Department of Biology, Lund University, Lund, Sweden.
Heredity (Edinb). 2012 Mar;108(3):264-72. doi: 10.1038/hdy.2011.70. Epub 2011 Sep 7.
Neo-sex chromosomes often originate from sex chromosome-autosome fusions and constitute an important basis for the study of gene degeneration and expression in a sex chromosomal context. Neo-sex chromosomes are known from many animal and plant lineages, but have not been reported in birds, a group in which genome organization seems particularly stable. Following indications of sex linkage and unexpected sex-biased gene expression in warblers (Sylvioidea; Passeriformes), we have conducted an extensive marker analysis targeting 31 orthologues of loci on zebra finch chromosome 4a in five species, representative of independent branches of Passerida. We identified a region of sex linkage covering approximately the first half (10 Mb) of chromosome 4a, and associated to both Z and W chromosomes, in three Sylvioidea passerine species. Linkage analysis in an extended pedigree of one species additionally confirmed the association between this part of chromosome 4a and the Z chromosome. Markers located between 10 and 21 Mb of chromosome 4a showed no signs of sex linkage, suggesting that only half of the chromosome was involved in this transition. No sex linkage was observed in non-Sylvioidea passerines, indicating that the neo-sex chromosome arose at the base of the Sylvioidea branch of the avian phylogeny, at 47.4-37.6 millions years ago (MYA), substantially later than the ancestral sex chromosomes (150 MYA). We hypothesize that the gene content of chromosome 4a might be relevant in its transition to a sex chromosome, based on the presence of genes (for example, the androgen receptor) that could offer a selective advantage when associated to Z-linked sex determination loci.
新性染色体通常起源于性染色体-常染色体融合，是研究性染色体背景下基因退化和表达的重要基础。新性染色体已在许多动物和植物谱系中被发现，但在鸟类中尚未报道，鸟类的基因组组织似乎特别稳定。在莺科（Sylvioidea；雀形目）中出现了性连锁和意外的性别偏向基因表达的迹象后，我们针对 5 种雀形目 passerida 独立分支的代表物种中的 31 个斑马雀 4a 染色体同源基因进行了广泛的标记分析。我们在 3 种 Sylvioidea 莺科物种中鉴定出一个大约覆盖染色体 4a 前半部分（10 Mb）的性连锁区域，与 Z 和 W 染色体都有关联。在一个物种的一个扩展家谱的连锁分析中，还证实了这部分 4a 染色体与 Z 染色体之间的关联。位于染色体 4a 的 10-21 Mb 之间的标记没有表现出性连锁的迹象，这表明只有一半的染色体参与了这种转变。在非 Sylvioidea 莺科中没有观察到性连锁，这表明新性染色体是在莺科鸟类系统发育的 Sylvioidea 分支的基础上产生的，时间大约在 4740-3760 万年前（MYA），远远晚于祖先性染色体（150 MYA）。我们假设，基于与 Z 连锁性别决定基因座相关的基因（例如雄激素受体）的存在，染色体 4a 的基因含量可能与其向性染色体的转变有关。
