Nonrandom mating in Discaria americana (Rhamnaceae)

Nonrandom mating takes place when seed paternity differs from that produced by random success of available pollen. To test if some form of this phenomenon operates in the self-incompatible perennial Discal-ia americana, we performed one-donor hand pollinations involving seven individuals, and measured pollen-tube performance and seed viability and germination. A significant level of heterogeneity in reproductive traits was present in our sample. Mating was nonrandom as a consequence of complete self-incompatibility (intrafloral and geitonogamous pollinations were unsuccessful) Linked with partial cross-compatibility (some inter-individual pollinations consistently failed to produce offspring while other parental combinations were fecund). Pollen tubes of successful pollen parents exhibited higher growth rate than those of infecund ones. Stylar-ovarian inhibition of incompatible pollen tubes and a wet stigma type suggests that a gametophytic incompatibility system operates in the species.