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Abstract

Arabidopsis thaliana is primarily selfing, but outcrossing – however rare – may nevertheless play an important role in generating phenotypic novelty and averting genetic meltdown due to accumulation of deleterious alleles. To begin gaining insight into how outcrossing affects progeny of lineages that are largely independently diverging, we intercrossed 293 wild A. thaliana accessions in 1430 combinations. 21 combinations showed strongly deleterious phenotypes attributable to hyperactivation of the plant immune system, and in at least two cases caused by known or suspected resistance (R) genes. Because R genes are especially diverse, it is logical that they might be among the earliest causes of genetic incompatibility among diverging lineages. Since we observe both strong positive and negative effects of hybridization in the laboratory, we are currently investigating the role that outcrossing may play in generating phenotypic novelty and adaptive opportunity in natural populations. We collected 1000 A. thaliana individuals from 95 ecologically diverse sites near Tübingen, Germany, and are genotyping them with 540 SNP markers across all 5 chromosomes. Among 950 plants genotyped to date at 149 SNP markers, we have identified 275 distinct whole-genome genotypes. 15 candidate heterozygotes had parental genotypes clearly identifiable in the same locale; the marker composition allowed identification of F1 as well as later generation offspring. The picture emerging from these results is that crossing can increase genetic diversity in A. thaliana; we are currently investigating how this impacts phenotypic diversity. Thus, though there may be some negative effects, spurts of heterozygosity and genetic novelty could also serve to reinvigorate A. thaliana populations and mitigate the disadvantages of selfing.