Conflicting origins of intra-genomic variation in
Symbiodinium. Red and green colourations represent divergent ITS2 sequences, with monocoloured cells featuring homogeneous ITS2 arrays and bicoloured cells hosting polymorphic ribotypes. Schematics show (a) competing hypotheses of sequence homology within P. damicornis-associated Symbiodinium, with a single clonal population hosting a NDP under H0, two genetically isolated populations under H1 and inter-lineage recombination under H2; and; and (b) introgression, with fitness disparities between F1 and later generations (shown in a size gradient). Many backcross generations (n) may occur before an increase in fitness is realized. Genetic isolation occurs when one or more classes suffer from insurmountably low mean fitness.