Aspects of sexual failure in the reproductive processes of a rare bulbiliferous plant, Titanotrichum oldhamii (Gesneriaceae), in subtropical Asia

Titanotrichum oldhamii produces both flowers and asexual bulbils on its inflorescences. However, field observations and herbarium collections indicate that seed set is infrequent and that most reproduction is from vegetative bulbils. We have investigated the failure of sexual reproduction and identified four major causes: (1) in the wild, the seed:ovule ratio for open pollination was only 1.9%, in contrast to 10.1% for artificial cross-pollination, implying poor pollinator services. (2) The overall reproductive success was 5–10 times greater in glasshouse pollination treatments compared to field treatments. This suggests suboptimal environmental conditions for seed set in natural habitats, which may eliminate natural seed set completely. (3) Pollination experiments showed that outcrossed populations set significantly more seed and had higher germination rates than intra-populational and self crosses, in both field and glasshouse experiments. T. oldhamii thus appears to benefit from wide outcrossing. Pollen transfer between populations, however, seems to be infrequent because of the rarity and scattered distribution of Titanotrichum populations. (4) Flowers near the apex of the inflorescence are less likely to set seed, especially late in the season when inflorescences convert to bulbil production. In these late flowers, pollen tubes showed poor guidance as they approach the micropyle of the ovules. Even under optimal glasshouse conditions, the average outcrossing success was only 0.229. Almost one-half of the ovules remained undeveloped and 13.5% of ovules aborted after pollination, indicating a strong shift of resource allocation toward vegetative bulbils and rhizome development. Efficient reproduction from asexual bulbils may thus have released Titanotrichum from strong selection for efficient sexual reproduction. However, occasional seed set observed in the wild may be very important for maintaining some genetic diversity in populations, and promoting overall fitness.