Candidate genes within a 143 kb region of the flower sex locus in Vitis
Wild Vitis species are dioecious plants, while the cultivated counterpart, Vitis vinifera subspec. vinifera, generally shows hermaphroditic flowers. In Vitis the genetic determinants of flower sex have previously been mapped to a region on chromosome 2. In a combined strategy of map-based cloning and the use of the publicly available grapevine reference genome sequence, the structure of the grapevine flower sex locus has been elucidated with the subsequent identification of candidate genes which might be involved in the development of the different flower sex types. In a fine mapping approach, the sex locus in grapevine was narrowed down using a population derived from a cross of a genotype with a Vitis vinifera background (‘Schiava Grossa’ × ‘Riesling’) with the male rootstock cv. ‘Börner’ (V. riparia × V. cinerea). A physical map of 143 kb was established from BAC clones spanning the 0.5 cM region defined by the closest flanking recombination break points. Sequencing and gene annotation of the entire region revealed several candidate genes with a potential impact on flower sex formation. One of the presumed candidate genes, an adenine phosphoribosyltransferase, was analysed in more detail. The results led to the development of a marker for the presence or absence of the female alleles, while the male and hermaphroditic alleles are still to be differentiated. The impact of other candidate genes is discussed, especially with regard to plant hormone actions. The markers developed will permit the selection of female breeding lines which do not require laborious emasculation thus considerably simplifying grapevine breeding. The genetic finger prints displayed that our cultivated grapevines frequently carry a female allele while homozygous hermaphrodites are rare.