紅果蠅新 Y 染色體的早期演化

Abstract

在許多物種中，其性染色體都是由同源體染色體演化而來。一般來說， Y 染色體喪失了性別決定因子外大部份的基因， X 染色體卻保留原本染色體上的基因 。由於在 XY 染色體之間停止互換，導致有害突變累積，因此造成 Y 染色體的退化。 在晚近形成的新性染色體中，屬於原體染色體的部份可供我們研究同源染色體演化為性染色體的過程。為了探究早期新性染色體的演化，我們比較具有新性染色體的紅果蠅中序列的差異和全轉錄體的模式。我們發現，雖然大部份的轉錄序列仍然有很強的負向選汰，但在 2344 個轉錄序列中，分別有 11.5% 和 2.7% 的轉錄序列顯示位於新 Y 基因座和新 X 基因座的基因表現量明顯下降。新性染色體表現量的分歧，是由於新 X 基因座過量表現以及新 Y 基因座表現量降低所造成的。此外，我發現這些具有表現量分歧的基因在新 X 和新 Y 基因座的序列間具有較高的差異。進而我們觀察到向來在雌性表現都較高的基因，其新 Y 基因座表現量往往降低；相反的，新 Y 基因座表現量升高會使基因在雄性中表現較高。我的結果顯示基因表現量在新性染色體的分歧在早期新性染色體演化扮演重要的角色。

Sex chromosomes evolved independently from autosomes in various lineages. In general, Y chromosome lost most of its genes except for sex determination factors while X chromosome remains gene-rich. The degeneracy of Y-linked genes is caused by accumulation of deleterious mutations as a consequence of recombination inhibition between the sex-chromosome pairs. To decipher the transition from homologous autosomes to differentiated sex chromosomes, the recently fused autosomal regions of the neo-sex chromosomes in Drosophila albomicans provide a good model. Both of its sequence divergence and whole transcriptome profiles were compared to understand the early neo-sex chromosome evolution. I find that most neo-X and neo-Y linked alleles remain under purify selection (dN/dS=0.001/0.021). However, among 1,798 transcripts, 19.0% and 4.0% show significantly reduced gene expression in neo-Y and neo-X linked alleles, respectively. The expression divergence between neo-sex alleles was attributed to both upregulation of neo-X alleles and downregulation of neo-Y. In addition, the genes with expression divergence would have larger sequence divergence between neo-sex alleles. Further analyses reveal that old female-biased genes were prone to reduce the expression of neo-Y alleles, whereas upregulation of neo-Y alleles might shape new male-biased genes. My results indicate that the expression divergence between neo-sex chromosomes is an important step of ealy neo-sex chromosome evolution in D. albomicans.