Herkogamy, a Principal Functional Trait of Plant Reproductive Biology

Abstract

Premise of the research: Phenotypic traits that consistently mediate species' responses to environmental variation ('functional traits') provide a promising approach towards generalizing ecological and evolutionary patterns, and thereby gaining insights into the processes generating them. In the plant functional ecology literature, most trait-based studies have focused on traits mediating either resource competition or responses to variation in the abiotic environment, while traits mediating reproductive interactions have often been neglected. Methodology: Here, I discuss the value of herkogamy (spatial separation of male and female functions in flowers) as a functional trait in plant reproductive biology and review the evidence relevant to the hypothesis that taxa exhibiting greater herkogamy have historically experienced more reliable pollination, and more outcrossed mating systems. Pivotal results: A large body of work in the field of plant reproductive biology has identified a set of nearly ubiquitous correlations between average herkogamy and features of plant mating systems, notably autofertility (seed set in the absence of pollinators), and outcrossing rate. Herkogamy often varies extensively among populations and species, and the adaptive interpretation is that herkogamy exhibits local adaptation to the reliability of the pollination environment. Conclusions: These results underline the value of herkogamy as a functional trait representing variation in mating histories. Many important insights are likely to emerge from studies leveraging herkogamy as an easily measured proxy of plant mating systems, as already demonstrated in comparative studies, and in studies of reproductive interactions. Going forward, greater consideration of herkogamy and other reproductive-function traits in studies

No evidence that seed predators constrain pollinator‐mediated trait evolution in a tropical vine

“…To quantify realized pollination reliability, we recorded allogamous pollen loads on the stigmas of female‐phase blossoms ( n = 14–101 blossoms, mean = 43.2, median = 36), when autogamous selfing is not yet possible. We measured upper bract area and anther‐stigma distance (herkogamy), a key floral trait mediating mating‐system variation among populations and species (Opedal et al., ; Opedal, ), on randomly selected blossoms in early bisexual condition ( n = 4–55 blossoms, mean = 23.9, median = 24). Trait differences among populations are largely genetically determined, as demonstrated by the correlation of phenotypic traits among populations measured in the wild and under common‐environment greenhouse conditions (Opedal et al., ).…”

“…In our study system, female‐phase stigmatic pollen loads is a strong predictor of current outcrossing rate (Opedal et al., ). However, because the present analysis is at the level of evolved relationships among populations, we used population‐mean herkogamy as a proxy of the long‐term mating system of each population, assuming that this would average out annual fluctuations in outcrossing rates (Opedal, ). This approach is justified by the observation that, across the study populations, herkogamy is positively correlated with pollination reliability (stigmatic pollen loads), bee abundance on perfume baits, outcrossing rate, and allelic diversity at microsatellite loci (Opedal et al., ).…”

Severe inbreeding depression is predicted by the “rare allele load” inMimulus guttatus*

“…Plants have evolved complex mechanisms to prevent self‐fertilization, such as molecular self‐incompatibility (Takayama and Isogai ), herkogamy including flexistyly and heterostyly (Ganders ; Li et al. ; Opedal ), and dichogamy (Bertin and Newman ). Classical theory predicts that a population should maintain outcrossing if inbreeding depression (ID) is strong enough, specifically that δ > 0.5 where δ equals one minus the fitness of selfed relative to outcrossed progeny (Kimura ; Lande and Schemske ; Charlesworth and Charlesworth ).…”

Geographic variation of reproductive traits and competition for pollinators in a bird‐pollinated plant

“…Insufficient pollinator service can reduce the quantity of seeds produced and may result in selection on floral traits. For example, low visitation can select for reproductive assurance to maintain maternal fertility resulting in reduced flower size and herkogamy (Barrett & Harder, ; Eckert et al, ; Lloyd, ; Opedal, ; Sicard & Lenhard, ). Although we found no evidence for a relation between pollinator visitation rate and seed production in our study, this is probably because of the capacity for autonomous seed production in B. ringens (de Waal, Anderson, et al, , see Figure ), which would have maintained some degree of maternal fertility despite limited sunbird visitation.…”