Sex Plant Reprod (2011) 24:97–112 Driving forces in the evolutionary transition toward differentiated multicellularity Many previous studies address the question why and how multicellularity and germ–soma differentiation evolved in volvocine algae (and elsewhere) (Bonner 1965, 1974, 1988, 1993, 1998, 2000, 2003; Kirk 1988, 1994, 1995, 1997, 1998, 1999, 2000, 2001, 2003, 2005; Kirk et al. 1990, 1993; Koufopanou and Bell 1991, 1993; Desnitski 1993, 1995; Koufopanou 1994; Bell 1998; Schmitt 2001, 2003; Miller 2002, 2010; Stark and Schmitt 2002; Hallmann 2002, 2003a, b, 2006b; Michod et al. 2003, 2006; Pfeiffer and Bonhoeffer 2003; Kirk and Kirk 2004; Nedelcu and Michod 2004; Cheng et al. 2005; Solari et al. 2006a, b, 2007; Grosberg and Strathmann 2007; Willensdorfer 2008, 2009; Sachs 2008; Rokas 2008; Herron et al. 2009; Gavrilets 2010). It appears to be a matter of course that there must be a combination of costs and beneﬁts that accompany increased body size, such that under certain conditions the beneﬁts of larger size out- weigh its costs; it was argued that size increase came ﬁrst, and the possible advantages that this change might provide would follow (Bonner 1998). One such beneﬁt of size is predation, which spares larger organisms (see below) (Bell 1985; Kirk 1998). Following the origin of multicellularity in the above-mentioned groups of organisms (Fig. 2), e.g., land plants, green algae, red algae, brown algae, fungi, and animals, there evolved more than one cell type. It has been assumed that also early differentiation is related to the size of the organism: the larger the organism, the more cell types (Bonner 2003). Thus, size plays a critical role in inﬂuencing the degree of differentiation (Bonner 2003). But how can groups of cells become individuals? It was hypothesized that ﬁtness trade-offs drive the transition of a cell group into a multicellular individual through the evo- lution of cells specialized at reproductive and vegetative functions of the group (Michod 2007). Somatic cells spe- cialized at vegetative functions describe a reproductive altruism, which may have evolved through co-option of life-history trade-off genes present in a unicellular ances- tor; the selective pressures leading to reproductive altruism in differentiated, multicellular volvocine algae obviously stem from the increasing cost of reproduction with increasing group size (Michod 2007). Life cycles of the volvocine algae Volvocine algae are able to reproduce both asexually and sexually (Figs. 4, 5, 6); however, in these algae, the prin- cipal mode of reproduction is asexual (Kirk 1998; Harris et al. 2009). The multicellular alga V. carteri is dioecious (i.e., it produces separate male and female organisms), but 101 the asexual life cycles of both sexes (mating types) are indistinguishable (Figs. 5, 6); the same is true for the unicellular alga C. reinhardtii (Fig. 4) (Kirk 1998; Harris et al. 2009). Volvocine algae switch from their principal, asexual mode of reproduction to a sexual mode when life-threat- ening conditions approach (Figs. 4, 6) (Kirk 1998; Harris et al. 2009). Thus, sexuality is a form of emergency response program for volvocine algae. Although the sexual cycle results in the production of resistant zygospores, as well as genetic recombination and repair, it does not result in any signiﬁcant amount of increase in population size. Key evolutionary innovations in the asexual life cycle When multicellular organisms evolved from unicellular ones, previous achievements had to be modiﬁed and developed to fulﬁll the requirements of multicellularity (Szathma´ry and Smith 1995; Bonner 1998; Grosberg and Strathmann 2007; Prochnik et al. 2010). A comparison of the developmental and reproductive traits of the modern multicellular alga V. carteri with those of the extant uni- cellular alga C. reinhardtii and species with intermediate complexity provides insight into the evolutionary innova- tions that were required for multicellular life. The cell division cycle of most asexually reproducing volvocine algae is different from the cycles seen in most other organisms (Kirk 2003, 1998). The asexual repro- ductive cells of most volvocine algae grow 2n-fold in size and then divide rapidly n times by multiple ﬁssion to produce 2n offspring cells. This type of cell division is known as palintomy and multiple ﬁssion (Sleigh 1989; Desnitski 1995). The n has a value from 2 to 15 depending on the volvocine species and, to some extent, the envi- ronmental conditions (Kirk 2003, 1998). In C. reinhardtii, the n is usually 2 or 3 (Fig. 4a, b), and in V. carteri, it is usually 11 or 12 (Fig. 5). However, there is an important difference between the multiple ﬁssion programs of these two species: the offspring cells in C. reinhardtii almost always separate from each other (complete cytokinesis) and eventually become unicellular individuals, while the offspring cells in multicellular volvocine algae stay linked to each other by cytoplasmic bridges throughout the rest of embryogenesis due to an incomplete cytokinesis (Fig. 1). Hence, incomplete cytokinesis and an increase in the number of cleavage divisions led to an evolutionary rise in adult cell number in volvocine algae, ranging from one cell in C. reinhardtii to many thousands of cells in various species of Volvox (Figs. 1, 6a, b). At the end of cleavage, each embryo of colonial or multicellular volvocine algae contains all of the cells that will be present in an adult of the next generation. However, 123