. "7 Evolution of Individuality During the Transition from Unicellular to Multicellular Life--RICHARD E. MICHOD." In the Light of Evolution: Volume 1. Adaptation and Complex Design. Washington, DC: The National Academies Press, 2007.
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In the Light of Evolution, Volume I: Adaptation and Complex Design
a transcriptional repressor (Kirk et al., 1999) thought to suppress several nuclear genes coding for chloroplast proteins (Meissner et al., 1999). Consequently, the cell growth (dependent on photosynthesis) and division (dependent on cell growth) of somatic cells are suppressed. Because they cannot divide, they do not participate directly in the offspring but contribute to the survival and reproduction of the colony through flagellar action (Short et al., 2006; Solari et al., 2006a,b). In other words, the somatic cells express an altruistic behavior, and regA [whose expression is necessary and sufficient for this behavior (Kirk et al., 1999)] is an altruistic gene. Which cells express regA and differentiate into somatic cells is determined early in development through a series of asymmetric cell divisions. The asymmetric divisions ensure that some cells (i.e., the germ-line precursors) remain above the threshold cell size associated with the expression of regA (Kirk, 1995). As with all forms of cooperation, this altruistic behavior is also susceptible to defection and selfish mutants; indeed, mutations in regA result in the somatic cells regaining reproductive abilities, which in turn results in them losing their flagellar capabilities (Kirk et al., 1987). Because motility is important for these algae (flagellar activity is required to maintain themselves in the water column at an optimum position relative to sunlight intensity), the survival and reproduction of V. carteri individuals in which such mutant somatic cells occur are negatively affected (Solari et al., 2006b).
How can an altruistic gene such as regA originate, and can its evolutionary origin be traced back to the unicellular ancestor of this group? The basic life cycle in Chlamydomonas reinhardtii (presumed to be similar to the unicellular ancestor of this group) involves a flagellated and motile vegetative stage, during which the cell grows in size, followed by absorption of the flagella and cell division to produce daughter cells. It seems reasonable to expect that life-history genes would exist in C. reinhardtii that would allocate effort to these different stages depending on environmental conditions and, in particular, allocate effort away from reproduction toward survival in conditions not promoting growth. Such a gene could become altruistic in the context of a cell group if it was turned on developmentally in some cells and if its vegetative functions also benefited the group.
Nedelcu and Michod (2006) showed that reproductive altruism (i.e., a sterile soma) in the multicellular green alga V. carteri (Fig. 7.1D) evolved via the cooption of a life-history gene whose expression in the unicellular ancestor was conditioned on an environmental cue (as an adaptive strategy to enhance survival at an immediate cost to reproduction) through shifting its expression from a temporal (environmentally induced) into a spatial (developmental) context as summarized in Fig. 7.2. The regA-like gene in C. reinhardtii (Fig. 7.1A) belongs to a diverged and structurally heterogeneous multigene family sharing a SAND-like domain (a DNA-