We have constructed a genetic map of Arabidopsis lyrata, a self-incompatible relative of the plant model species A. thaliana. A. lyrata is a diploid (n ϭ 8) species that diverged from A. thaliana (n ϭ 5) ‫5ف‬ MYA. Mapping was conducted in a full-sib progeny of two unrelated F 1 hybrids between two European populations of A. lyrata ssp. petraea. We used the least-squares method of the Joinmap program for map construction. The gross chromosomal differences between the two species were most parsimoniously explained with three fusions, two reciprocal translocations, and one inversion. The total map length was 515 cM, and the distances were 12% larger than those between corresponding markers in the linkage map of A. thaliana. The 72 markers, consisting of microsatellites and gene-based markers, were spaced on average every 8 cM. Transmission ratio distortion was extensive, and most distortions were specific to each reciprocal cross, suggesting cytoplasmic interactions. We estimate locations and most probable genotype frequencies of transmission ratio distorting loci (TRDL) with a Bayesian method and discuss the possible reasons for the observed distortions.A RABIDOPSIS thaliana has become the plant molec-B. nigra and A. thaliana, the conserved segments were ular biology model organism partly because of its only ‫8ف‬ cM in length (Lagercrantz 1998). As the diververy small genome size. It has also become a focus of gence time between Brassica and A. thaliana is estimated plant comparative genomics studies (Schmidt 2002).to be ‫02ف‬ MY, this gives rise to a high rate of chromoSince the sequencing of the A. thaliana genome (Arabisomal evolution. The rapid rate of chromosomal evoludopsis Genome Initiative 2000), the genomic organition was assigned to the polyploid Brassica lineage. A zation of A. thaliana has been compared with that of comparison of A. thaliana and the even closer relative, many other species. This work has revealed a complex Capsella rubella, has shown extensive microsynteny (Acarhistory of genome-wide duplication and subsequent kan et al. 2000;Rossberg et al. 2001). However, the gene loss (Lan et al. 2000;Vision et al. 2000; Simillion closest relatives of A. thaliana have not been mapped et al. 2002). Even in rather remote relatives, such as rice, comparatively at the whole-genome level. tomato, and potato, syntenic stretches have been foundIn this article we construct a genetic map of A. lyrata, (Devos et al. 1999;Ku et al. 2000;Gebhardt et al. 2003).the closest relative of A. thaliana (Koch et al. 1999). This Comparisons with more closely related species have species is estimated to have diverged from A. thaliana shown that the rate of chromosomal evolution in the ‫5ف‬ MYA (Koch et al. 2000). The synonymous divergence Brassicaceae is quite rapid. Kowalski et al. (1994)

Genetic variation was studied in quantitative traits and molecular markers in six natural Scandinavian populations of Arabidopsis thaliana. Only two of the populations had several molecular marker haplotypes and significant between-family variance components in quantitative traits. There was no genetic variation in the other four populations. The differentiation between the populations was high in both molecular markers and quantitative traits, with FST estimates of above 0.60 in almost all traits. The patterns of variation of the neutral markers and morphological and phenological traits were consistent in all the analyses, as opposed to what has been found in predominantly outcrossing species. The general picture of the level and distribution of genetic variance agrees with the information from other predominantly inbreeding species.

We examined microsatellite variation in two diploid, outcrossing relatives of Arabidopsis thaliana, Arabis petraea and Arabis lyrata. The primer sequences were derived from A. thaliana. About 50% (14 loci) of the A. thaliana primers could successfully amplify microsatellites in the related species. Analysis of microsatellite structure in the related species showed that there had been large changes in the microsatellites: there were large differences in repeat numbers and many of the A. thaliana simple repeats were shorter in the related species. For the loci we compared, the related species had a much lower level of variability at the microsatellites than Japanese wild populations of A. thaliana. This is presumably related to the different microsatellite structures, because allozyme data showed that the outcrossing relatives were highly polymorphic compared to other outcrossing herbaceous species. Use of microsatellites in assessing variability or phylogenetic relationships between different species requires caution, because changes in microsatellite structure may alter evolutionary rates.

Species share homologous genes to a large extent, but it is not yet known to what degree the same loci have been targets for natural selection in different species. Natural variation in flowering time is determined to a large degree by 2 genes, FLOWERING LOCUS C and FRIGIDA, in Arabidopsis thaliana. Here, we examine whether FRIGIDA has a role in differences in flowering time between and within natural populations of Arabidopsis lyrata, a close outcrossing perennial relative of A. thaliana. We found 2 FRIGIDA sequence variants producing potentially functional proteins but with a length difference of 14 amino acids. These variants conferred a 15-day difference in flowering time in an association experiment in 2 Scandinavian populations. The difference in flowering time between alleles was confirmed with transformation to A. thaliana. Because the north European late-flowering populations harbor both late- and early sequence variants at intermediate frequencies and the late-flowering variant is most frequent in the southern early flowering European population, other genetic factors must be responsible for the flowering time differences between the populations. The length polymorphism occurs at high frequencies also in several North American populations. The occurrence of functional variants at intermediate frequencies in several populations suggests that the variation may be maintained by balancing selection. This is in contrast to A. thaliana, where independent loss-of-function mutations at the FRIGIDA gene are responsible for differences between populations and local adaptation.

Inbreeding depression may be caused by (partially) recessive or overdominant gene action. The relative evolutionary importance of these two modes has been debated; the former mode is emphasized in the "dominance hypothesis," the latter in the "overdominance hypothesis." We analyzed the genetic basis of inbreeding depression in the self-incompatible herb Arabis petraea (L.) Lam.: In the selfed progeny of twelve parental plants, we studied the proportion of chlorophyll-deficient seedlings, the genotypic distributions of marker genes, and associations of marker genotypes with viability and quantitative traits. Early components of fitness were examined by scoring seed size, germination time, and early growth rate and by observing the proportion of chlorophyll-deficient seedlings. Later components of fitness, flowering, and root and aboveground biomass were also measured. Marker genotypes of young seedlings were scored for 11 enzyme loci and three microsatellite markers. We found a high proportion (about 70%) of families with chlorophyll-deficient seedlings, indicating a high mutational load. We found six significant deviations from 1:2: 1 ratio at marker loci of 60 tests in seedlings, with three of these significant at the experimentwide level. Deviations from the expected ratio were assumed to be due to linked viability loci. A graphical and a Bayesian method were used to distinguish between the overdominance and dominance hypotheses. Most of the deviant segregation ratios suggested overdominance instead of recessivity of the deleterious allele. Neither the early (seed size, germination time, or early growth trait) nor the late quantitative traits (flowering, and root and aboveground biomass) showed significant linkage to markers at the experimentwide level. Presence of significant associations between markers and early viability, but lack thereof for quantitative traits expressed late, suggests either that there may be relatively low inbreeding depression in later life stages or that individual quantitative trait loci may have smaller effects than loci contributing to early viability.