Comparative pollen–pistil interactions and insect pollination in two Hypoxis species (Hypoxidaceae) in China and North America

Abstract

We compared the floral ecology and pollen–pistil interactions in Hypoxis hirsuta (L.) Coville from North America and Hypoxis aurea Loureiro from China. Both species are vernal-flowering herbs, with yellow perianths, providing pollen as their only reward. In H. hirsuta, hand self-pollinated, emasculated and bagged control flowers failed to set fruit. When cross-pollinated, 77% of the pistils produced seed-filled capsules while 72% of open, insect-pollinated flowers set seed. Epifluorescence showed that most germinating grains failed to penetrate the stigmatic surface > 24 h after hand self-pollination suggesting early-acting self-incompatibility (SI). The majority of pollinators of H. hirsuta were female bees representing 21 species distributed in four families. Pollen load analyses indicated that most bees were polylectic and carried grains of H. hirsuta mixed with pollen of co-blooming, nectariferous species. In contrast, capsule production of H. aurea was 60% for open, insect-pollinated flowers. Pollen tubes of H. aurea also entered ovules within 24 h, and SI of this species was partial. Evidence of a late-acting rejection in this species following self-pollination included abnormal growth of pollen tubes in styles with fewer tubes penetrating ovules. In contrast, abnormal tube growth was not observed in styles of H. hirsuta. Hypoxis aurea received fewer insect visitations. Pollen was dispersed primarily by hoverflies (Syrphidae) followed by bees. Floral presentation, generalist pollination and two modes of SI found in these two Hypoxis species contribute to our understanding of floral evolution for congeners isolated on different continents.