| According to the biological species concept, species are distinct populations that are reproductively isolated from each other. Reproductive isolation may be manifested as either sterility or inviability of the interspecific hybrid. We used the Arabidopsis thaliana X A. arenosa hybrid system to study the molecular and genetic basis of hybrid inviability in the F1 generation.; Parental genome dosage overwhelmingly determines the outcome of the hybrid cross. We showed that maternal A. thaliana genomic excess suppresses seed death from approximately 95%, in a genomically balanced hybrid cross, to 20%. Parental genome dosage correlated with uniparental loss of gene silencing in the hybrid seed. Genes regulated by heterochromatin silencing were activated in a dosage-dependent manner in the hybrid cross, with high lethality correlating with high loss of silencing. This observation suggests that maternally provided chromatin regulators are limited in the hybrid cross and that increasing maternal genome dosage increases the contribution of such factors.; We carried out a genetic analysis to determine the number and location of factors on the maternal A. thaliana genome that influence hybrid seed survival. Composite interval mapping, using a recombinant inbred line population of A. thaliana, revealed 12 additive quantitative trait loci (QTL) that ranged in effect on phenotype from weak to strong. Epistatic interaction analysis showed that eight of the additive loci affected hybrid seed survival through interacting in a highly connected genetic network. Three QTL candidate genes with known roles in seed development were tested for effects on hybrid seed development. Mutations in TTG2, IKU1 and IKU2 modulated hybrid seed set in distinct ways, promoting early (ttg2) and late (iku1, iku2) hybrid seed development. Our study has established a framework that will enable future cloning and identification of individual genes and pathways governing hybrid seed set. That information will be of value to our understanding of how hybridization barriers between species are enforced, and should prove useful for efforts to improve domesticated species by introgression of wild germplasm. |
