POLLINATION BIOLOGY OF NIVENIA (IRIDACEAE) AND THE PRESENCE OF HETEROSTYLOUS SELF-COMPATIBILITY

ABSTRACT

Nivenia, a shrubby genus of the Cape Province, South Africa, comprises five distylous species (N. binata, N. capitata, N. corymbosa, N. dispar, N. stenosiphon) and four monomorphic species. Hand pollinations of natural populations of seven Nivenia species have confirmed previous reports of self-compatibility. Pollen deposited on the stigma of all species germinates and pollen tubes penetrate the ovary within 8–20 h whether the pollen comes from the same flower or inflorescence, or from different morphs. The nature of self-compatibility appears to be similar in the two monomorphic species studied (N. levynsiae, N. stokoei) and in the five distylous species. However, in N. capitata a few abortive pollen tubes (±5%) were found in the styles of two long-styled morphs that had been selfed and in one short-styled flower pollinated with pollen from a long-styled plant. Four out of five distylous species have pollen size but not exine dimorphism. The flowers of Nivenia species are unscented, tubular, and nectariferous, encouraging a “trapline” pollination syndrome. The major pollinators appear to be large-bodied, long-tongued bees of the family Anthophoridae or long-tongued flies of the family Nemastrinidae; the latter evidently are the exclusive pollinators of N. binata and N. stenosiphon. Both insects insert their tongues down the long floral tubes and collect nectar while hovering. The exserted anthers of flowers of the monomorphic Nivenia species and the short-styled flowers deposit pollen on the dorsal and ventral surfaces of the insects' thorax and abdomen. Pollen deposition from anthers of the long-styled flowers appears to be confined to the insects' head. Differential deposition of pollen from the two floral morphs thus facilitates intermorph pollinations. Analysis of nectar sugars indicates high sucrose:hexose ratios and relatively high sugar concentrations, consistent with insect pollination. The combination of self-compatibility, adaptive floral morphology, and ratio of floral morphs within populations does not suggest that Nivenia is a model of either conventional (Primula-type) distyly or andromonoecy. We suggest that the genus is derived from self-incompatible ancestors, perhaps now extinct, but the self-incompatibility response has declined, while dimorphic flowers are retained in five of the nine extant species. We presume that self-compatibility and the loss of distyly reflect a trend in the adaptive radiation of the genus towards harsher, xeric environments and a sublithophytic habitat.