100 Nakada et al. 2008; Herron and Michod 2008; Herron et al. 2010). However, the evolutionary tree of volvocine algae is highly branched, and several taxa, including the genus Volvox, are found on more than one branch. The structure of volvocine taxonomy results from the fact that early classiﬁcations of this group were based on morphological correlations rather than phylogenetic relationships; several nominal taxa appear to represent convergent morphologies rather than monophyletic relationships. Therefore, the genus and species names of volvocine algae probably should be revised based on molecular phylogenetic reconstructions (Fig. 3). A detailed molecular phylogenetic analysis indicates that multicellularity evolved only once in volvocine algae (Fig. 3) (Larson et al. 1992; Nozaki et al. 1995, 1999, 2000, 2002; Kirk 1998; Coleman 1999; Nozaki 2003; Nakada et al. 2008; Herron and Michod 2008; Herron et al. 2010). After this transition to multicellularity, a partial germ–soma division of labor emerged independently in three different lineages. Organisms are characterized as Sex Plant Reprod (2011) 24:97–112 exhibiting a ‘‘partial division of labor’’ if their gonidia are derived by redifferentiation of cells that had initially dif- ferentiated as biﬂagellate (somatic-like) cells and contrib- uted to the organism’s motility, whereas organisms are characterized as exhibiting a ‘‘full’’ or ‘‘complete division of labor’’ if their gonidia differentiate without ﬁrst devel- oping functional ﬂagella and contributing to the motility of the organism (Herron et al. 2010). In volvocine algae, a full germ–soma division of labor evolved three times (within one of the three lineages that show partial germ–soma division) and led to the species Volvox gigas, Volvox africanus, Volvox obversus, and V. carteri (Fig. 3) (Nozaki et al. 1997; Herron and Michod 2008; Herron et al. 2009, 2010). The evolution of germ–soma division also reﬂects the evolution of a sterile soma, which involves the emer- gence of mortality and the production of the ﬁrst dead ancestors in the evolution of this lineage. Remarkably, the innovation ‘‘division of labor’’ was actually lost twice in volvocine algae (Nozaki et al. 1997; Herron and Michod 2008; Herron et al. 2009, 2010). Fig. 3 Evolutionary tree of volvocine algae based on the nucleotide sequences of ﬁve chloroplast genes. This phylogenetic analysis indicates that multicellularity evolved only once in this group. In contrast, a partial germ–soma division of labor evolved independently in three different lineages and was lost twice (Nozaki et al. 1997; Herron and Michod 2008; Herron et al. 2009). A full germ–soma division also evolved three times. The meanings of symbols and letters are given in the ﬁgure. Names of strains are given in gray behind the species names. The species shown on the photomicrographs in Fig. 1 are indicated with ﬁlled black circles. This tree was adapted from Herron and Michod (2008) and others (Sachs 2008; Herron et al. 2010); some additional information was added (Lerche and Hallmann 2009; Ueki et al. 2010) 123