Pollination Efficiencies of Insects Visiting Magnolia obovata, as Determined by Single-Pollen Genotyping

Abstract

The genetic composition of pollen grains that are transported to flowers affects the reproductive success and fitness of the plants and the genetic structure of the plant population. For example, for plants that suffer inbreeding depression, the pollen from another conspecific individual is essential for effective reproduction. Many researchers have focused on the pollination processes in various plants and have sought to understand relationships between plants and pollinators. Here, we show the latest approach, which evaluates the contribution of flower-visiting insects by direct genotyping of pollen grains. The genotypes of pollen grains adhering to flower beetles, small beetles, and bumblebees that visited flowers of Magnolia obovata were determined directly. The genetic traits of transported pollen differed by insect type. Most of the pollen adhering to small beetles and bumblebees was self-pollen (pollen transported to a different region on the same tree). On the other hand, an average of 70% of pollen grains adhering to flower beetles was transported from other reproductive trees. We also described the patterns of pollen movements by performing a paternity analysis on pollen grains using trees in our study populations as candidate male parents. Although most of the pollen that adhered to small beetles and bumblebees was moved within a short range of distance, pollen grains that adhered to flower beetles tended to travel longer distances. Our results showed that small beetles and bumblebees rarely move between plants, cause geitonogamous pollen flow, and may negatively affect reproduction in M. obovata, which undergoes substantial inbreeding depression in the early life stages. In contrast, flower beetles contribute to the outcrossing of M. obovata, transporting genetically diverse outcross pollen. Our evaluation is consistent with the traditional idea that the flowers of Magnoliaceae have features of a beetle pollination syndrome. The direct genotyping of pollen grains provides powerful evidence for the traditional idea of pollination syndrome and offers new insights, shedding light on the mutualism and coevolution of plants and flower visitors.