Primrose homostyles: A classic case of possible balancing selection revisited

Abstract

In this issue of Molecular Ecology, Mora-Carrera et al. (2022) revisit a case of the loss of an outcrossing system in primroses, which has been studied as an example of balancing selection in the wild since the 1940s. Molecular variants in the gene involved in the mutant self-fertile phenotype, which is now known, help towards understanding this textbook example of breakdown of an outcrossing system. However, as often happens, new information also raises further questions.

FIGURE 1

The genetics and evolution of distyly and an XY sex chromosome pair, showing the two haplotypes in both situations. (a) the distyly genome region, or S‐locus. At least three S‐locus genes probably control the phenotype differences between the long‐ and short‐styled morphs in distylous plants. The S‐locus region is hemizygous in the S/s genotype (short‐styled), and five genes have been identified in the roughly 280‐kb region, none of which is present in the s haplotype (long‐styled). The S haplotype genes are therefore hemizygous in the short‐styled genotype. Long homostyles can arise by a mutation causing loss of a functional CYP^T gene, which controls the gynoecium character of the short‐styled morph; the other s^H haplotype genes are hemizygous in the initial mutant (S^H/s), but the haplotype can become homozygous through self‐fertilization or after the haplotype spreads to a high frequency in the population. (b) X‐linked genes are present in diploid coverage in females, but may be hemizygous in males, due to loss of copies from the Y, so that homozygotes for the Y chromosome are lethal. (c) Frequencies of homostyled individuals and the two other morphs in primrose natural populations sampled in Somerset; distylous populations, with no homostyles, are also shown on the y‐axis