Abrupt shortening of bird W chromosomes in ancestral Neognathae

As a result of suppressed recombination, heterogametic sex chromosomes (either Y or W) are usually assumed to gradually shorten over evolutionary time as a way to remove accumulated mutations. However, suppressed recombination removes the most obvious mechanism for excising portions of sex chromosomes. We examined ratios of W/Z chromosome size across 224 bird species from 146 genera. Much of the data were obtained from a previous study (Rutkowska et al. 2012. Biology Letters 8: 636–638), who, similar to ourselves, found no gradual decrease in W chromosome length over evolutionary time. However, we show an abrupt decrease in W chromosome length at or just after the phylogenetic split between the two extant bird superorders, Paleognathae and Neognathae, indicating that the key to understanding sex chromosome evolution may have little to do with gradual suppression of recombination.由于重组受到抑制，异配性别染色体（heterogametic sex chromosomes，即Y或W染色体）通常被认为会在进化历程中逐渐缩短，以此清除累积的突变。然而，重组抑制恰恰移除了切除性染色体片段最显而易见的机制。我们针对146个属的224种鸟类，分析了其W/Z染色体的大小比例。其中大部分数据取自此前的一项研究（Rutkowska等人，2012年，《生物学快报》8卷：636–638），该研究与我们的研究思路相仿，并未发现W染色体长度在进化过程中出现逐渐缩短的现象。但我们的研究结果显示，在现存鸟类两大总目——古颚总目（Paleognathae）和今颚总目（Neognathae）的系统发育分歧之时或之后不久，W染色体长度出现了急剧缩短。这表明，阐释性染色体进化的关键或许与重组的渐进式抑制关联甚微。