Volvox carteri

Volvox carteri F. Stein 1878 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. 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: 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.  regA encodes 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 inhibiting cell growth via downregulation of chloroplast biosynthesis, and represses expression of genes necessary for germ cell formation.  Chlamydomonas reinhardtii, a unicellular relative of V. carteri, is known to possess genes related to regA.  This suggests that the regA gene originated before proper cellular differentiation in Volvox and was likely present in an undifferentiated ancestor.  In this case, the function of regA in V. carteri most likely arose due to changes in expression pattern from a temporal (environmental response) state to a spatial (developmental) state. The V. carteri genome consists of 138 million base pairs and contains ~14,520 protein-coding genes.  Like many other multicellular organisms, this alga has a genome rich in introns; approximately 82% of the genome is non-coding.  The V. carteri genome has a GC content of approximately 55.3%.