Format: ORAL

Anita Cisternas-Fuentes1,2; Matthew Koski 2.

1. Departamento de Botánica, Facultad de Ciencias Naturales y Oceanográficas, Universidad de Concepción, Chile. 2. Department of Biological Sciences, Clemson University, Clemson, SC, USA

Achieving reproductive success in self-incompatible (SI) populations can be difficult. Flowers must receive enough pollen to produce seeds, and that pollen must be genetically compatible. The lack of available mates can reduce population-level reproductive fitness, have negative impacts on population growth, and increase extinction risk. The effects of low mate availability are exacerbated in small populations, which often harbor fewer S-alleles. So, even with substantial pollination, reproduction may fail. The breakdown of SI systems, however, may rescue population declines. Evaluating mate availability across populations can help us to understand the factors and conditions leading to mate limitation. We tested the predictions that populations with small effective population size have reduced mate availability and exhibit signatures of a breakdown in their SI mechanism. Additionally, we evaluated mechanisms at the pollen-pistil interface that contribute to the failure of outcrossed flowers and success of self-pollinated flowers. We performed self and outcross pollinations in 13 populations of Argentina anserina with known genomic estimates of effective population size to estimate mate availability and the SI index, and used fluorescence microscopy to enumerate pollen deposition, germination, and tube extension. Results show that genetically depauperate populations have reduced mate availability but not a lower SI index. Unsuccessful outcrosses did not receive fewer pollen grains or have lower pollen germination than successful ones. Barriers to outcross fertilization occurred in the style, specifically pollen tubes were arrested at the S-allele recognition site. Pistils from self-crosses showed that a higher proportion of self-pollen grains extend tubes past the location of the SI mechanism in the style in effectively smaller populations. However, this does not necessarily result in seed production. Results highlight strong mate limitation in genetically small populations of A. anserina and suggest that leaky-SI in small populations may represent early stages of a breakdown in SI.