The Genetics of Fitness Reorganization during the Transition to Multicellularity: The Volvocine regA-like Family as a Model.

The evolutionary transition from single-celled to multicellular individuality requires organismal fitness to shift from the cell level to a cell group. This reorganization of fitness occurs by re-allocating the two components of fitness, survival and reproduction, between two specialized cell types in the multicellular group: soma and germ, respectively. How does the genetic basis for such fitness reorganization evolve? One possible mechanism is the co-option of life history genes present in the unicellular ancestors of a multicellular lineage. For instance, single-celled organisms must regulate their investment in survival and reproduction in response to environmental changes, particularly decreasing reproduction to ensure survival under stress. Such stress response life history genes can provide the genetic basis for the evolution of cellular differentiation in multicellular lineages. The regA-like gene family in the volvocine green algal lineage provides an excellent model system to study how this co-option can occur. We discuss the origin and evolution of the volvocine regA-like gene family, including regA-the gene that controls somatic cell development in the model organism Volvox carteri. We hypothesize that the co-option of life history trade-off genes is a general mechanism involved in the transition to multicellular individuality, making volvocine algae and the regA-like family a useful template for similar investigations in other lineages.

Characterization and Transformation of reg Cluster Genes in Volvox powersii Enable Investigation of Convergent Evolution of Cellular Differentiation in Volvox.

The evolution of germ-soma cellular differentiation represents a key step in the evolution of multicellular individuality. Volvox carteri and its relatives, the volvocine green algae, provide a model system for studying the evolution of cellular differentiation. In V. carteri, the regA gene controls somatic cell differentiation and is found in a group of paralogs called the reg cluster, along with rlsA, rlsB, and rlsC. However, the developmental program of V. carteri is derived compared to other volvocine algae. Here we examine Volvox powersii which possesses an ancestral developmental program and independent evolution of the Volvox body plan. We sequenced the reg cluster from V. powersii wild-type and a mutant with fewer cells and altered germ-soma ratio. We found that the mutant strain's rlsB gene has a deletion predicted to cause a truncated protein product. We developed a genetic transformation procedure to insert wild-type rlsB into the mutant strain. Transformation did not result in phenotypic rescue, suggesting the rlsB mutation is insufficient for generating the mutant phenotype. The transformation techniques and sequences described here provide essential tools to study V. powersii, a species well suited for studying the evolution of cellular differentiation and convergent evolution of Volvox morphology.