Discovery of Sex Linkage Markers – a Molecular Proof for the Genetic Model for of Sex Determination in Fig (Ficus carica L.)

The Fig (Ficus carica) is a gynodioecious species with bisexual trees (functional male caprifigs) and unisexual female trees. Cytological results suggest that there are no classical sex chromosomes in Fig. Sex determination in fig was suggested to be controlled by a sex locus that contains two genes that are genetically linked, with an allele combination that determines the plant sex. Genetic experiments support this hypothesis.

In order to establish a molecular genetics system in fig we sequenced the transcriptome of male and female figs. The sequence reads were de-novo assembled and a set of SNPs was discovered.  Implementing bulk segregation approach with self-developed SNP array we compared DNA pools of males and females from our mapping population and germplasm collection.  Statistical analysis and SNP analysis were applied to find two genetic markers that were linked to the sex determination locus (homozygous in female heterozygous in male). This result supports the model for sex determination in fig, proposing that this trait is controlled by one locus. The low number of markers that was discovered reinforces the model that there are no sex chromosomes where one would expect to find numerous markers linked to the sex locus. In order to validate our results we established a population that is based on artificial pollination of caprifigs, functional male trees. In the progenies of this cross the sex linked markers identifies males and females as expected. We suggest that fig can be further used for studying molecularly early stages of development of the sex determination system.