Experimental insights on Darwin's cross-promotion hypothesis in tristylous purple loosestrife (Lythrum salicaria).

PREMISE OF THE STUDY: Darwin proposed that the reciprocal arrangement of anthers and stigmas in heterostylous plants promotes cross-pollination through pollen segregation on pollinators' bodies. The floral tube in most heterostylous species constrains the feeding posture of pollinators determining the site of contact with sex organs located at different heights within a flower. Here, we evaluate Darwin's hypothesis in tristylous Lythrum salicaria, a species with a partially tubular corolla, and examine the extent to which the location of sex organs within a flower influence compatible and incompatible pollination. We predicted that the proficiency of cross-pollination would increase for more inserted sex organs due to the restrictions imposed by the floral tube on pollinator positioning.

METHODS: We used experimental trimorphic and monomorphic arrays and emasculated flowers to quantify intermorph pollen transfer and capture among all sex-organ heights, and estimated the contribution of intraflower self-pollination, geitonogamous self-pollination, and intramorph outcross pollination to total intramorph pollination.

KEY RESULTS: As predicted, disassortative pollination varied significantly with sex-organ height and was highest for short-level organs and lowest for long-level organs. In monomorphic arrays, most intramorph pollination resulted from outcross pollination followed by intraflower and geitonogamous self-pollination.

CONCLUSIONS: Our results provide evidence for the Darwinian hypothesis. Reciprocal herkogamy promoted varying degrees of disassortative pollination with the magnitude strongly influenced by sex-organ height within a flower.