Pollination and Reproductive Ecology of Three Lowland Tropical Pitcher Plants (Nepenthes) and Their Hybrids

Christopher K. Frazier

Biology Department • University of New Mexico • Albuquerque, NM 87131 • USA

Tropical pitcher plants have been the subject of substantial scientific scrutiny for their prey digestion (Frazier, 2000), but other aspects of their biology, particularly their reproductive ecology have garnered much less attention. Species in the genus Nepenthes are interfertile under greenhouse conditions, yet in the field demonstrate a high degree of sympatry. This suggests that differences in reproductive strategies may play an important role in reducing hybridization and maintaining species distinction under natural conditions. I studied the three species of Nepenthes present in Johore, Malaysia and Singapore, N. gracilis, N. ampullaria and N. rafflesiana, to determine if there are significant differences among species in phenology, floral and nectar characteristics and pollinators. I also addressed the fitness of naturally occuring hybrids. Differences in flowering period represents at best a partial barrier to cross-pollination among species. Each species has three flowering periods between January and July, but the timing of these episodes is offset among species such that at any given time one, two or all three species are in flower. Floral presentation differs among species in such characters as the number of flowers per inflorescence, flower size and color and sepal shape (Frazier, in press). N. ampullaria flowers present pollen during the day, while the other two species open their flowers in the evening. Nectar is produced nocturnally in all three species, but they differ in sugar composition and quantity of nectar produced. These differences suggest the species are adapted to attract different suites of pollinators. All species are pollinated by moths and small Diptera, but to differing degrees. N. rafflesiana is pollinated almost entirely by moths and all of its pollination is nocturnal. N. gracilis is pollinated almost equally by moths and small Diptera at night and also by wasps during the night and day. By day, N. ampullaria flowers have a foul odour and are pollinated by large flies, wasps and butterflies. The flowers produce a sweet odour at night and are then visited by moths. These differences are significant between species, but still not sufficient to constitute a complete barrier to cross-pollination.

Low hybrid fitness may also act as a barrier to gene flow between the parent species. I found that hybrids in the wild abort significantly more flowers, produce fewer seeds per inflorescence and have a lower efficiency of seed production (seeds produced per gram of inflorescence) than the parent taxa. Nevertheless, some hybrid inflorescences demonstrated zero flower abortion and produced seed quantities on par with the parent taxa, suggesting that ecological phenomena and not intrinsic sterility barriers are the main factors limiting hybrid fitness.

My results show that many factors interact to limit, but not preclude hybrid production and that lower hybrid fitness comprises a further, though incomplete barrier to gene flow. I suggest that the combination of such partial barriers allows sympatric species to retain species distinction without intrinsic sterility barriers, although some degree of interspecific gene flow is still likely.