Comparison of Flowering Time Genes in Brassica rapa , B . mpus and Arabidopsis thulium

The major difference between annual and biennial cultivars of oilseed Brassicn napus and B. rapa is conferred by genes controlling vernalization-responsive flowering time. These genes were compared between the species by aligning the map positions of flowering time quantitative trait loci (QTLs) detected in a segregating population of each species. The results suggest that two major QTLs identified in B. rapa correspond to two major QTLs identified in B. napus. Since B. rapa is one of the hypothesized diploid parents of the amphidiploid B. napus, the vernalization requirement of B. napus probably originated from B. rapa. Brassica genes also were compared to flowering time genes in Arabidopsis thaliana by mapping RFLP loci with the same probes in both B. napus and Arabidopsis. The region containing one pair of Brassica QTLs was collinear with the top of chromosome 5 in A. thaliana where flowering time genes FLC, mand CO are located. The region containing the second pair of QTLs showed fractured collinearity with several regions of the Arabidopsis genome, including the top of chromosome 4 where FRI is located. Thus, these Brassica genes may correspond to two genes (FLC and FRI) that regulate flowering time in the latest flowering ecotypes of Arabidopsis. T HE genus Brassica includes species with many morphological forms that are cultivated for vegetables, oils, fodder and condiments. Major differences among crop types are regulated, in part, by genes that control flowering time. Forms such as broccoli and spring oilseed rape are annuals and flower in the seeding year; other forms, such as cabbage and winter oilseed rape, are biennials and require several weeks of exposure to low temperatures (vernalization) to induce flowering. We previously identified genes controlling vernalization-responsive flowering time in B. napus (FERREIRA et al. 1995) and B. rapa (TEUTONICO and OSBORN 1995) by analyzing segregating populations derived from crosses of annual and biennial oilseed cultivars. Two major quantitative trait loci (QTL) were positioned on linkage groups (LG) 2 and 8 of a B. rapa RFLP map. For B. napus, we identified a single major QTL on LG 9 and minor QTL effects associated with LG 12 and LG 16. We also found preliminary evidence that the QTLs in the two species might represent equivalent genes (TEUTONICO and OSBORN 1995). Many loci with late-flowering alleles have been identified in the related crucifer Arabidopsis thaliana, some of which confer phenotypes that are responsive to vernalization (KOORNNEEF et al. 1991). The latest flowering ecotypes of Arabidopsis have late-flowering alleles at the FHlocus on the top of chromosome 4 (BURN et al. Correspondingauthor: Thomas C. Osborn, Department ofAgromony, 1575 Linden Dr., University of Wisconsin, Madison, WI 53711. E-mail: osborn@calshp.cals.wisc.edu Genetics 146: 1129-1129 (July, 1997) 1993; LEE et al. 1993; CLARKE and DEAN 1994) and at FLC on the top of chromosome 5 (KOORNNEEF et al. 1994; LEE et al. 1994a). These two loci account for the late-flowering phenotype in progeny of crosses to Landsberg erecta, and plants with the late-flowering alleles are responsive to vernalization. The tops of chromosomes 4 and 5 also contain other loci ( u > on chromosome 4 and FYand CO on chromosome 5) for which recessive ( L D and or semidominant (CO) mutations confer late-flowering phenotypes (KOORNNEEF et al. 1991; LEE et al. 199413). Mutants with late-flowering alleles at FY and are responsive to vernalization, but the CO lateflowering mutant is only weakly responsive. Arabidopsis is an attractive species for studying the genetics of flowering time because of the relative ease of cloning specific genes. Two genes controlling flowering time have been cloned: LD (LEE et al. 1994b) and CO (PUTTERILL et al. 1995), and there are ongoing efforts to clone other genes. Some of the genes that have been or will be cloned in Arabidopsis may have homologues that regulate this trait in Brassica species. LAGERCRANTZ et al. (1996) presented evidence for homology of CO with flowering time genes in B. nigra. Information on the homology of Arabidopsis and Brassica genes provides insight on the evolution of traits and new avenues for manipulating traits in Brassica crops. In this article, we report on a further analysis of the B. napus flowering time data using additional RFLP and AFLP marker loci detected in this population and on a new analysis of a recombinant inbred population derived from the same B. rapa cross studied previously.