106 Volvocine algae are haplonts, and the only diploid stage of development is the zygote. In the unicellular alga C. reinhardtii, haploid vegetative cells of both sexes, the plus and minus mating types, dif- ferentiate into gametes in response to nitrogen starvation, and the isogamous gametes of opposite mating types can agglutinate and fuse to form zygotes (Harris et al. 2009). The zygote further matures into a dormant zygospore. At the time of spore germination, meiosis occurs to form four haploid progeny cells, each of which is able to enter the asexual life cycle (Fig. 4). In the multicellular alga V. carteri, heat shock causes the production and release of a sex-inducer glycoprotein (Starr 1970; Starr and Jaenicke 1974; Kirk and Kirk 1986; Tschochner et al. 1987; Mages et al. 1988), which is one of the most potent biological effector molecules known because it can act at concentrations as low as 10-16 M (Gilles et al. 1984; Starr 1970; Sumper et al. 1993). Reproductive cells of asexually grown individuals (Fig. 6a, b) that have been exposed to the sex-inducer switch to the sexual mode (Hallmann et al. 1998). In sexually induced female embryos, the ﬁrst asymmetrical cell division is postponed from the sixth to the seventh division cycle (Starr 1969, 1970; Hallmann et al. 1998). After the asym- metric cell division, the somatic cell initials undergo fur- ther cleavage (as in asexual embryos). The large reproductive cell initials develop into eggs, such that the sexual female ends up with *32 eggs and *2,000 somatic cells (Fig. 6d), compared to *16 gonidia and *2,000 somatic cells in asexual females (Fig. 6b). Sexually induced male embryos exhibit yet another pattern of asymmetric division (Starr 1969, 1970; Hallmann et al. 1998): the asymmetric cell division is postponed from the sixth to the eighth division cycle, and thereafter, the somatic cell initials no longer divide. Then about a day later, the large reproductive cell initials each undergo a new round of up to seven symmetric divisions, to form sperm packets containing up to 128 sperm. The sexual male thus ends up with 128 sperm packets and 128 somatic cells (Fig. 6c), compared with *16 gonidia and *2,000 somatic cells in asexual males (Fig. 6a) (Starr 1969, 1970; Hallmann et al. 1998). Eventually, the sperm packets, consisting of biﬂagellate sperm (Fig. 6e), are released from the spheroid. As soon as a swimming sperm packet makes contact with a sexual female, which seems to happen by chance rather than by chemoattraction (Kirk 1998), the packets break up into individual sperm, and the sperm penetrate the ECM of the female to reach the eggs inside the sphere (Fig. 6f) and fertilize them. Following fertil- ization, the resulting diploid zygotes mature and build tough cell walls (Fig. 6g). After a rest period, favorable conditions cause the zygotes to undergo meiosis and ger- mination to form only a single viable germling and three 123 Sex Plant Reprod (2011) 24:97–112 nonviable small polar bodies (Starr 1975). The germling will produce a haploid female or male, depending on its genetic sex, which then reproduces asexually. The sexual dimorphism, i.e., male–female dichotomy and anisogamy (oogamy), observed in V. carteri (Fig. 6) and many other volvocine species with an increased level of organismal complexity (Fig. 1) evolved from a Chla- mydomonas-like, isogamous unicellular ancestor. Phylo- genetic analyses indicate that either oogamy evolved twice or it evolved once and was lost once in the Eudorina- Pleodorina-Volvox lineage (Kirk 2006). Oogamy also evolved independently in almost all the lineages of eukaryotes that evolved multicellularity, e.g., in other Chlorophyta (green algae), Embryophyta (land plants), Rhodophyta (red algae), Phaeophyceae (brown algae), fungi, and animals (Kirk 2006). However, except for one lineage, there are no molecular genetic data relating the sex-determining loci of oogamous organisms to the mating types of their isogamous ancestors. The only lineage for which we have such data is the volvocine lineage. Sexual development in V. carteri and C. reinhardtii is controlled by a large, multigenic, haploid sex-determining locus (mating-type locus) that is located on homologous chromosomes in the two species and that segregates as a single Mendelian trait in each. In C. reinhardtii, the mat- ing-type locus is a 200- to 300-kb region within which the order of genes is rearranged between sexes (plus and minus mating type); thus, meiotic recombination is suppressed in this region (Ferris et al. 2002; Merchant et al. 2007). Relative to C. reinhardtii, the sex-determining locus of V. carteri has undergone a remarkable divergence in both sexes and has expanded ﬁvefold (Kianianmomeni et al. 2008; Ferris et al. 2010; Prochnik et al. 2010). Only the sex-speciﬁc genes MID (Ferris and Goodenough 1997) and MTD1 (Lin and Goodenough 2007) from C. reinhardtii have clear homologs in V. carteri, namely in the male mating-type locus (Nozaki et al. 2006; Ferris et al. 2010). The MID gene of C. reinhardtii was shown to be both necessary and sufﬁcient to cause cells to differentiate as gametes of the minus mating type (Ferris and Goodenough 1997). Thus, the MID homologs indicate that maleness in the oogamous V. carteri might have evolved from the minus mating type of its isogamous unicellular ancestor (Nozaki et al. 2006; Nozaki 2008). Further analysis of the mating-type locus of V. carteri identiﬁed several male- and female-speciﬁc genes without detectable homologs in C. reinhardtii (Ferris et al. 2010), but their functions remain to be investigated. A key protein in sexual development seems to be the gender-speciﬁc retinoblastoma-related protein RBR1 (Kianianmomeni et al. 2008; Hallmann 2009a). The RBR1 gene maps to the sex-determining locus and exhibits divergent evolution in females and males; in addition,