Fitness and Complexity in Volvocalean Green Algae

As a means to understand the emergence of individuality at a new higher level, a model about the transition from undifferentiated cell-groups to multicellular organisms with germ-soma separation is developed. We argue that the increase in complexity is a consequence of the trade-offs between the two basic fitness components –fecundity and viability– as size increases. We use volvocalean green algae as a model system to compare the fitness, as size increases, of four hypothetical colony types with different degrees of germsoma differentiation and show that soma evolves first, and, as size increases further, complete germ-soma specialization is achieved. Our results show that the cost of reproduction plays an important role in the evolution of multicellularity in Volvocales. Two general principles emerge from our work that may apply to other lineages: a cell group has to reach a specific number of cells to overcome the high cost of soma specialization, and soma, as the first specialization step, contributes to the integrity and individuality of the organism and increases viability, whereas germ, as the first specialization step, disrupts the integrity and individuality of the organism and decreases viability.