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：相关理论指出，群体形成方式会对合作演化以及合作群体能否向更高层级个体实现重大跃迁产生关键影响。与通过聚集潜在非亲属个体形成群体的半社会（semisocial）形成路径相比，通过留存于亲代身边形成克隆群体的亚社会（subsocial）群体形成方式，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诱导形成多细胞群体，但目前尚不清楚其群体形成是通过彼此留存聚集（亚社会方式）还是通过个体聚集（半社会方式）。
实验生物：淡水藻类包括椭圆小球藻（Chlorella sorokiniana）、普通小球藻（Chlorella vulgaris）与斜生栅藻（Scenedesmus obliquus），淡水甲壳类捕食者为大型溞（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体捕食者的诱导下，这些藻类同时以亚社会与半社会两种方式形成群体。此外，不同藻类物种可在捕食压力下形成跨物种多细胞混合群体。
研究结论：在这些兼性多细胞藻类中观察到半社会甚至跨物种的群体形成现象，其意义在于：（1）凸显了通过形成群体以躲避捕食所带来的直接适合度收益；（2）强化了不同物种间群体形成方式与多细胞性为兼性还是专性之间的相关性。