COS 98-6
Population size and inbreeding depression in a native biennial: Unexpected patterns
Inbreeding depression (ID) is thought to influence population viability, and accordingly has been a major concern for conservation biologists. This concern is resurfacing as recent meta-analyses demonstrate increased selfing rates and reduced fitness in small, fragmented plant populations compared to large ones. However, the magnitude of ID may also vary with population size. ID may be positively correlated with population size if small populations have purged their inbreeding load or if stronger drift in small populations leads to fixation of deleterious alleles (‘drift load’, DL). Alternatively, ID might not vary with population size if sizes fluctuate or have only recently become reduced. Our previous population genetic work in a native biennial, Sabatia angularis (Gentianaceae), suggested a third unexpected possibility: a negative correlation between ID and population size. These possibilities and their underlying mechanisms have very different implications for population viability. Here, we use an experimental approach to quantify ID and DL for six natural S. angularis populations and to evaluate whether these vary with population size. We performed experimental pollinations (self, within-population, and between-population) on maternal individuals in situ and examined performance of offspring in a common garden to estimate ID and DL for components of and cumulative lifetime fitness.
Results/Conclusions
Inbreeding depression (ID) was significant and high for cumulative lifetimes fitness across the six natural populations studied; on average, individuals resulting from selfing events experienced a 72% (± 0.32 SD) reduction in fitness compared to those resulting from within-population crosses. We found a strong, significant negative relationship between ID and populations size across the range of population sizes examined, consistent with our previous results based on population genetics. The strong lifetime ID in all populations was attributable to significant ID found across all life stages, including germination, survival, and reproduction. The magnitude of ID and the stages at which it manifests suggests that inbreeding has the potential to affect the demography and population viability of S. angularis. In addition, comparison of lifetime fitness of individuals resulting from within- vs. between-population crosses revealed significant drift load. We discuss possible explanations for the surprising relationship between ID and population size, including the role of seed banks. While seed banks are classically seen as a source of genetic rescue in plant populations, introducing novel genetic variation when populations are small, this same mechanism can reintroduce genetic load and so maintain high levels of ID in small populations under certain conditions.