Reduced fitness at early life stages in peripheral versus core populations of Swiss stone pine (Pinus cembra) is not reflected by levels of inbreeding in seed families

Abstract

Small and fragmented populations are prone to mating among related individuals, increasing homozygosity and likely negatively affecting offspring fitness. Such a trend may be enforced by environmental changes in species with narrow ecological niches because inbred populations are more prone to become maladapted as compared to outcrossed populations. Here, we studied differences in offspring fitness and inbreeding levels between core and peripheral populations of Swiss stone pine (Pinus cembra). We collected open-pollinated progenies of P. cembra in large, contiguous and in small, isolated populations (core vs. periphery). Seeds were germinated and grown in a common garden to test for differences in fitness parameters, whereas subsamples of seed lots were genotyped at seven nuclear microsatellites to calculate selfing rates. We found significantly lower seed production, higher embryo abortion rates and lower germination success in small peripheral compared to large core populations of P. cembra. In contrast, winter survival and first-year growth of seedlings did not significantly differ between the two population types. These results indicate higher inbreeding depression at the earliest life stages in small and fragmented populations compared to populations from the contiguous range of P. cembra. However, we found no correlation between any fitness parameters and progeny-derived selfing rates. We explain this discrepancy by the fact that inbreeding depression mostly affects embryo abortion rates in Pinaceae. This cannot be genetically quantified because non-developed embryos cannot be genotyped. We infer that population fragmentation in the long term negatively affects natural regeneration in this long-lived, wind-pollinated conifer species.