98 Sex Plant Reprod (2011) 24:97–112 Fig. 1 Rough approximation of the evolution of volvocine green algae from unicellular forms to colonial and multicellular forms with increasing complexity. Six representative species with characteristic developmental traits were arranged such that there is a progressive increase from left to right in morphologic and developmental complexity (Kirk 1998, 2000; Prochnik et al. 2010). Check marks indicate that a given trait is present in the respective species. Graded differences in a given trait are indicated by 1–5 plus signs Volvocales (Chlorophyta). This group spans the full range of organismal complexity, from unicellular organisms, such as Chlamydomonas reinhardtii, to colonial organisms without a division of labor, such as Gonium pectorale, Pandorina morum, and Eudorina unicocca, to multicellu- lar organisms with a partial or full germ–soma division of labor, such as Pleodorina californica and V. carteri, respectively (Fig. 1). To facilitate detailed molecular analyses in volvocine algae, an array of important modern analytical tools has been developed. For example, stable nuclear transforma- tion is possible (Ferna´ndez et al. 1989; Debuchy et al. 1989; Kindle et al. 1989; Mayﬁeld and Kindle 1990; Schiedlmeier et al. 1994; Lerche and Hallmann 2009), unselectable markers can be efﬁciently co-transformed together with selectable markers (Kindle et al. 1991; Gruber et al. 1992; Schiedlmeier et al. 1994; Minko et al. 1999; Lerche and Hallmann 2009), suitable reporter genes and both inducible and constitutive promoters are available (Goldschmidt-Clermont 1991; Hallmann and Sumper 123 and ± indicates ambiguity or occasional occurrence. The photomi- crographs show Chlamydomonas reinhardtii (SAG 11-32b), Gonium pectorale (SAG 12.85), Pandorina morum (SAG 60-1d), Eudorina unicocca (SAG 24-1c), Pleodorina californica (SAG 32.94), and Volvox carteri (Eve). The exact phylogenetic position of these six species is indicated with ﬁlled black circles in the evolutionary tree in Fig. 3 1994; Lerche and Hallmann 2009), expression of chimeric genes works (Blankenship and Kindle 1992; Hallmann and Sumper 1994; Lerche and Hallmann 2009), foreign genes can be expressed (Hallmann and Sumper 1996; Stevens et al. 1996; Hallmann and Rappel 1999; Lerche and Hallmann 2009), dominant selectable markers are available (Nelson et al. 1994; Hallmann and Rappel 1999), gene replacement by homologous recombination is feasible (Sodeinde and Kindle 1993; Hallmann et al. 1997), plas- mids or transposons can be used for gene tagging (Tam and Lefebvre 1993; Schnell and Lefebvre 1993; Kirk et al. 1999; Miller and Kirk 1999; Ueki and Nishii 2008), and the genomes of C. reinhardtii and V. carteri have been sequenced (Merchant et al. 2007; Prochnik et al. 2010). In summary, the existence of a germ–soma division of labor in Volvox, the range of complexity from unicellular to multicellular species in volvocine algae, and the avail- ability of an extensive molecular toolkit make volvocine green algae useful model organisms for studying the evo- lution of multicellularity and of sterile somatic cells.