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.