title: Rapid divergence and expansion of the X chromosome in papaya.
abstract: X chromosomes have long been thought to conserve the structure and gene content of the ancestral autosome from which the sex chromosomes evolved. We compared the recently evolved papaya sex chromosomes with a homologous autosome of a close relative, the monoecious Vasconcellea monoica, to infer changes since recombination stopped between the papaya sex chromosomes. We sequenced 12 V. monoica bacterial artificial chromosomes, 11 corresponding to the papaya X-specific region, and 1 to a papaya autosomal region. The combined V. monoica X-orthologous sequences are much shorter (1.10 Mb) than the corresponding papaya region (2.56 Mb). Given that the V. monoica genome is 41% larger than that of papaya, this finding suggests considerable expansion of the papaya X; expansion is supported by a higher repetitive sequence content of the X compared with the papaya autosomal sequence. The alignable regions include 27 transcript-encoding sequences, only 6 of which are functional X/V. monoica gene pairs. Sequence divergence from the V. monoica orthologs is almost identical for papaya X and Y alleles; the Carica-Vasconcellea split therefore occurred before the papaya sex chromosomes stopped recombining, making V. monoica a suitable outgroup for inferring changes in papaya sex chromosomes. The papaya X and the hermaphrodite-specific region of the Y(h) chromosome and V. monoica have all gained and lost genes, including a surprising amount of changes in the X.
authors.affiliation: Department of Plant Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
authors.forename: Andrea R
authors.initials: AR
authors.lastname: Gschwend
authors.forename: Qingyi
authors.initials: Q
authors.lastname: Yu
authors.forename: Eric J
authors.initials: EJ
authors.lastname: Tong
authors.forename: Fanchang
authors.initials: F
authors.lastname: Zeng
authors.forename: Jennifer
authors.initials: J
authors.lastname: Han
authors.forename: Robert
authors.initials: R
authors.lastname: VanBuren
authors.forename: Rishi
authors.initials: R
authors.lastname: Aryal
authors.forename: Deborah
authors.initials: D
authors.lastname: Charlesworth
authors.forename: Paul H
authors.initials: PH
authors.lastname: Moore
authors.forename: Andrew H
authors.initials: AH
authors.lastname: Paterson
authors.forename: Ray
authors.initials: R
authors.lastname: Ming
mesh_terms: D000483:Alleles; D029441:Carica / Q000235:genetics*; D002503:Centromere / Q000648:ultrastructure; D022202:Chromosomes, Artificial, Bacterial; D032461:Chromosomes, Plant; D004251:DNA Transposable Elements; D017343:Genes, Plant; D008957:Models, Genetic; D011995:Recombination, Genetic; D012091:Repetitive Sequences, Nucleic Acid; D012730:Sex Chromosomes; D014158:Transcription, Genetic
chemical_list: D004251:DNA Transposable Elements