indicated in Fig. 7.3 B and C. This bonus can be obtained only in groups and is the basis for the synergistic effects of specialization according to our hypothesis. Alternatively, the bonus of specialization in soma may be viewed as an initial cost when somatic cells dedifferentiate into reproductive cells. Below we present evidence for this cost in terms of decreased flagellar force in regA mutants in which somatic cells have flagella for a day before dedifferentiating into reproductive cells.
Solari and colleagues developed a hydrodynamics approach using videotaping of colonies to understand motility and its determinants in volvocine algae (Solari, 2005; Solari et al., 2006b). The swimming force exerted by a single motile cell for the benefit of group motility can be calculated for different species and mutants by these techniques. Single gene mutations in life-history traits can be a powerful approach to understanding the cost of reproduction and tradeoffs between life-history traits (Reznick, 1985; Roff, 2000, 2002). In the V. carteri regA mutant, ≈235 cells change their phenotype from being somatic (S) with no reproductive function back to the ancestral state of having both somatic and reproductive functions (being flagellated first and then absorbing the flagella and reproducing). As a result of these changes in reproductive effort at the cell level, the size and motility capacities of the group change. The striking result is that as specialized somatic cells (cells with b = 0 in Fig. 7.3) prepare to exert reproductive effort (cells with b > 0), there is not only a large decrease in colony motility, but there is a large decrease in the motility force contributed by a single flagellated cell. For example, the average force exerted for group motility by a single motile cell is approximately half in the regA– mutant of what it is in wild type (4.9 × 10−8 dynes versus 8.0 × 10−8 dynes). The cost of reproduction to motility that underlies the convex nature of the fitness tradeoffs (Fig. 7.3) is real and directly measurable in these organisms and is attributable to a change in the effort exerted by single cells within the cell group. There is a caveat in that we do not know whether there are genetic differences (other than a mutation at the regA locus) between the regA− mutant strain we have obtained from the Culture Collection of Algae at the University of Texas (Austin, TX) and the wild-type strain.
In summary, comparative data indicate that reproductive effort increases with colony size and that as the investment in reproduction increases, motility declines. The regA mutant indicates that flagellar force declines if somatic cells are to dedifferentiate and start reproducing. In addition, during development, as reproductive cells increase in size, motility does not change for small species, but declines for the larger species (Solari, 2005; Solari et al., 2006b). Apparently, because the length of the flagella increases as cells increase in size, this allows the smaller