Herron Matthew D, Ghimire Susma, Vinikoor Conner R, Michod Richard E
Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona, USA.
Evol Ecol Res. 2014;16(3):203-221.
The evolution of mortal somatic cells was a critical step in the evolution of complex body plans and the major radiations of multicellular life. In the volvocine green algae, somatic cells are hypothesized to mitigate an increasing cost of reproduction as colony size increases, primarily by providing motility to the colony during reproduction.
Does selection on colony size cause an evolutionary response in proportion of somatic cells? Does the effect of selection on colony size differ in environments that differ in the importance of motility?
We subjected an outcrossed population of the volvocine alga to selection on colony size in still and mixed environments. After approximately 40 generations with periodic selection, we estimated the relationship between colony size and proportion of soma in evolved colonies from both environments.
In the largest size category, colonies selected in the still environment (in which motility is hypothesized to be more important) had a higher proportion of soma than those from the mixed environment. Within-strain variation in cell number was surprisingly large: up to 16-fold for some genotypes. The positive among-species relationship between colony size and proportion of soma was paralleled within the larger (16- to 64-celled) colonies of , but not within the smaller (4- and 8-celled) colonies, which had the highest proportions of soma, suggesting the existence of an evolutionary constraint preventing optimization of soma in the smallest size classes.
必死的体细胞的进化是复杂身体结构进化以及多细胞生命主要辐射进化中的关键一步。在团藻目绿藻中，体细胞被假定为随着群体大小增加减轻繁殖成本的增加，主要是通过在繁殖期间为群体提供运动能力来实现。
对群体大小的选择是否会导致体细胞比例的进化响应？在运动重要性不同的环境中，对群体大小的选择效果是否不同？
我们让一种团藻目藻类的异交群体在静止和混合环境中接受群体大小的选择。经过大约40代的定期选择后，我们估计了来自两种环境的进化群体中群体大小与体细胞比例之间的关系。
在最大的尺寸类别中，在静止环境（据推测运动在其中更重要）中选择的群体比来自混合环境的群体具有更高的体细胞比例。细胞数量的菌株内变异惊人地大：某些基因型高达16倍。在较大（16至64细胞）的群体中，群体大小与体细胞比例之间的种间正相关关系与之平行，但在较小（4和8细胞）的群体中则不然，这些群体具有最高的体细胞比例，这表明存在一种进化限制，阻止最小尺寸类别的体细胞优化。
