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Exploration of molecular methods for detecting individual inbreeding levels in mimulus Leblanc, Marissa Erin

Abstract

Inbreeding directly affects fitness and population dynamics by way of inbreeding depression and is particularly prevalent in plants and in small populations. While the importance of measuring fitness and inbreeding levels in natural populations is clear, it is difficult to do in practice. As an alternative, molecular methods have been developed for estimating individual inbreeding levels. These methods may also offer insight into fitness because in the presence of inbreeding depression, inbreeding level will negatively correlate with fitness. While some molecular methods for estimating inbreeding levels apply to any codominant marker, others have been specifically constructed for microsatellites, a type of genetic marker that roughly follows the stepwise mutation model (Ohta and Kimura, 1973). This model has provided the foundation for microsatellite-specific measures. Unlike conventional measures of inbreeding levels (e.g individual heterozygosity), microsatellite-specific measures possibly detect historical levels of inbreeding. The purpose of this thesis is to explore the correlation between various inbreeding measures and fitness. A literature review of empirical and theoretical treatments of the first microsatellite-specific measure, d² (Coulson et al., 1998), revealed that d² was sometimes, but not always, more correlated with fitness than was individual heterozygosity. The pattern in the literature suggests that populations may differ in their extent of historical inbreeding. The literature review also indicated that exploration of d² in closely related taxa of different selfing rates was still warranted. Furthermore, the analytical modeling of Tsitrone etal. (2001) provided the inspiration for the development of new microsatellite-specific measures that distinguish between small repeat differences but pool larger repeat differences together. An empirical study of microsatellite-specific measures for detecting individual inbreeding levels was carried out in four taxa with known differences in selfing rates in the Mimulus guttatus species complex. Genotype-fitness correlations are driven by inbreeding levels, extent of inbreeding depression, and linkage disequilibrium, all of which are influenced by selfing rate. It was hypothesized that different measures might best correlate with fitness in inbreeding taxa than in outbreeders. However no genotype-fitness correlations were observed for any measure or in any of the species. This study shows that even in the presence of within-population variation in inbreeding levels, genotype-fitness correlations may not be detectable. Although inbreeding issues are most pressing in small populations, molecular methods for detecting inbreeding level do not perform well under these circumstances (Ritland, 1996; Tsitrone et al., 2001).

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