Genetic Basis of Inbreeding Depression in Arabis petraea

Abstract

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.

Inbreeding depression in self-incompatible North-American Arabidopsis lyrata: disentangling genomic and S-locus-specific genetic load

“…Although we did not assess later life history traits related to flowering in much detail, we found no inbreeding depression for flowering rate. It remains to be tested whether this pattern can be generalised across North-American populations, but it contrasts with patterns of inbreeding depression described for the European subspecies of A. lyrata, where the level of inbreeding depression was in fact higher for growth (around 0.4) and flowering rate (d40.75, Karkkainen et al, 1999). Differences in timing of inbreeding depression may have consequences for the efficiency of purging: genetic load expressed in early life history stages may be purged more easily than that in later life history stages (Husband and Schemske, 1996).…”

“…petraea), the most recessive S-haplotype had a transmission advantage over more dominant ones, which was attributed to selection at the gametic stage (Bechsgaard et al, 2004). An earlier study had found that forced self-pollinations led to considerable inbreeding depression (Karkkainen et al, 1999), suggesting that mutational load and potentially also segregation load helps to maintain SI and outcrossing in European A. lyrata.…”

Transmission ratio distortion in Arabidopsis lyrata: effects of population divergence and the S-locus

“…The crosses have typically been conducted between highly divergent lines of cultivated species or outcrossing plants have been selfed (Carr and Dudash, 2003 and references therein). In a study of selfed progenies of A. lyrata deviations of marker segregation from Mendelian segregation were assumed to be due to linkage to viability loci (Kärkkäinen et al, 1999). That study however did not describe the general level of transmission ratio distortion in outcrossing A. lyrata populations.…”

Analysis of the effects of inbreeding on lifespan and starvation resistance in Drosophila melanogaster

“…Which of the two hypotheses underlies the cause of inbreeding depression is still open to debate, although the partial dominance hypothesis has replaced the once more popular overdominance hypothesis, and is now the most favored in explaining inbreeding depression at least for most fitness traits Charlesworth 1987, 1999;Barrett and Charlesworth 1991;Dudash and Carr 1998;Roff 2002;Wright et al 2008 but see e.g. Karkkainen et al 1999;Li et al 2001; see also Willis 1999;Frankham et al 2001;Crnokrak and Barrett 2002;Radwan 2003;Swindell and Bouzat 2006c, d). …”