Unidirectional hybridization in heterostylous primroses

Heteromorphy in flowers has a profound effect on breeding patterns within a species, but little is known about how it affects reproductive barriers between species. Ma et al.  study a natural hybrid zone between two heterostylous primrose species, Primula beesiana and P. bulleyana, and find that all hybrid derivatives examined are backcrosses of first or later generations to P. bulleyana, and have the cpDNA of that species.Viable F₁s can not be produced with P. bulleyana pollen on P. beesiana mothers. They conclude that partial ecological isolation, inhibition of heterospecific pollen and possibly complete barriers to F₁ formation on P. beesiana may be enough to make F₁ formation very rare in these species. The hybrid zone may thus have a finite lifespan as successive generations become more similar to P. bulleyana.