Reproductive biology of the andromonoecious shrub Trevoa quinquenervia (Rhamnaceae)

Various aspects of the reproductive biology of the andromonoecious shrub Trevoa quinquenervia Gill. & Hook, were studied in a population located in central Chile. Flowering extends from mid August to the end of December. The small, white, entomophilous flowers are incompletely protandrous and last c. 5 days. A weak, pleasant odour along with nectar and pollen attracts 17 diurnal insect species, seven of which are probable pollinators: these include Diptera (Syrphidae and Bombyliidae), Hymenoptera (Apidae and Colletidae) and Coleoptera (Melyridae). Besides perfect flowers T. quinquenervia individuals possess c. 10% staminate flowers, which are lighter, seem to be shorter-lived, and show a smaller gynoecium with abnormal ovules. Reproduction is exclusively sexual and apparently xenogamous, with pollen transfer depending on insect vectors. Although flower morphology and individual flower phenology do not fully prevent self-pollination (and geitonogamy can easily take place) the level of autogamy is low. Therefore, some self-incompatibility mechanism seems operative in this species. Fruit set of open-pollinated flowers is 6.6%, and ripe fruits contain on average 0.66 viable seeds. Predispersal (maternal) reproductive success (percent ovules becoming viable seeds) is 1.45%. For medium to full-sized individuals this corresponds to c. 2600 to 8600 potential offspring per year. Beyond the production of male flowers, four other aspects of the species' reproductive biology seem directed to enhance pollen export, partly implying increased male allocation: (a) a high pollen:ovule ratio, (b) protection of anthers against pollen-robbing by means of the cucullate petals, (c) secondary pollen presentation in a hairy barrier located above the nectary, and (d) a reduction in female allocation achieved by the regular abortion of two out of the three ovules in each ovary, thus producing a mostly one-seeded fruit provided with a thin, energy-poor pericarp. It is proposed that the advantages of enhanced pollen flow for a self-incompatible organism, which is moreover structured in isolated populations, might explain the increased male effort, including the joint evolution of andromonoecy and one-seededness.