Abnormalities in sexual development and pollinator limitation in Michelia coriacea (Magnoliaceae), a critically endangered endemic to Southeast Yunnan, China

Michelia coriacea (Magnoliacae) is a critically endangered tree, endemic to Southeast Yunnan province, China. Most of the individuals in the extant populations normally bear flowers, but fruit set and fertile seed production rates were only 6.7% and 0.2%, respectively. To identify possible causes of reproductive barriers, the following studies were carried out: examination of stamen and pistil development; experiments using controlled pollination; observations of behavior and frequency of floral visitors in both cultivated and natural populations. The results revealed that about 60% of pollen was abnormal and approx. 70% of ovules had delayed development. Hand pollination could effectively enhance the fruit set (F4,25=35.139, P<0.0001) and seed set per fruit (F4,25=85.022, P<0.0001). Both cultivated and wild M. coriacea had an extremely low frequency of floral visitors. Some beetles, a few species of Andrenidae and some Bombus sp. are likely to be the effective pollinators. The fruit set and seed set per fruit from controlled self-pollination and cross-pollination were significantly different (P<0.05), and thus it is inferred, that inbreeding depression may be a contributing factor in the very low seed production. It appears that low seed set in M. coriacea is due to a combination of factors: abnormalities in pollen and ovules, low number of effective pollinators, and inbreeding depression.