Solari Cristian A, Kessler John O, Michod Richard E
Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona 85721, USA.
Am Nat. 2006 Apr;167(4):537-54. doi: 10.1086/501031. Epub 2006 Feb 14.
During the unicellular-multicellular transition, there are opportunities and costs associated with larger size. We argue that germ-soma separation evolved to counteract the increasing costs and requirements of larger multicellular colonies. Volvocalean green algae are uniquely suited for studying this transition because they range from unicells to multicellular individuals with germ-soma separation. Because Volvocales need flagellar beating for movement and to avoid sinking, their motility is modeled and analyzed experimentally using standard hydrodynamics. We provide comparative hydrodynamic data of an algal lineage composed of organisms of different sizes and degrees of complexity. In agreement with and extending the insights of Koufopanou, we show that the increase in cell specialization as colony size increases can be explained in terms of increased motility requirements. First, as colony size increases, soma must evolve, the somatic-to-reproductive cell ratio increasing to keep colonies buoyant and motile. Second, increased germ-soma specialization in larger colonies increases motility capabilities because internalization of nonflagellated germ cells decreases colony drag. Third, our analysis yields a limiting maximum size of the volvocalean spheroid that agrees with the sizes of the largest species known. Finally, the different colony designs in Volvocales reflect the trade-offs between reproduction, colony size, and motility.
在单细胞向多细胞的转变过程中，细胞体积增大既带来了机遇，也产生了代价。我们认为生殖细胞与体细胞的分离是为了应对多细胞群体增大所带来的成本增加和需求变化而进化出来的。团藻目绿藻非常适合用于研究这一转变过程，因为它们涵盖了从单细胞到具有生殖细胞与体细胞分离的多细胞个体。由于团藻目生物需要鞭毛摆动来移动并避免下沉，因此利用标准流体动力学对它们的运动性进行了建模和实验分析。我们提供了一个由不同大小和复杂程度的生物体组成的藻类谱系的比较流体动力学数据。与库福帕努的观点一致并加以拓展，我们表明随着群体大小的增加，细胞特化的增加可以用运动需求的增加来解释。首先，随着群体大小的增加，体细胞必须进化，体细胞与生殖细胞的比例增加以保持群体的浮力和运动性。其次，较大群体中生殖细胞与体细胞特化程度的增加提高了运动能力，因为无鞭毛生殖细胞的内化减少了群体的阻力。第三，我们的分析得出了团藻目球体的一个极限最大尺寸，这与已知最大物种的尺寸相符。最后，团藻目中不同的群体设计反映了繁殖、群体大小和运动性之间的权衡。
