The distribution of individual inbreeding coefficients and pairwise relatedness in a population of Mimulus guttatus

Abstract

In order to infer population structure at the individual level, we estimated individual inbreeding coe cients and examined the relationship between geographical distance and genetic relatedness from polymorphic microsatellite data for a population of Mimulus guttatus that has an intermediate sel®ng rate. Expected heterozygosities for ®ve microsatellites ranged from 0.79 to 0.93. The population inbreeding coe cient was calculated to be 0.19 (SE 0.023). A method-of-moments estimator developed by Ritland (1996b) was used to estimate the distribution of inbreeding among and relatedness between individuals of a natural population. The mean individual inbreeding coe cient (F 0.16) did not di er signi®cantly from the population-level estimate. Most of the individuals appeared to be outbred, and there were very few plants that had estimated inbreeding coe cients greater than one-half. Individuals sampled from one transect showed signi®cantly more inbreeding than individuals sampled along the other (P 0.005). There was no apparent relationship between interplant distance (range: 0±14 m) and mean genetic relatedness between individuals. These results represent the ®rst application of polymorphic microsatellites to estimate ®ne-scale genetic population structure.Keywords: inbreeding, isolation by distance, microsatellites, Mimulus, population structure, selffertilization. IntroductionThe genetic structure of a population can have profound e ects on its evolutionary potential. Through the use of polymorphic genetic markers such as allozymes, there is now a large number of studies that describe population structure in terms such as the average level of inbreeding within a population or the average genetic di erentiation between populations or subpopulations (Hamrick & Godt, 1990). In contrast, very few studies use markers to infer population structure at the level of individual organisms. This is unfortunate, because knowledge of the extent to which individuals di er in their inbreeding histories and the degree of genetic relatedness between pairs of individuals is important for many areas of ecology and evolution.For example, if individuals vary in their inbreeding histories, then we expect statistical associations to exist between diploid genotypes at di erent loci (Haldane, 1949; Kimura, 1958). Knowing the variance in individual inbreeding coe cients, and therefore the relative magnitude of these associations (referred to as identity disequilibrium), is important for the interpretation of studies of natural selection at individual loci (Ohta & Cockerham, 1974; Charlesworth, 1990; Houle, 1994) and on quantitative traits . In addition, the existence of individual variation in inbreeding coe cient implies that traditional quantitative genetic methods for predicting the evolutionary response to selection in outbred (Lande & Arnold, 1983) or fully inbred populations (Mather & Jinks, 1982) are not appropriate. Instead, one can use the distribution of inbreeding histories in a partially inbred population, in conjunction...

Inbreeding and the genetic variance in floral traits of Mimulus guttatus

“…Of course, the evolutionary relevance of C AD and V DI depends not only on their magnitudes, but also on the level of inbreeding in the population. M. guttatus is a mixed mating species and the estimated selfing rate varies from 0 to 0.75 among populations (Ritland and Ganders, 1987;Willis, 1993;Awadalla and Ritland, 1997;Sweigart et al, 1999). As a consequence, we expect the nonadditive components to impact substantially evolutionary change in some populations but make only a minor contribution in others.…”

Pleiotropic Quantitative Trait Loci Contribute to Population Divergence in Traits Associated With Life-History Variation in Mimulus guttatus

“…The well-studied IM population consists of small-flowered, diminutive annuals that live on Iron Mountain, in Oregon's western Cascades (Willis 1993). These plants are predominantly outcrossing (Willis 1993;Sweigart et al 1999) and have a short period of growth and reproduction, with germination occurring in either the fall or spring, flowering occurring over a 3-to 5-week period in June through early July. All plants at this site die by mid-July.…”

Mating System and the Evolution of Quantitative Traits: An Experimental Study of Mimulus Guttatus

“…The species is mixed mating, the estimated selfing rate varies from 0 to 0.75 among populations, and it may be annual, winter annual, or perennial (Ritland and Ganders 1987;Willis 1993;Awadalla and Ritland 1997;Sweigart et al 1999). Previous genetic studies have demonstrated substantial genetic variation in floral traits (Carr and Fenster 1994;Robertson et al 1994) and inbreeding depression in fitness components (Dole and Ritland 1993;Willis 1993;Latta and Ritland 1994;Willis 1999a,b;Carr and Dudash 1996;Dudash and Carr 1998).…”