DISTYLY AND VARIATION IN HETEROMORPHIC INCOMPATIBILITY IN GAERTNERA VAGINATA (RUBIACEAE) ENDEMIC TO LA REUNION ISLAND

Documenting the floral biology of species throughout the Rubiaceae fam ily is of particular interest since heterostyly and dioecy may have ev olved more than once in this large family. Unfortunately many species in several tropical regions remain unstudied. The purpose of this pape r is to describe the floral biology, the nature of self-incompatibilit y, morph ratios, and fecundity in natural populations of Gaertnera vag inata, a small tree endemic to the island of La Reunion in the Indian Ocean. Measurements of floral characters in populations across the ent ire distribution of this species showed that G. vaginata exhibits a re ciprocal stigma height and anther height dimorphism characteristic of a distylous species. Pollen grain size and corolla tube length are con sistently greater in short-styled plants and long-styled plants produc e more pollen per flower. Controlled pollinations in a natural populat ion showed that 25% of the short-styled plants gave at least one fruit on intra-morph (illegitimate) pollination, whereas no long-styled pla nts set fruit on illegitimate pollination. In total, 19.4% of illegiti mate pollinations produced fruit on short-styled plants. No self-polli nation gave fruit on either morph and between-morph pollinations produ ced 92.2 and 92.8% for short and long-styled plants, respectively. Ove rall, short-styled plants were significantly more abundant than long-s tyled plants. Short-styled plants outnumbered long-styled plants in 16 of the 19 populations. In three of these populations the morph ratio was significantly different from 1:1. In two natural populations, frui t set was significantly higher on long-styled plants, although the num ber of seeds per fruit was not significantly different between the two morphs. The possible effect of variation in the strength of heteromor phic incompatibility on observed variation in morph abundance and the possible causes for the variation in fruit set are discussed.