the surrounding medium to aid in uptake of metabolites and elimination of waste (Short et al., 2006; Solari et al., 2006a).
The first factor that leads to a cost of reproduction to flagellar action is the so-called “flagellation constraint” (Koufopanou, 1994). The flagellation constraint refers to the fact that, because of their rigid cell wall, the basal bodies cannot take the position expected for centrioles during cell division while still remaining attached to the flagella (as they do in naked green flagellates). The flagellation constraint becomes critical at the 32-cell colony size, because a flagellum may beat for up to five cell divisions without the basal bodies attached. The second factor leading to a tradeoff between reproduction and motility is that the increasing mass of the reproductive cells and embryos during reproduction decreases motility by increasing drag (Solari et al., 2006b). This increasing mass is especially noticeable in the larger species.
Large germ cells are required to form large colonies because of the unusual and likely ancestral form of cell division found in most volvocine species, known as palintomy or multiple fission. Instead of growing to twice their initial size and dividing in two, reproductive cells in palintomic species grow to many times their initial size before undergoing up to ≈13 rounds of division in rapid succession, with little or no growth between divisions. For a reproductive cell to undergo d rounds of (symmetric) division without interspersed growth, it must begin mitosis at a minimum of 2d times the initial size of the daughter cells.
Koufopanou (1994) argued for the volvocine green algae that soma evolved to keep larger colonies afloat and motile while reproductive cells divide and develop. She showed that the soma-to-reproductive-cell ratio increases with colony size and that the investment in somatic tissue increases twice as fast with colony size as does the investment in germ tissue. However, no direct evidence was given as to why a higher investment in somatic cells is needed for motility as colony size increases. Although the between-species trend is consistent with an increasing cost of reproduction with increasing group size, what selective factors operate within species?
We have modeled the hypothesis that life-history tradeoffs drive evolutionary transitions in individuality by selecting for cell specialization by considering how cells should change their allocation to reproduction and viability as colony size increases (Michod, 2006; Michod et al., 2006). Our theoretical results predict that in unicellular organisms the tradeoff curve between viability and fecundity should be concave, but as groups form and increase in size the curve should become increasingly convex (Fig. 7.3A) as a result of the increasing cost of reproduction to survival as colonies increase in size (Fig. 7.3 B and C). A central focus of Solari’s hydrodynamic work (Solari, 2005; Solari et al., 2006b) is to quantify this hypothesized increasing cost of reproduction.