Sex Plant Reprod (2011) 24:97–112 several splice variants of the RBR1 transcript exist (Kianianmomeni et al. 2008). An RBR1-related homolog exists not only in C. reinhardtii, where it is called MAT3 (Gillham et al. 1987; Armbrust et al. 1995; Umen and Goodenough 2001), but also in almost all eukaryotes (Hallmann 2009b), which highlights the importance of RBR1. The information about the function of the genes described above is promising, but clearly more work is required to understand the evolution and functionality of the male–female dichotomy at a molecular level. Conclusions and prospects The gradual evolutionary transition in volvocine algae from unicellular species to multicellular species with a full division of labor between germline and somatic cells and with male–female dichotomy not only exempliﬁes the evolution of cellular cooperation from cellular autonomy but is also a prime example of Darwin’s notion that com- plex traits arise via evolution by small steps (Darwin 1872). However, the observed disappearance of innovations in the evolution of volvocine algae also indicates that evolution is not a one-way street from simple to complex organisms. On the basis of the above-mentioned traits characteristic of volvocine algae, and taking into account previous con- siderations by Hoops (1997), Herron and Michod (2008) and Kirk (2005), the transition to differentiated multicel- lularity in asexual volvocine algae encompasses the fol- lowing nine developments: 1. an increase in the number of cleavage divisions and an incomplete cytokinesis that generates a cytoplasmic bridge network between cells; 2. inversion of the (spheroidal) cell sheet to correct a maladaptive arrangement produced during cleavage; this inversion is the solution to a special problem found only in volvocine embryos; 3. development and expansion of a multifunctional ECM, which displays a dynamic and multifunctional inter- face; the ECM holds cells in the appropriate places on the surface of the spheroid, and developmentally controlled ECM-speciﬁc enzymes allow for ECM remodeling during ontogenesis; 4. modiﬁcation of the breaststroke-type swimming motion of unicellular species to a beating mode in which the effective strokes of both ﬂagella of each cell beat in the same direction, which is accomplished by rotation of the basal bodies; 5. modiﬁcation of ﬂagellar beating direction in such a way that it causes rotation of the spheroid, which is required for phototaxis, rather than rotation of the cell; 107 6. invention of organismal polarity with gradients in cell size, cell-to-cell distance, eyespot size, and light sensitivity along the anterior–posterior axis; 7. invention of partial germ–soma division of labor; all cells ﬁrst differentiate into motile biﬂagellate (somatic-like) cells and then later at least some cells redifferentiate and turn into nonmotile gonidia, while all the other cells remain as motile biﬂagellate cells and eventually die; 8. evolution of full germ–soma division of labor between somatic and germline cells (germ–soma dichotomy); gonidia differentiate without ﬁrst developing func- tional ﬂagella and contributing to the motility of the organism; all the other cells develop as sterile somatic cells and eventually die; 9. evolution of asymmetric cell division producing large and small daughter cells and bifurcation of the cell division program. The stepwise transition to multicellularity with differ- entiated cell types in volvocine algae likely has parallels in the transitions to multicellularity that occurred in other eukaryotic lineages (Fig. 2). The frequent and convergent evolution of differentiated multicellularity seems to indi- cate that it can be achieved relatively easily. This specu- lation is corroborated by a recent mathematical model, which shows that complete germ–soma differentiation can be achieved easily and quickly (within a million gen- erations) via the evolution of developmental plasticity (Gavrilets 2010). Volvocine algae that exhibit a partial or complete divi- sion of labor in the asexual cycle exhibit a similar division of labor in the sexual cycle. Thus, for example, in V. carteri asexual gonidia, eggs and sperm-forming cells are all set apart from somatic cells by asymmetric cleavage divisions, whereas Volvox species that form gonidia without beneﬁt of asymmetric division form prospective eggs and sperm- forming cells by similar mechanisms. Sex seems to provide the following advantages for volvocine algae (Maynard Smith 1978; Dawes 1981; Coleman 1983; Bernstein et al. 1985; Kirk and Kirk 1986; Goodenough et al. 1995; Kirk 1998; Burt 2000; Cavalier-Smith 2002; Colegrave et al. 2002; Colegrave 2002; Kaltz and Bell 2002; Hallmann 2003b): a. survival during life-threatening conditions by the production of stress-resistant zygotes (zygospores); b. creation of genetic variation, which is required for evolution; c. recombinational DNA repair during meiosis because homologous chromosomes pair at that time. Like differentiated multicellularity in asexual repro- duction, different cell types in sexual males and females 123