Department of Plant Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
Proc Natl Acad Sci U S A. 2012 Aug 21;109(34):13716-21. doi: 10.1073/pnas.1121096109. Epub 2012 Aug 6.
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.
X 染色体一直被认为保存了祖先常染色体的结构和基因内容，性染色体就是从这些祖先常染色体进化而来的。我们比较了新近进化的木瓜性染色体和与其密切相关的雌雄同体植物 Vasconcellea monoica 的同源常染色体，以推断自重组停止以来木瓜性染色体所发生的变化。我们对 12 个 Vasconcellea monoica 细菌人工染色体进行了测序，其中 11 个对应于木瓜 X 特有的区域，1 个对应于木瓜常染色体区域。与相应的木瓜区域（2.56 Mb）相比，组合的 Vasconcellea monoica X 同源序列要短得多（1.10 Mb）。鉴于 Vasconcellea monoica 基因组比木瓜大 41%，这一发现表明木瓜 X 发生了相当大的扩张；与木瓜常染色体序列相比，X 具有更高的重复序列含量，这支持了扩张的观点。可比对的区域包括 27 个转录编码序列，其中只有 6 个是功能性的 X/Vasconcellea 基因对。木瓜 X 和 Y（h）染色体的等位基因与 Vasconcellea monoica 同源序列的序列差异几乎相同；因此，木瓜性染色体停止重组之前就已经发生了 Carica-Vasconcellea 分化，使得 Vasconcellea monoica 成为推断木瓜性染色体变化的合适外群。木瓜 X 和 Y（h）染色体的雌雄同体特化区域以及 Vasconcellea monoica 都获得和丢失了基因，包括 X 染色体发生了令人惊讶的大量变化。
