Duplication of the Brassica oleracea APETALA1 floral homeotic gene and the evolution of domesticated cauliflower

Abstract

Development of the cauliflower phenotype in Arabidopsis thaliana requires mutations at both the CAULIFLOWER and APETALA1 loci. BoAP1 is the Brassica oleracea orthologue to the Arabidopsis AP1 gene, and is present in two copies in Brassica genomes. The BoAP1-A gene appears to encode a full- length protein, but BoAP1-B alleles in B. oleracea contain insertions that lead to premature translation termination. The BoAP1-B allele found in most B. oleracea subspecies, including B. oleracea ssp. botrytis (domesticated cauliflower) contains a 9 bp insertion in exon 4. This insertion leads to the formation of an in-frame translation termination codon, and these alleles can encode a protein that is truncated at the K domain of this MADS-box transcriptional activator. The allele in B. oleracea ssp. oleracea (wild cabbage) lacks this insertion and instead contains a downstream 4 bp frameshift mutation resulting in the formation of a nonsense mutation. The structure of the BoAP1-B alleles suggests that they are impaired in their ability to perform their floral meristem identity function. These mutations, in conjunction with mutations at the BoCAULIFLOWER (BoCAL) locus, may be associated with the evolution of domesticated cauliflower.