On the rediscovery of Volvox perglobator (Volvocales, Chlorophyceae) and the evolution of outcrossing from self-fertilization

Hypothesis: Genetic recombination underlies the frequent origins of self-fertilization and rare reversions to outcrossing. Organisms: Volvox perglobator, Gonium pectorale, volvocine green algae. Methods: We report the rediscovery of a volvocine alga, obligate outcrossing V. perglobator, including morphological characteristics and molecular phylogeny. We use ancestral-state reconstruction and phylogenetic simulation to demonstrate multiple transitions between self-fertilization and outcrossing. We sequence sex-determining genes in multiple volvocine algae. Results: We find significant support for numerous origins of self-fertilization and multiple reversions to outcrossing. Volvox perglobator is one of these reversions to outcrossing. Combination of ancestrally sex-restricted and sex-determining genes into the same genotype correlates with the evolution of self-fertilization. Re-segregation of these same sex-determining loci into separate sexes correlates with the evolution of outcrossing. These results suggest that recombination underlies the evolutionary transitions between self-fertilization and outcrossing in the volvocine algae.