Molecular Cytogenetics (FISH, GISH) of Coccinia grandis: A ca. 3 myr-Old Species of Cucurbitaceae with the Largest Y/Autosome Divergence in Flowering Plants

The independent evolution of heteromorphic sex chromosomes in 19 speciesfrom 4 families of flowering plants permits studying X/Y divergenceafter the initial recombination suppression. Here, we documentautosome/Y divergence in the tropical Cucurbitaceae Coccinia grandis,which is ca. 3 myr old. Karyotyping and C-value measurements show thatthe C. grandis Y chromosome has twice the size of any of the otherchromosomes, with a male/female C-value difference of 0.094 pg or 10%of the total genome. FISH staining revealed 5S and 45S rDNA sites onautosomes but not on the Y chromosome, making it unlikely that rDNAcontributed to the elongation of the Y chromosome; recent end-to-endfusion also seems unlikely given the lack of interstitial telomericsignals. GISH with different concentrations of female blocking DNAdetected a possible pseudo-autosomal region on the Y chromosome, andC-banding suggests that the entire Y chromosome in C. grandis isheterochromatic. During meiosis, there is an end-to-end connectionbetween the X and the Y chromosome, but the X does not otherwise differfrom the remaining chromosomes. These findings and a review of plantswith heteromorphic sex chromosomes reveal no relationship betweenspecies age and degree of sex chromosome dimorphism. Its relativelysmall genome size (0.943 pg/2C in males), large Y chromosome, andphylogenetic proximity to the fully sequenced Cucumis sativus make C.grandis a promising model to study sex chromosome evolution