Secondary homostyly in Primula L.; evidence for the model of the ‘S’ supergene



In Primula, in which 9% of the species are homostyle, it is estimated that 54% are polyploid, while only 4% of the cytologically examined heterostyle species are polyploid.

To investigate the supposed secondary and recombinational nature of homostyly in polyploid Primulas of section Aleuritia, crosses were made between three diploid heterostyle and five polyploid homostyle species. Patterns of pollen germination, pollen tube growth and seed set in interspecific illegitimate crosses were the same as those observed for intraspecific crosses. However, interspecific legitimate crosses showed a late-acting incompatibility not seen in the intraspecific crosses. In all cases, homostyles acted as it they were of thrum mating type when male, but as if they were of pin mating type when female. We conclude that for all five homostyle species, homostyly has arisen secondarily by recombination within the ‘S’ complex linkage group. The relationship between secondary recombinational homostyly and polyploidy in Primula is discussed.

Although the small quantities of seed produced in crosses between four species of different ploidy levels were shown to be viable, substantial crossability barriers were demonstrated between all eight species. Thus, evidence of crossability supports the maintenance of these closely similar taxa as distinct species of disjunct distribution.