102 Sex Plant Reprod (2011) 24:97–112 Fig. 4 Asexual and sexual reproduction in C. reinhardtii. Under favorable conditions of growth, Chlamydomonas reproduces asexu- ally; only when the conditions are unfavorable does it reproduce sexually. a and b During asexual reproduction, cells grow and undergo two or more rounds of mitosis and cytokinesis before the daughter cells hatch from the old cell wall. Both mating types, the plus mating type (mt?) (a) and the minus mating type (mt-) (b), show identical asexual life cycles. c Sexual reproduction is induced by unfavorable conditions (nitrogen limitation), and asexual cells of both the orientation of the cells of the embryo is inside out with respect to the adult conﬁguration; that is, the cells have their ﬂagellar ends pointing toward the interior, rather than toward the exterior where they will be needed to function in locomotion. To correct this maladaptive arrangement and to achieve the adult conﬁguration, the embryo turns itself right-side out in a gastrulation-like morphogenetic process called (embryonic) inversion (Figs. 1, 5) (Hallmann 2006a). The cytoplasmic bridges that link all embryonic cells play a major role in inversion (Kirk 1998; Kirk and Nishii 2001). The shift to organisms with more than one cell also required the elaboration of a multifunctional ECM out of the simple ECM (usually called cell wall) of the Chla- mydomonas-like ancestor. In all volvocine algae, the structured ECM meshwork is assembled mainly from hydroxyproline-rich glycoproteins (Miller et al. 1974; Kirk et al. 1986; Sumper and Hallmann 1998; Hallmann 2003b, 2006b). In volvocine algae with high levels of organismal complexity like Volvox, the ECM not only embeds the cells in the surface of a transparent sphere, holds them together and allows for their suitable orientations but also represents a rather dynamic and multifunctional interface between each cell and its neighboring cells and/or environment. The ECM is continuously adapted to changing intracellular, 123 mating types develop into gametes. Gametes of opposite mating types are able to form aggregates via ﬂagellar agglutination. After the release of cell walls, mating structures are formed, and the gametes fuse (plasmogamy and karyogamy) to form a zygote. The zygote further matures into a dormant, heavily walled, and diploid zygospore. At the time of spore germination, meiosis occurs to form four haploid progeny cells (two of each mating type), each of which grows into an asexual cell of the respective mating type organism-speciﬁc or environmental conditions, and ECM remodeling is essential for embryonic development, mor- phogenesis, and reproduction (Nagase and Woessner 1999; Hallmann 2003b). As the number of cells increased during the evolution of the volvocine lineage, the size of organ- isms increased even more rapidly due to an exponential increase in the volume of ECM per cell (Fig. 1). In adult V. carteri spheroids in the stage shortly before the release of juveniles (Fig. 5), the ECM constitutes up to 99% of the spheroid volume, whereas C. reinhardtii has only a rela- tively thin ECM (Fig. 1). The evolution of large multicel- lular spheroids with a voluminous ECM might result in an improved protection of the next generation, which develops inside the parent spheroid (Fig. 5). Additionally, predation spares larger organisms: small green ﬂagellates are subject to predation by invertebrate ﬁlter feeders such as rotifers and small crustaceans, but most colonial volvocine algae are too large to be consumed by these ﬁlter feeders (Bell 1985; Kirk 1998). Traits that impact forward swimming and phototaxis also required evolutionary adaptation during the transition to an organism with more than one cell. In multicellular organisms, phototactic swimming requires a proper and stable orientation of cells with respect to one other and some kind of coordination between cells (Ueki et al. 2010).