No 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...

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. abstract: Genetic correlations are the most commonly studied of all potential constraints on adaptive evolution. We present a comprehensive test of constraints caused by genetic correlation, comparing empirical results to predictions from theory. The additive genetic correlation between the filament and the corolla tube in wild radish flowers is very high in magnitude, is estimated with good precision ( ), and is caused by pleiotropy. Thus, evolu-0.85 ‫ע‬ 0.06 tionary changes in the relative lengths of these two traits should be constrained. Still, artificial selection produced rapid evolution of these traits in opposite directions, so that in one replicate relative to controls, the difference between them increased by six standard deviations in only nine generations. This would result in a 54% increase in relative fitness on the basis of a previous estimate of natural selection in this population, and it would produce the phenotypes found in the most extreme species in the family Brassicaceae in less than 100 generations. These responses were within theoretical expectations and were much slower than if the genetic correlation was zero; thus, there was evidence for constraint. These results, coupled with comparable results from other species, show that evolution can be rapid despite the constraints caused by genetic correlations.

Progeny produced by inbreeding were compared to progeny derived from outcrosses for gynodioecious Schiedea salicaria and subdioecious S. globosa to assess fitness consequences of breeding system on parental fecundity (seeds per capsule) and progeny measures of fitness (germination, survival, biomass, and number of flowers). Results from both species indicated that inbreeding depression occurred at all measured stages of the life history. In both species, different females showed different levels of inbreeding depression. Multiplicative fitness functions of the ratio of values for selfed and outcrossed progeny in S. salicaria resulted in inbreeding depression values of 0.62–0.94. Within‐ vs. between‐family crosses of S. globosa also resulted in inbreeding depression values as high as 0.49. These values suggest that inbreeding depression may promote the evolution of dioecy within S. globosa and S. salicaria, depending on the levels of natural outcrossing.

The importance of pollinator visitation for determining both maternal reproductive success and outcrossing rates was investigated in the facultatively autogamous annual, Lupinus nanus. Sixty plants in each of two adjacent sites were assigned to either a pollinator‐exclusion, pollen‐augmentation, or open‐pollinated (control) treatment. Flower, fruit, and seed production were recorded for each plant, and outcrossing rates were determined for plants in all three treatments at each site. Pollinator‐exclusion reduced the reproductive success of plants at Site 1, but had no effect on reproduction for plants at Site 2. Pollen‐augmentation increased fruit production by 22% and seed production by 45% at Site 2, but had no effect on reproduction for plants at Site 1. Plants in the open‐pollinated, control treatment outcrossed at intermediate rates at both sites, with plants at Site 2 (outcrossing rate, t = 0.66) outcrossing at a significantly higher rate than plants at Site 1 (t = 0.40). The pollen‐augmentation treatment increased the outcrossing rate for plants at both sites (Site 1, t = 0.72; Site 2, t = 0.78). These results indicate that pollinator availability can limit maternal reproductive success for this facultatively autogamous plant species. Additionally, the intermediate outcrossing rates observed in nature for this species are not a consequence of plant characters alone, but instead result from plant characters combined with the local pollination ecology.

Our work provides convincing evidence that intermediate anther exsertion values are adaptive, and that this is probably an adaptation to a subset of the pollinator fauna, small bees. The picture for anther height dimorphism is much less clear, as the weight of current evidence suggests that current values of this trait might actually be maladaptive; however, if this is true it is difficult to understand how the dimorphism is maintained across the family Brassicaceae.

To determine how the capability of pollen is affected by lengthy exposure to field conditions, we obtained pollen samples (from Erythronium grandiflorumi that were fresh (from newly dehisced anthers) or aged, from 6 to 24 hr. Without pretreating the collected grains, we compared their abilities I) to retain viability, as indicated by the Heslop-Harrison fluorochromatic reaction (FCR) test; 2) to retain germinability, as indicated by pollen-tube growth in vitro on Brewbaker-Kwack medium; 3) to sire seeds when used as a pollen donor on recipient flowers in the field; and 4) to sire seeds competitively when mixed with another donor pollen in competitive pollinations. FCR scores declined drastically with pollen age. Germinability typically declined with age, but less drastically; the correlation between FCR and germinability ranged from strong to nil in different trials. The sharp decline in FCR was probably due to nonlethal, reversible desiccation. Seed-siring success, both in competitive and noncompetitive trials, bore no relation to pollen age or to germinability. Any loss in seed-siring ability in E. grandiflorum pollen over 24 hr is apparently overwhelmed by other sources of variation. The lack of correlation between seed-siring ability and FCR or germinability scores means that those tests should not be used to estimate overall pollen competence in this species. They should be so used in other species only if experiments have demonstrated their greater reliability.

The effect of cattle grazing on the genetic structure of native grass populations has received little attention. We investigated the effect of cattle grazing on genetic variation in Idaho fescue (Festuca idahoensis Elmer) using ISSR (inter-simple sequence repeat) DNA markers. The ISSR markers are hypervariable and are generally interpreted as being selectively neutral. Idaho fescue tillers were sampled from inside (N = 31) and outside (N = 34) a 64-year-old cattle exclosure in southeastern Oregon. We extracted DNA and used 2 ISSR primers to determine the genotypes for grazed and ungrazed plants at 60 variable loci. No statistically significant differences were observed between grazed and ungrazed samples for percent polymorphic loci (grazed = 85 %; ungrazed = 80 %), mean expected heterozygosity (grazed = 0.1393; ungrazed = 0.1365), or for a measure of loci dissimilarity