Reproductive biology of the Andean-disjunct genus Retanilla (Rhamnaceae)

Various aspects of the reproductive biology of the Andean-disjunct genus Re tanilla (DC.) Brongn. were studied in populations of R. trinervia and R. ep hedra located in central Chile and of R. patagonica in southern Argentina. Flowering extends from -austral- winter (early spring in R. patagonica) to early summer. The small, white, entomophilous flowers are incompletely prot androus and last four to five days. A weak pleasant odor along with nectar and pollen attract thirty-seven diurnal insect species, ten of which are pr obable pollinators: these include the honey bee, eight species of solitary bees pertaining to Anthophoridae (1 sp.), Colletidae (3 spp.) and Halictida e (4 spp.), and a nemestrinid fly. Although flower morphology and individua l flower phenology do not fully prevent self-pollination (and geitonogamy c an easily take place), the level of autogamy is low. Therefore, some self-i ncompatibility mechanism seems operative in Retanilla. Fruit set of open-po llinated flowers is extremely low, with a maximal value of c. 3% in R. pata gonica. In this species, ripe fruits contain on average 1.37 viable seeds. Predispersal (maternal) reproductive success (percent ovules becoming viabl e seeds) is 2.3%. For medium to full-sized individuals this corresponds to c. 4600 potential offspring per year. As presently known, Retanilla is a re productively uniform group, in which the Andean disjunction seems to have e xerted no particular impact, which is consistent with the view that pollina tion generalization exerts some stabilizing influence on floral morphology and other reproductive traits. Retanilla expresses a basic rhamnacean set o f traits (including protandry and self-incompatibility) showing also high p ollen production and secondary pollen presentation. The latter two traits s eem characteristic of the Retanilla-Trevoa an clade, suggesting that a tren d to increased male effort and, perhaps, dependence upon pollen-eating has evolved within the tribe Colletieae.