Sex Plant Reprod (2011) 24:97–112 A phylogenetic perspective on the transition from unicellularity to multicellularity In many textbooks, volvocine algae are used to illustrate the evolutionary transition from unicellular to multicellular organisms, and some readers might therefore mistakenly arrive at the conclusion that this transition happened only once in the evolution of all multicellular life. However, multicellularity has arisen independently again and again in the tree of life, over billions of years. In fact, these tran- sitions were not even restricted to eukaryotes but occurred in prokaryotes as well: The ﬁrst evidence of a shift to multicellularity comes from fossilized Cyanobacteria-like prokaryotes, which lived 3–3.5 billion years ago (Schopf 1993; Knoll 2003), and the ﬁrst multicellular eukaryotes may have existed about 1 billion years ago (Knoll et al. 2006). Between about 530 and 570 million years ago, a burst of diversiﬁcation occurred, with the eventual appearance of the lineages of almost all metazoa living today (Carroll 2001; Knoll 2003; King 2004; Maynard Smith and Szathma´ry 1995; Pfeiffer and Bonhoeffer 2003; Grosberg and Strathmann 2007). Among the eukaryotes, multicellular organisms independently originated at least 25 times from unicellular ancestors (followed in some cases by secondary losses) (Buss 1987; Cavalier-Smith 1991; Maynard Smith and Szathma´ry 1995; Kirk 1998; Bonner 1998, 2000; Carroll 2001; Kaiser 2001; Medina et al. 2003; Baldauf 2003; King 2004; Grosberg and Strathmann 2007; Rokas 2008; Prochnik et al. 2010). Almost every lineage of the eukaryotic tree of life includes multicellular forms: the most well-known lineages are the Embryophyta (land plants), Chlorophyta (green algae), Rhodophyta (red algae), Phaeophyceae (brown algae), Bacillariophyta (diatoms), fungi, and animals (Fig. 2). For most multicellular lineages, it has been quite chal- lenging to investigate the shifts to multicellularity because the transitions occurred so long ago (Herron and Michod 2008). Therefore, a molecular understanding of the genetic changes underlying this transition might even be unat- tainable in these lineages. In contrast, the volvocine algae represent a unique opportunity to study the transition to multicellularity at a molecular level because the multicel- lular members of this group diverged relatively recently from their unicellular relatives: the last common ancestor of the unicellular alga C. reinhardtii and the multicellular alga V. carteri lived only 50–200 million years ago (Rausch et al. 1989; Herron and Michod 2008; Herron et al. 2009). Moreover, extant volvocine species display a range of intermediate grades between unicellular forms and multicellular forms with a complete separation of germ and soma (Fig. 1). Taxonomically, the volvocine algae belong to the order Volvocales. Together with orders such as the Dunaliellales, 99 Fig. 2 Phylogenetic distribution of multicellularity among eukary- otes. Large U, only unicellular forms; large M, only colonial/ multicellular forms; middle-sized U and M, clade with both unicellular and colonial/multicellular forms; large U and small M, mainly unicellular forms with rare colonial/multicellular forms (Baldauf 2003; King 2004; Grosberg and Strathmann 2007; Prochnik et al. 2010) Chlorococcales, Oedogoniales, and Chaetophorales, the Volvocales belong to the Chlorophyceae, one of the four classes of Chlorophyta (green algae) (Fig. 2). Detailed molecular phylogenetic analyses of the volvocine algae have revealed that the history of the whole group is quite complex, and not just a simple, linear progression in size and complexity (Nozaki et al. 2000; Nozaki 2003; Nakada et al. 2008; Herron and Michod 2008; Herron et al. 2010). Therefore, the few volvocine species arranged in order of increasing complexity in Fig. 1 reﬂect only a rough approximation of the evolution of volvocine green algae. The detailed molecular phylogenetic reconstruction of volvocine algae shown in Fig. 3 conﬁrms that volvocine algae constitute a robust monophyletic group with two major families within this group, the Goniaceae and the Volvocaceae (Larson et al. 1992; Nozaki et al. 1995, 1999, 2000, 2002; Kirk 1998; Coleman 1999; Nozaki 2003; 123