Volvox carteri is a species of colonial green algae in the order Volvocales. The V. carteri life cycle includes a sexual phase and an asexual phase. V. carteri forms small spherical colonies, or coenobia, of 2000–6000 Chlamydomonas-type somatic cells and 12–16 large, potentially immortal reproductive cells called gonidia. While vegetative, male and female colonies are indistinguishable; however, in the sexual phase, females produce 35-45 eggs and males produce up to 50 sperm packets with 64 or 128 sperm each. The genome of this species of algae was sequenced in 2010. Volvox carteri is a significant model organism for research into the evolution of multicellularity and organismal complexity, largely due to its simple differentiation into two cell types, versatility in controlled laboratory environments, and natural abundance. Differentiation Volvox carteri is a useful model organism for understanding the evolution and developmental genetics of cellular differentiation, in part because asexual colonies possess only two cell types. Approximately 2000 biflagellated somatic cells form a monolayer at the surface of the extracellular matrix (ECM) and cannot divide, rendering them mortal. They facilitate motility in response to changes in light concentration (phototaxis), which is detected via an orange photoreceptor-containing eyespot. Gonidia, by contrast, are immobile, embedded in the ECM interior, and are potentially immortal due to their ability to divide and participate in reproduction. Three key genes are known to play significant roles in the somatic-gonidium dichotomy: glsA (gonidialess A); regA (regenerator A); and lag (late gonidia). These genes are believed to carry out germ-soma differentiation during development in a general order: gls specifies cell fate based on size lag genes facilitate gonidial development in large cells reg genes facilitate somatic development in small cells The glsA gene contributes to asymmetric cell division that results in the designation of large cells that develop into gonidia and small cells that develop into somatic cells. Gls mutants do not experience asymmetric division, a key component for creating gonidia, and thus are composed only of somatic swimming cells. The lag gene plays a role in specialization of gonidial initials. If mutations disable the lag gene, large cells specified by glsA will develop as somatic cells initially but then de-differentiate to become gonidia. Determination of somatic cells is controlled by the transcription factor regA. The regA geneencodes a single 80 amino acid-long DNA-binding SAND domain that is expressed in somatic cells after embryonic development. regA acts to prevent division by