Abstract
BackgroundPlants adopt different reproductive strategies as an adaptation to growth in a range of climates. In Arabidopsis thaliana FRIGIDA (FRI) confers a vernalization requirement and thus winter annual habit by increasing the expression of the MADS box transcriptional repressor FLOWERING LOCUS C (FLC). Variation at FRI plays a major role in A. thaliana life history strategy, as independent loss-of-function alleles that result in a rapid-cycling habit in different accessions, appear to have evolved many times. The aim of this study was to identify and characterize orthologues of FRI in Brassica oleracea.ResultsWe describe the characterization of FRI from Brassica oleracea and identify the two B. oleracea FRI orthologues (BolC.FRI.a and BolC.FRI.b). These show extensive amino acid conservation in the central and C-terminal regions to FRI from other Brassicaceae, including A. thaliana, but have a diverged N-terminus. The genes map to two of the three regions of B. oleracea chromosomes syntenic to part of A. thaliana chromosome 5 suggesting that one of the FRI copies has been lost since the ancient triplication event that formed the B. oleracea genome. This genomic position is not syntenic with FRI in A. thaliana and comparative analysis revealed a recombination event within the A. thaliana FRI promoter. This relocated A. thaliana FRI to chromosome 4, very close to the nucleolar organizer region, leaving a fragment of FRI in the syntenic location on A. thaliana chromosome 5. Our data show this rearrangement occurred after the divergence from A. lyrata. We explored the allelic variation at BolC.FRI.a within cultivated B. oleracea germplasm and identified two major alleles, which appear equally functional both to each other and A. thaliana FRI, when expressed as fusions in A. thaliana.ConclusionsWe identify the two Brassica oleracea FRI genes, one of which we show through A. thaliana complementation experiments is functional, and show their genomic location is not syntenic with A. thaliana FRI due to an ancient recombination event. This has complicated previous association analyses of FRI with variation in life history strategy in the Brassica genus.
Highlights
Plants adopt different reproductive strategies as an adaptation to growth in a range of climates
Two FRI genes are present in the Brassica oleracea genome The BoFRI genes were isolated from the JBo Bacterial artificial chromosome (BAC) library of the B. oleracea Chinese kale genotype A12DHd [32] through hybridization with an A. thaliana FRI genomic clone
The data we present here suggest that the chromosomal rearrangements that occurred during the evolution of the ancestral Brassicaceae genome into A. thaliana included a recombination/rearrangement event that relocated a genomic region containing A. thaliana FRI (AtFRI) to a position near the distal end of the short arm of chromosome 4, close to the nucleolar organiser region, leaving a nonfunctional remnant in the genomic position on chromosome 5 that is syntenic with FRI in the other Brassicaceae (A. lyrata, B. oleracea; [25,52])
Summary
Plants adopt different reproductive strategies as an adaptation to growth in a range of climates. In Arabidopsis thaliana FRIGIDA (FRI) confers a vernalization requirement and winter annual habit by increasing the expression of the MADS box transcriptional repressor FLOWERING LOCUS C (FLC). Variation at FRI plays a major role in A. thaliana life history strategy, as independent loss-of-function alleles that result in a rapid-cycling habit in different accessions, appear to have evolved many times. Flowering influences the pattern of growth throughout the seasons and affects many agronomic characters including the quantity and quality of crop production. This is apparent in cultivated brassicas, where variation in the flowering process has been selected to produce a diverse array of economically important morphological forms. To date natural variation in vernalization requirement has been associated with FRI polymorphism in A. lyrata [8] and allelic variation in one orthologue in Brassica napus (BnaA.FRI.a) has been associated with flowering time variation [22]
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