Project
Viability of Fern Edge Populations as Founding Source Populations Under Global Climate ShiftsNorthern range-edge populations are important for species persistence as they are potential sources for range expansion under climate warming. These populations are often small and more isolated compared to those at the centre of the species range, which can result in reduced gene flow, higher inbreeding, and stronger drift. However, it remains unclear whether these edge populations maintain inbreeding load through relaxed selection and increased drift or if prolonged inbreeding enhances purifying selection, purging deleterious mutations over time. We quantify inbreeding load in five northern range-edge and five core populations of the fern Osmundastrum cinnamomeum. Analysis of germinating spores and the haploid gametophyte stage revealed 19% higher mortality, 33% more developmental abnormalities and increased frequencies of segregating recessive lethals in edge populations, indicating elevated expression of deleterious mutations. Fitness assays measuring the presence and number of diploid sporophyte offspring produced through selfing and cross-fertilization of gametophytes were used to calculate inbreeding depression values (δ). Edge populations exhibited higher inbreeding depression compared to core populations. Overall, this suggests that these populations at the northern range-edge have not undergone purging of deleterious mutations, allowing inbreeding load to persist. Our findings reveal that isolation, drift, and inbreeding reduce edge population fitness, raising concerns about their ability to effectively contribute to range expansion under climate warming.
