I COVERED the volvocine algae recently and promised a post on some of the genes involved in the evolution of multicellularity in this group. We still know relatively little about the genomes of volvocine algae, but research has picked out three genes involved in the evolution of multicellularity. These are invA, glsA, and regA.
The first gene, invA, is involved in embryo inversion. Volvocine algae have developed two types of inversion, complete and incomplete. This gene was studied in Volvox carteri, which has complete inversion, but the gene may be involved in incomplete inversion as well.
When a Volvox embryo begins to invert, it first forms a cup shape. Then two changes of the cells at the lips of the cup are required. First, they must elongate to become flask shaped, with the narrow stems on the outer rim of the cup. Second, they must move relative to their cytoplasmic bridges, which initially connect all cells at the midpoint. These changes attach the cells at their most narrow point and move the bulk of the cell to the inner edge of the rim, forcing it to curl outwards. A gene called invA was discovered to be involved in this process, and found to be a kinesin that localizes at the cytoplasmic bridges.1 By knocking out invA, Nishii and coworkers discovered that it was involved in the microtubule-mediated movement of cells relative to their cytoplasmic bridges. Embryos without invA could produce a cup shape and the cells at the rim changed to a flask-shape, but they remained attached at the midpoint and the result was only a slight curl of the embryo rim. This produces an adult colony that is shaped kind of like a Derby hat and can’t control its direction of travel, but spins in circles.
A survey of the genome of the volvocine’s closest unicellular relative, Chlamydomonas reinhardtii, discovered a similar gene named iar1 (from invA-related-1). This gene has 82% overall sequence identity, and the motor domain and C-terminus are over 90% identical. The similarity is so great that iar1 can rescue inversionless V. carteri mutants (Nishii and Kirk, unpublished data). C. reinhardtii has no need for embryo inversion, being unicellular, so the ancestral kinesin must have some other role in the single-celled ancestor of volvocine algae. The modern role of invA in embryo inversion is an exaptation.
The second case is the gene glsA. This gene is involved in asymmetrical cell division, which is used in some volvocine algae to produce small somatic cells and large germ cells.2 Most volvocine algae produce some dedicated somatic cells, and other cells of similar size that act initially as somatic cells before switching to a reproductive role. Only a few types of volvocines have developed dedicated somatic and germ lineages, and these control cell fate by the use of glsA. Embryos which have glsA knocked out produce adults with only somatic cells, and no germ cells. glsA has homologs in a wide range of organisms. One of these homologs is found in C. rheinhardtii, and was named gar1 (from glsA related). Cheng and coworkers found that when this gene is cloned into embryos with nonfunctional glsA, gar1 is able to correct the absence of glsA and a normal phenotype results. C. rheinhardtii does not undergo asymmetric cell division itself, so glsA must have its effect by interacting with an unknown protein to control cell division. The multicellular volvocine algae again adapted a pre-existing protein to serve a novel role.
The final gene is regA. This gene appears again in V. carteri, where it maintains the terminal differentiation of somatic cells. It is thought that its product is a transcription factor that serves to suppress chloroplast division in somatic cells and thus suppresses cell growth. A search for homologs in V. carteri subspecies and C. reinhardtii discovered a conserved ~100 amino acid domain named the VARL domain (Volvocine Algae RegA-Like) with a SAND domain, used for binding DNA.3 C. reinhardtii possesses several proteins containing VARL domains, and the closest match to regA is C_170011. Since regA, V. carteri regA homolog rlsA, and C_170011 share an intron at the same position in the VARL domain, these genes probably share a recent common ancestor. Terminal differentiation of somatic cells appears to have evolved by the exaptation of a VARL-containing transcription factor present in the unicellular ancestor of the volvocines.
We might expect that these genes involving embryonic inversion and cellular differentiation would be completely new in the volvocine algae, but very similar genes were present in the unicellular ancestors of these algae. The evolution of multicellularity in volvocine algae adapted several genes to serve new roles. In two cases the gene is changed so little that the homolog preserved in a unicellular relative can replace the multicellular organism’s gene. Evolution often proceeds by using old genes in new ways.
- Nishii, I.; Ogihara, S.; Kirk, D. L. “A Kinesin, InvA, Plays an Essential Role in Volvox Morphogenesis.” Cell 2003, 113, 743-753. doi:10.1016/S0092-8674(03)00431-8
- Cheng, Q.; Fowler, R.; Tam, L.; Edwards, L.; Miller, S. M. “The role of GlsA in the evolution of asymmetric cell division in the green alga Volvox carteri.” Development Genes and Evolution 2003, 213, 328-335. doi:10.1007/s00427-003-0332-x
- Duncan, L.; Nishii, I.; Howard, A.; Kirk, D.; Miller, S. M. “Orthologs and paralogs of regA, a master cell-type regulatory gene in Volvox carteri.” Current Genetics 2006, 50, 61-72. doi:10.1007/s00294-006-0071-4