The role of phenotypic plasticity and pollination environment in the cleistogamous, mixed mating breeding system of Triodanis perfoliata

The role of variable pollination environments in maintaining mixed mating systems is an active area of research. Dimorphic cleistogamy, in which a plant reproduces by both open, facultative outcrossing chasmogamous (CH) flowers and closed, cleistogamous (CL) flowers presents an excellent opportunity to study mixed mating. For example, plastic responses in allocation to an optimal floral type could serve as an adaptive strategy that maintains mixed mating under variable pollination environments. We tested for pollen limitation and plastic responses in allocation to different floral types under manipulated pollination conditions in the dimorphic cleistogamous, mixed mating annual, Triodanis perfoliata. Using a field population, we quantified pollen limitation, auto-fertility and plastic responses in the breeding system by measuring allocation to flower number and seed set of floral types. We found no evidence for pollen limitation for CH flowers, and CH flowers had low efficacy of autonomous selfing. Importantly, we found that T. perfoliata alters floral number following changes in pollination conditions, with pollen-supplemented plants having lower relative CH flower number than non-supplemented plants. Breeding system plasticity may allow for benefits from outcrossing through CH flowers, but also increased overall fitness through relatively cheap CL reproduction. After CH flowers receive pollen, subsequent production of CH flowers was reduced, which may be due to resource limitation. Our findings did not support a theoretical model predicting increased CH flowers with high pollination levels. These results increase our understanding of the role of pollination services and resource allocation in the maintenance of mixed mating systems, which also warrants further investigation.

Small and surrounded: population size and land use intensity interact to determine reliance on autonomous selfing in a monocarpic plant

BACKGROUND AND AIMS

Habitat fragmentation has transformed landscapes globally, leaving remnants embedded within a complex matrix that is rapidly becoming more developed. For many plant populations, the associated factors of decreased size and intensification of land use surrounding them are expected to increase pollen limitation ('PL'), unless autonomous self-pollination provides reproductive assurance ('RA'). Decreased pollinator visitation is often assumed to drive these patterns, but other, less studied mechanisms might include increased heterospecific pollen transfer or decreased conspecific pollen availability via florivory. I investigate how PL and RA and their potential underlying mechanisms vary with population size and land use intensity surrounding populations in the biennial Sabatia angularis (Gentianaceae).

METHODS

I estimated the capacity for seed production via autonomous self-pollination (i.e. autofertility). Over 2 years in 22 S. angularis populations across a fragmented landscape, I performed emasculation and pollen supplementation experiments measuring RA and PL, and quantified visitation rates of potential pollinators and a pollen consumer, conspecific pollen loads and rates of heterospecific pollen deposition.

KEY RESULTS

Autofertility based on fruit mass was 93 % under PL but only 51.6 % relative to maximal conditions. PL and RA were significant on average across populations in the first year of study. Variation in RA was significantly influenced by the interaction between population size and land use intensity, which in turn rendered PL independent of these factors. Visitation and heterospecific pollen deposition rates were greatest in small populations and declined with population size, while conspecific pollen loads were greatest in intermediate sized populations.

CONCLUSIONS

Increased reliance on RA is predicted in small S. angularis populations surrounded by intense development, which can explain elevated selfing rates in fragmented populations of plant species more generally. Results from this study point toward forces such as heterospecific pollen transfer, self-pollen limitation or resource availability influencing the need and ability to rely on RA.

Distinct effects of pollinator dependence and self-incompatibility on pollen limitation in South African biodiversity hotspots

Global synthesis indicates that limitation of plant fecundity by pollen receipt (pollen limitation) is positively related to regional plant diversity and is higher for self-incompatible than self-compatible species. While self-incompatible species are always dependent on pollinating agents, self-compatible species may be pollinator-dependent or autofertile. This should cause variation in pollen limitation among self-compatible species, with lower pollen limitation in autofertile species because they do not depend on pollinators. We hypothesized that the intensity of pollen limitation in self-incompatible compared with pollinator-dependent self-compatible species should depend on whether pollen limitation is determined more by quantity than quality of pollen received. We compared pollen limitation between these three groups using a dataset of 70 biotically pollinated species from biodiverse regions of South Africa. Comparison with a global dataset indicated that pollen limitation in the South African biodiversity hotspots was generally comparable to other regions, despite expectations of higher pollen limitation based on the global plant diversity-pollen limitation relationship. Pollen limitation was lowest for autofertile species, as expected. It was also higher for pollinator-dependent self-compatible species than self-incompatible species, consistent with increased pollen-quality limitation in the former group due to negative consequences of pollinator-mediated self-pollination. However, there was a higher frequency of plants with zygomorphic flowers, which were also more pollen-limited, among pollinator-dependent self-compatible species. Thus, we could not attribute this difference in pollen limitation exclusively to a difference in pollen quality. Nevertheless, our results indicate that comparative studies should control for both pollinator dependence and self-incompatiblity when evaluating effects of other factors on pollen limitation.