104 Sex Plant Reprod (2011) 24:97–112 Fig. 6 The switch from asexual to sexual reproduction in V. carteri. Asexual development (see Fig. 5) and the phenotypes of males and females are indistinguishable. a Photomicrograph of an asexual male. b Photomicrograph of an asexual female. When the sex inducer is present, the gonidia of both sexes undergo a modiﬁed embryogenesis, which is also different in males and females. c In males, embryonic cleavage divisions (including asymmetric cell division) result in juveniles with somatic cells and sperm packets in a 1:1 ratio. d In females, juveniles with *32 eggs arise. Mature sperm packets (e) are released and swim around until they contact a sexual female with mature eggs. The sperm packets then break up into individual sperm, and the sperm penetrate the ECM of the female and fertilize the eggs (f). g The resulting diploid zygotes develop an orange coloration and secrete a thick, crenellated cell wall. When the zygote germinates, it undergoes meiosis to produce either a single haploid asexual male or female redifferentiate as nonmotile cells, and then undergo mul- tiple ﬁssion to form daughter colonies. Once all cells of these daughter colonies have developed ﬂagella, the daughters hatch and swim away. In volvocine algae with an intermediate level of organismal complexity namely, certain species and strains of Eudorina, a few of the cells at the anterior pole of the embryo retain their ﬂagella and fail to divide. In other words, the ﬁrst sterile somatic cells emerged in evolution of this lineage (Fig. 1). The formation of sterile somatic cells is also graded along the anterior–posterior axis, with the sterile cells at the anterior pole (Kirk 2005). The ﬁrst clear sign of cell differentiation and germ– soma dichotomy in the volvocine lineage becomes appar- ent in P. californica (Fig. 1; Gerisch 1959). This alga shows a partial germ–soma division of labor: all cells ini- tially differentiate into biﬂagellate cells, but then only cells of the posterior hemisphere redifferentiate as gonidia, while the cells of the anterior hemisphere remain as sterile somatic cells and eventually die. In V. carteri, cell differentiation and germ–soma dichotomy have been fully established, i.e., this alga shows a complete germ–soma division of labor between *2,000 somatic cells and *16 germline cells (Fig. 1). During 123 embryogenesis of volvocine species with a complete germ– soma division of labor, some of the cleavage divisions are asymmetric, producing large–small sister-cell pairs (Fig. 1) (Kirk 1995, 1998). In V. carteri, asymmetric cell division is accompanied by a bifurcation of the cell division program (Fig. 1; Kirk 2005): V. carteri embryos ﬁrst cleave sym- metrically ﬁve times to form a 32-cell embryo with iden- tical cells, and then 16 cells divide asymmetrically to produce one large gonidial cell initial and one small somatic cell initial each (Kirk and Kirk 2004). These gonidial initials divide asymmetrically two more times and produce additional somatic initials at each division. The gonidial initials then temporarily stop any cleavage activ- ity, while the somatic initials divide symmetrically about three more times. At the end of embryogenesis in V. car- teri, the volume of the gonidial initials is about 30-fold larger than that of the somatic initials (Figs. 1, 7); how- ever, the cells are only different in size. Then, by an as-yet- unknown mechanism, the size of each sister cell leads to the activation of either a somatic or germline program (Fig. 7) (Kirk et al. 1993). Thus, small cells develop as biﬂagellate somatic cells for ECM biosynthesis, motility, and phototaxis, and large cells develop as nonmotile, germline gonidia specialized for growth and reproduction