Gene and pathway co-option and the origins of asymmetric cell division and cellular differentiation in Volvox

(A) The function of glsA appears to have been co-opted without change from an unknown function in the unicellular ancestor of Volvox, so that it is now part of a pathway (shaded green) that is required for asymmetric cell division. This may have happened because some not yet identified gene (X) that acted in the same pathway (shaded gray) as the ancestor of glsA ("proto-glsA") changed to take on a new function, generating the new asymmetric division pathway. The dashed arrow indicates that the ancestral pathway may or may not exist in Volvox. (B) The evolution of the somatic cell fate appears to have involved gene duplication and then change (divergence) of one of the gene copies, regA. Scientists hypothesize that the ancestor of regA, "proto-regA," acted in a stress-activated pathway (shaded gray) that led to the repression of growth and cell division. Duplication of proto-regA produced regA* and regA, both of which may initially have functioned in that same pathway, in an intermediate species. In Volvox, regA acts in a pathway (shaded green) that represses growth and cell division in response to developmental cues; while the descendent of regA*, known in Volvox as rlsD, might act in the stress-activated pathway that represses growth and cell division. Thus, regA could have gained its cell fate function because it changed in a way that permitted it to co-opt an existing pathway that repressed growth and cell division.