Analysis of transposable elements and organellar DNA in male and female genomes of a species with a huge Y chromosome reveals distinct Y centromeres

Few angiosperms have distinct Y chromosomes. Among those that do are Silene latifolia (Caryophyllaceae), Rumex acetosa (Polygonaceae) and Coccinia grandis (Cucurbitaceae), the latter having a male/female difference of 10% of the total genome (female individuals have a 0.85pg genome, male individuals 0.94pg), due to a Y chromosome that arose about 3 million years ago. We compared the sequence composition of male and female C.grandis plants and determined the chromosomal distribution of repetitive and organellar DNA with probes developed from 21 types of repetitive DNA, including 16 mobile elements. The size of the Y chromosome is largely due to the accumulation of certain repeats, such as members of the Ty1/copia and Ty3/gypsy superfamilies, an unclassified element and a satellite, but also plastome- and chondriome-derived sequences. An abundant tandem repeat with a unit size of 144bp stains the centromeres of the X chromosome and the autosomes, but is absent from the Y centromere. Immunostaining with pericentromere-specific markers for anti-histone H3Ser10ph and H2AThr120ph revealed a Y-specific extension of these histone marks. That the Y centromere has a different make-up from all the remaining centromeres raises questions about its spindle attachment, and suggests that centromeric or pericentromeric chromatin might be involved in the suppression of recombination.