A shift in long-proboscid fly pollinators and floral tube length among populations of Erica junonia (Ericaceae)

Abstract

Macroevolutionary studies frequently emphasize the importance of pollinator shifts for driving speciation within the angiosperms. Pollinators have been identified as a driver of diversification in the mega-diverse genus Erica in the Cape Floristic Region of South Africa, but basic information on pollinator-driven divergence at the initial stages of speciation in this genus is lacking. We focus on two populations of Erica junonia (var. junonia and var. minor) that occur on adjacent mountain ranges separated by c. 10 km and which differ in floral tube length, and ask whether these differences in tube length are associated with a shift between pollinators with different proboscis lengths. In addition, we assess whether the two forms differ in their ability to reproduce in the absence of pollinators. Our findings show that different Erica junonia varieties are visited by different nemestrinid fly species in the genus Moegistorhynchus, and that their floral tube lengths correspond to the proboscis lengths of these flies. Pollinator exclusion experiments show that the short-tubed var. minor is capable of producing seeds in the absence of pollinators, whereas the long-tubed var. junonia depends fully on pollinators. Our study provides an example of pollinator-driven divergence on a small spatial scale and contributes to an understanding of diversification of the genus Erica.