School of Life Sciences, University of Dundee, Dundee, DD1 5EH, UK.
Department of Biological Sciences, Mississippi State University, Mississippi State, MS, USA.
Sci Rep. 2020 May 29;10(1):8797. doi: 10.1038/s41598-020-65709-3.
Unicellular protozoa that encyst individually upon starvation evolved at least eight times into organisms that instead form multicellular fruiting bodies with spores. The Dictyostelia are the largest and most complex group of such organisms. They can be subdivided into 4 major groups, with many species in groups 1-3 having additionally retained encystment. To understand fitness differences between spores and cysts, we measured long-term survival of spores and cysts under climate-mimicking conditions, investigated spore and cyst ultrastructure, and related fitness characteristics to species ecology. We found that spores and cysts survived 22 °C equally well, but that spores survived wet and dry frost better than cysts, with group 4 spores being most resilient. Spore walls consist of three layers and those of cysts of maximally two, while spores were also more compacted than cysts, with group 4 spores being the most compacted. Group 4 species were frequently isolated from arctic and alpine zones, which was rarely the case for group 1-3 species. We inferred a fossil-calibrated phylogeny of Dictyostelia, which showed that its two major branches diverged 0.52 billion years ago, following several global glaciations. Our results suggest that Dictyostelium multicellular sporulation was a likely adaptation to a cold climate.
当面临饥饿时，能够形成囊泡的单细胞原生动物会独自形成囊泡，而另一些则会形成具有孢子的多细胞生殖体。盘基网柄菌是此类生物体中最大和最复杂的群体。它们可以进一步细分为 4 个主要群体，其中第 1-3 组的许多物种还保留了囊泡形成能力。为了了解孢子和囊泡之间的适应能力差异，我们在模拟气候条件下测量了孢子和囊泡的长期存活率，研究了孢子和囊泡的超微结构，并将适应特征与物种生态学联系起来。我们发现，孢子和囊泡在 22°C 下的存活率相等，但孢子在潮湿和干燥的霜害下的存活率高于囊泡，其中第 4 组孢子的抵抗力最强。孢子壁由三层组成，而囊泡最多由两层组成，此外，孢子比囊泡更紧实，第 4 组孢子最为紧实。第 4 组物种经常从北极和高山地区分离出来，而第 1-3 组物种则很少出现在这些地区。我们推断出盘基网柄菌的化石校准系统发育树，表明其两个主要分支在 5.2 亿年前分化，这与几次全球冰川作用有关。我们的研究结果表明，盘基网柄菌的多细胞孢子形成可能是对寒冷气候的一种适应。
