Hermaphroditism promotes mate diversity in flowering plants

Abstract

Premise: Genetically diverse sibships are thought to increase parental fitness through a reduction in the intensity of sib competition, and through increased opportunities for seedling establishment in spatially or temporally heterogeneous environments. Nearly all research on mate diversity in flowering plants has focused on the number of fathers siring seeds within a fruit or on a maternal plant. Yet as hermaphrodites, plants can also accrue mate diversity by siring offspring on several pollen recipients in a population. Here we explore whether mate composition overlaps between the dual sex functions, and discuss the implications for plant reproductive success.

Methods: We established an experimental population of 49 Mimulus ringens (monkeyflower) plants, each trimmed to a single flower. Following pollination by wild bees, we quantified mate composition for each flower through both paternal and maternal function. Parentage was successfully assigned to 240 progeny, 98% of the sampled seeds.

Results: Comparison of mate composition between male and female function revealed high mate diversity, with almost no outcross mates shared between the two sexual functions of the same flower.

Conclusions: Dual sex roles contribute to a near doubling of mate diversity in our experimental population of Mimulus ringens. This finding may help explain the maintenance of hermaphroditism under conditions that would otherwise favor the evolution of separate sexes.

Keywords: hermaphrodite; male fitness; mate diversity; mating network; mating portfolio; multiple paternity; paternity; pollination; selfing; sexual system.

Figure 1

Mating portfolios for 10 focal plants with random positions in an experimental population of 49 plants. Note that all 49 individuals are included as potential pollen recipients (left panel) or pollen donors (right panel). The links between them show that the father on the left sired seeds on the individual labeled on the right. The thickness of the link is scaled to the number of seeds sired. The height of the colored bars (left) for each individual is scaled to show the proportion of that individual's contribution to the sum of the number of seeds sired for the 10 individuals. The “seeds mothered” (right panel) depicts the maternal individual (left) and the individuals who sired seeds on the maternal plant are linked on the right. The thickness of the link is scaled to the number of seeds from one fruit that were sired by the paternal individual. The height of the colored bar (left) for each individual is scaled to the proportion of the total number of seeds that were mothered by each individual.

Figure 2

Detailed mating portfolios for 3 of the 10 focal plants shown in Figure 1. The thickness of the link is scaled to the number of seeds sired (left panel) or mothered (right panel). (A) The matings for plant 20 from the male (left panel) and female (right panel) perspectives; (B) the matings for plant 30; and (C) the matings for plant 38.