Sex Plant Reprod (2011) 24:97–112 DOI 10.1007/s00497-010-0158-4 R E V I E W Armin Hallmann Evolution of reproductive development in the volvocine algae Received: 16 September 2010 / Accepted: 27 November 2010 / Published online: 21 December 2010 The Author(s) 2010. This article is published with open access at Springerlink.com Abstract The evolution of multicellularity, the separa- tion of germline cells from sterile somatic cells, and the generation of a male–female dichotomy are certainly among the greatest innovations of eukaryotes. Remarkably, phylogenetic analysis suggests that the shift from simple to complex, differentiated multicellularity was not a unique progression in the evolution of life, but in fact a quite frequent event. The spheroidal green alga Volvox and its close relatives, the volvocine algae, span the full range of organizational complexity, from unicellular and colonial genera to multicellular genera with a full germ–soma division of labor and male–female dichotomy; thus, these algae are ideal model organisms for addressing funda- mental issues related to the transition to multicellularity and for discovering universal rules that characterize this transition. Of all living species, Volvox carteri represents the simplest version of an immortal germline producing specialized somatic cells. This cellular specialization involved the emergence of mortality and the production of the ﬁrst dead ancestors in the evolution of this lineage. Volvocine algae therefore exemplify the evolution of cel- lular cooperation from cellular autonomy. They also serve as a prime example of the evolution of complex traits by a few successive, small steps. Thus, we learn from volvocine algae that the evolutionary transition to complex, multi- cellular life is probably much easier to achieve than is commonly believed. Communicated by Scott Russell. A. Hallmann (&) Department of Cellular and Developmental Biology of Plants, University of Bielefeld, Universita¨tsstr. 25, 33615 Bielefeld, Germany e-mail: armin.hallmann@gmx.de Keywords Cell differentiation Multicellularity Reproduction Green algae Volvox carteri Chlamydomonas reinhardtii Introduction Since the early work of August Weismann on the conti- nuity of germ plasm at the end of the nineteenth century (Weismann 1893, 1892, 1889), the spheroidal green alga Volvox and its close relatives have been seen as suitable model organisms for addressing fundamental issues in the evolution of multicellularity and the development of a germ–soma dichotomy (Kirk 1998, 2001, 2003, 2005; Nozaki 2003; Hallmann 2003b; Schmitt 2003; Kirk and Kirk 2004; Michod et al. 2006). Volvox carteri illustrates the concept of a germ–soma dichotomy with diagrammatic clarity (Fig. 1): an adult V. carteri spheroid has only two cell types, *2,000 small Chlamydomonas-like somatic cells, and *16 large asexual reproductive cells (gonidia). The potentially immortal gonidia are nonmotile cells that are specialized for growth and reproduction; these cells constitute the germline. In contrast, the biﬂagellate somatic cells are specialized for extracellular matrix (ECM) bio- synthesis, motility, and phototaxis. They are also incapable of dividing and are programmed to die when only a few days old. Therefore, of all living species, Volvox represents the simplest version of an immortal germline producing mortal soma. Particularly with regard to questions about the evolution of multicellularity and of the germ–soma dichotomy, both Volvox and its close relatives, the volvocine green algae are of special interest (Kirk 1998, 2005; Prochnik et al. 2010). The volvocine green algae form a group of genera closely related to the genus Volvox within the order 123