The biosystematics and evolution of the Polyalthia hypoleuca species complex (Annonaceae) of Malesia. III. Floral ontogeny and breeding systems

Abstract

Aspects of floral ontogeny, breeding systems, and hybridization potential for selected members of the monophyletic Polyalthia hypoleuca complex (Annonaceae) of Malesia were investigated. Complete intrafloral dichogamy (protogyny) was found in all five members examined. Further, these five tree species had flowers developing in several serial “cohorts,” the flowers of any one cohort maturing in synchrony resulting in complete intracohort dichogamy. Two modes of the timing of maturation of successive floral cohorts within an individual were observed. In the first (P. hypoleuca and P. sumatrana), the onset of stigma receptivity of successive cohorts was separated by 2 days, resulting in complete intercohort dichogamy within any given individual. This mode of ontogeny probably acts to reduce geitonogamy and stamen‐carpel interference within an individual. Two distinct “sets” of trees existed in these populations. The trees of one set were functionally carpellate at times when the trees of the other set were functionally staminate. Functional status for the two sets was reversed the next day. This pattern of complete intraset dichogamy with temporally shifted sets probably leads to enhanced pollen transfer by beetles between, but not within, the sets. The second mode of successive cohort maturation involved serial cohorts that achieved stigma receptivity day after day such that staminate and carpellate phase flowers were simultaneously present in a given individual. The second mode permits geitonogamy and intraindividual, interfloral stamen‐carpel interference. P. discolor, P. glauca, and P. multinervis exhibited both modes to varying degrees. In addition, autogamy was not detected for P. glauca, P. hypoleuca, and P. sumatrana. Results from manual pollination experiments are consistent with the hypothesis that P. glauca and P. hypoleuca are self‐incompatible, and a bidirectional hybrid cross between these two species yielded no fruit set.