Votaw H R, Ostrowski E A
Department of Biology and Biochemistry, University of Houston, Houston, TX, USA.
J Evol Biol. 2017 Nov;30(11):2017-2030. doi: 10.1111/jeb.13172. Epub 2017 Sep 25.
Reproductive division of labour is common in many societies, including those of eusocial insects, cooperatively breeding vertebrates, and most forms of multicellularity. However, conflict over what is best for the individual vs. the group can prevent an optimal division of labour from being achieved. In the social amoeba Dictyostelium discoideum, cells aggregate to become multicellular and a fraction behaves altruistically, forming a dead stalk that supports the rest. Theory suggests that intra-organismal conflict over spore-stalk cell fate can drive rapid evolutionary change in allocation traits, leading to polymorphisms within populations or rapid divergence between them. Here, we assess several proxies for stalk size and spore-stalk allocation as metrics of altruism investment among strains and across geographic regions. We observe geographic divergence in stalk height that can be partly explained by differences in multicellular size, as well as variation among strains in clonal spore-stalk allocation, suggesting within-population variation in altruism investment. Analyses of chimeras comprised of strains from the same vs. different populations indicated genotype-by-genotype epistasis, where the morphology of the chimeras deviated significantly from the average morphology of the strains developed clonally. The significantly negative epistasis observed for allopatric pairings suggests that populations are diverging in their spore-stalk allocation behaviours, generating incompatibilities when they encounter one another. Our results demonstrate divergence in microbial social traits across geographically separated populations and demonstrate how quantification of genotype-by-genotype interactions can elucidate the trajectory of social trait evolution in nature.
生殖分工在许多社会中都很常见，包括那些群居昆虫、合作繁殖的脊椎动物以及大多数形式的多细胞生物。然而，对于个体与群体而言什么是最优选择的冲突可能会阻碍实现最佳分工。在社会变形虫盘基网柄菌中，细胞聚集形成多细胞体，其中一部分细胞表现出利他行为，形成支撑其余细胞的死亡柄细胞。理论表明，关于孢子柄细胞命运的生物体内冲突可推动分配性状的快速进化变化，导致种群内的多态性或种群间的快速分化。在这里，我们评估了几种用于衡量柄细胞大小和孢子 - 柄细胞分配的指标，以此作为不同菌株间以及不同地理区域间利他性投资的度量。我们观察到柄细胞高度的地理差异，这部分可以由多细胞体大小的差异来解释，同时也观察到克隆孢子 - 柄细胞分配在不同菌株间的变化，这表明种群内利他性投资存在差异。对由来自相同或不同种群的菌株组成的嵌合体的分析表明存在基因型与基因型之间的上位性，即嵌合体的形态显著偏离克隆发育菌株的平均形态。对异域配对观察到的显著负上位性表明，不同种群在孢子 - 柄细胞分配行为上正在分化，当它们相遇时会产生不相容性。我们的结果证明了地理隔离种群间微生物社会性状的差异，并展示了基因型与基因型相互作用的量化如何能够阐明自然中社会性状进化的轨迹。
