Gender specialisation and stigma height dimorphism in Mediterranean Lithodora fruticosa (Boraginaceae)

Dimorphism in style height has evolved repeatedly in flowering plants, with some individuals having short and others long styles; in the case of distylous species, stigma position varies reciprocally with that of the anthers. Distyly can be associated with divergence in the functional gender between long- and short-styled individuals, but gender divergence has rarely been investigated in species with a simple stigma height polymorphism in the absence of reciprocal dimorphism in anther position. To evaluate the relation between stigma height polymorphism and gender, I measured the dimensions of floral morphology and seed production for the two morphs of a large population of the Iberian species Lithodora fruticosa (Boraginaceae). Results confirm the existence of a stigma height polymorphism in L. fruticosa, with long- and short-styled individuals at a 1:1 ratio in the studied population. Long-styled individuals produced substantially more seeds than did short-styled individuals, pointing to strong divergence in functional gender between the two morphs. The results of this study are puzzling in light of recent work that suggests that L. fruticosa has a multi-allelic self-incompatibility system. I discuss the significance of gender divergence in L. fruticosa and evaluate hypotheses that might explain it.