Submitted to: Plant and Animal Genome Conference
Publication Type: Abstract Only
Publication Acceptance Date: 10/2/2006
Publication Date: N/A
Interpretive Summary: Not applicable
Technical Abstract: Sex chromosomes in flowering plants, in contrast to those in animals, evolved relatively recently and only a few are heteromorphic. At cytological level, the sex chromosomes of papaya appear homomorphic, nevertheless, we are finding the papaya Y chromosome shows features of incipient sex chromosome evolution. To survey the state of Y chromosome differentiation, we sequenced two pairs of X and Y specific bacterial artificial chromosomes (BACs) and conducted functional analysis on two X BACs and seven Y BACs. Sequence comparison between the two paired X and Y BACs revealed three inversion events on the MSY. Aligning the paired BAC sequences showed that one of the Y BACs had expanded by 9.6% and the other Y BAC had expanded by 30%. One gene on the Y chromosome had been lost by deletion, while the corresponding gene on the X chromosome is still functioning. The average gene density was 1 gene per 35kb on 420 kb X BACs, but only 1 gene per 171 kb on 1.2 Mb Y BACs, a five fold reduction on the Y chromosome. Analysis of sequence divergence between X and Y gene pairs indicates that the majority of X-Y gene pairs investigated are functionally constrained, a characteristic of young sex chromosomes. Our estimate of the age of divergence between X-Y gene pairs ranges from 1.3-2.8 million years ago, supporting a recent origin of the primitive sex chromosomes in papaya. Our findings demonstrate the features and consequence of sex chromosome evolution in this primitive X and Y system of papaya.