Interspecific variation in pollen-ovule ratio is negatively correlated with pollen transfer efficiency in a natural community.

ABSTRACT

The pollination efficiency hypothesis has long been proposed as an explanation for interspecific variation in pollen-ovule (PO) ratios. However, no empirical study on PO ratios has directly and quantitatively measured pollen transfer efficiency (PE). Here, we use a PE index, defined as the proportion of pollen grains removed from anthers that are subsequently deposited on conspecific stigmas, as a direct and quantitative measure of PE. We investigated PO ratios, pollen removal and pollen deposition in 26 plant species in an alpine meadow, over three consecutive years. Our community survey showed that nearly 5% of removed pollen was successfully deposited on conspecific stigmas. The PE index ranged from 0.01% up to 78.56% among species, and correlated negatively with the PO ratio across years. This correlation was not changed by controlling for phylogenetic relationships among species, suggesting that the interspecific variation in PO ratios can be attributed to the probability of pollen grains reaching a stigma. The results indicate that the pollination efficiency hypothesis can help to explain interspecific variation in PO ratios.