Data from: How do algae form multicellular groups?

Background: Theory suggests that how groups are formed can be a major influence on the evolution of cooperation, and whether cooperative groups make the major transition to a higher level individual. The formation of clonal groups, by remaining with parents (subsocial group formation) leads to a greater kin selected benefit of cooperation, compared with formation of groups by aggregating, with potential non- relatives (semisocial). Freshwater algae form multicellular groups in response to the presence of predators, but it is not clear whether they form groups by remaining together (subsocial) or by aggregation (semisocial). Organisms: The freshwater algae Chlorella sorokiniana, Chlorella vulgaris and Scenedesmus obliquus, and the freshwater crustacean predator Daphnia magna. Results: Fluorescence microscopy and time-lapse photography revealed that, in response to predator supernatant/live predators, these algae form groups both subsocially and semisocially. Additionally, different algal species form mixed-species multicellular groups in response to predation. Conclusion: The observation of semisocial, and even between species, group formation in these facultatively multicellular algae: (i) emphasises the direct fitness benefits of forming groups to avoid predation; and (ii) strengthens the across species correlation between the method of group formation and whether multicellularity is facultative or obligate.

研究背景：理论研究表明，群体组建方式会对合作演化进程，以及合作群体能否向更高层级的个体单元发生重大过渡，产生决定性影响。与通过聚集潜在非亲缘个体形成的半社会性（semisocial）群体相比，通过留存于亲本身边形成的克隆群体——即亚社会性（subsocial）群体组建方式，能为合作带来更高的亲缘选择收益。淡水藻类可在捕食者存在时形成多细胞群体，但目前尚未明确其群体组建是通过彼此留存（亚社会性方式）还是聚集（半社会性方式）实现。
受试生物：包括淡水藻类原壳小球藻（Chlorella sorokiniana）、普通小球藻（Chlorella vulgaris）、斜生栅藻（Scenedesmus obliquus），以及淡水甲壳类捕食者大型溞（Daphnia magna）。
实验结果：通过荧光显微镜与延时摄影观测发现，在捕食者上清液或活捕食者的刺激下，上述藻类可同时以亚社会性和半社会性两种方式形成群体。此外，不同藻类物种还会在捕食压力下形成跨物种多细胞混合群体。
研究结论：针对这些兼性多细胞藻类的半社会性甚至跨物种群体组建现象的观测，可得到两点结论：其一，组建群体以躲避捕食的行为，其直接适合度收益得到了进一步证实；其二，群体组建方式与多细胞性属于兼性还是专性之间的跨物种相关性，得到了显著强化。