, I discuss the costs of mate finding and choice and demonstrate how most fungi have multiple ways of reducing these costs, which can explain why mating types are mostly limited to two per species. Nevertheless, it is perplexing that fungi have not evolved multiple mating types on more occasions nor evolved sexes. The few exceptions to these rules suggest that it is dictated by both molecular and evolutionary constraints.
真核生物在有性生殖过程中进化出了无数种结合配子的方式。一个反复出现的模式是，从形态相同的配子融合（同配生殖）的交配系统，向较大配子与较小配子融合（异配生殖）的交配系统趋同进化。在异配生殖物种中，性别被定义为只产生一种配子类型的个体。尽管性别在整个真核生物域中普遍存在，但在真菌中不存在生物学意义上的性别，因为即使在异配生殖物种中，个体也是雌雄同体的，会产生两种配子类型。因此，“交配型”一词比“性别”更可取，这样定义后，只有不同交配型的个体才能交配（等位基因纯合不相容）。在异配生殖的真菌物种中，几乎没有证据表明交配型超过两种，这可能与遗传限制有关，比如利用交配型来确定细胞质基因组的遗传。然而，蘑菇真菌（伞菌纲）很突出，它们在一个物种内有大量的交配型，这使得几乎所有个体都能相互兼容，并且在交配过程中细胞核会相互交换，这将避免细胞质混合和细胞核与细胞质的冲突。尽管大多数真菌将交配型限制为两种与细胞核与细胞质冲突模型一致，但伞菌纲的生命周期还有许多方面也表明它们需要很高的杂交效率。具体来说，它们大多是专性有性和异交的，栖息在复杂的竞争生态位中，并表现出广泛的孢子传播。随后，伞菌纲个体在遇到配偶时挑剔会付出高昂代价。在这里，我讨论寻找和选择配偶的成本，并说明大多数真菌如何有多种降低这些成本的方式，这可以解释为什么每个物种的交配型大多限制为两种。然而，令人困惑的是，真菌在更多情况下没有进化出多种交配型，也没有进化出性别。这些规则的少数例外表明，这是由分子和进化限制共同决定的。
