Oyama Ryan K, Silber Martina V, Renner Susanne S
Systematic Botany & Mycology, Ludwig-Maximilians-Universität (LMU Munich), 80638 Munich, Germany.
BMC Res Notes. 2010 Jun 14;3:166. doi: 10.1186/1756-0500-3-166.
Relatively few species of flowering plants are dioecious and even fewer are known to have sex chromosomes. Current theory posits that homomorphic sex chromosomes, such as found in Bryonia dioica (Cucurbitaceae), offer insight into the early stages in the evolution of sex chromosomes from autosomes. Little is known about these early steps, but an accumulation of transposable element sequences has been observed on the Y-chromosomes of some species with heteromorphic sex chromosomes. Recombination, by which transposable elements are removed, is suppressed on at least part of the emerging Y-chromosome, and this may explain the correlation between the emergence of sex chromosomes and transposable element enrichment.
We sequenced 2321 bp of the Y-chromosome in Bryonia dioica that flank a male-linked marker, BdY1, reported previously. Within this region, which should be suppressed for recombination, we observed a solo-LTR nested in a Copia-like transposable element. We also found other, presumably paralogous, solo-LTRs in a consensus sequence of the underlying Copia-like transposable element.
Given that solo-LTRs arise via recombination events, it is noteworthy that we find one in a genomic region where recombination should be suppressed. Although the solo-LTR could have arisen before recombination was suppressed, creating the male-linked marker BdY1, our previous study on B. dioica suggested that BdY1 may not lie in the recombination-suppressed region of the Y-chromosome in all populations. Presence of a solo-LTR near BdY1 therefore fits with the observed correlation between retrotransposon accumulation and the suppression of recombination early in the evolution of sex chromosomes. These findings further suggest that the homomorphic sex chromosomes of B. dioica, the first organism for which genetic XY sex-determination was inferred, are evolutionarily young and offer reference information for comparative studies of other plant sex chromosomes.
开花植物中相对较少的物种是雌雄异株的，已知具有性染色体的物种更少。当前理论认为，同型性染色体，如在雌雄异株的白泻根（葫芦科）中发现的，有助于深入了解性染色体从常染色体进化的早期阶段。关于这些早期步骤知之甚少，但在一些具有异型性染色体的物种的Y染色体上观察到了转座元件序列的积累。在至少部分新出现的Y染色体上，通过其可去除转座元件的重组受到抑制，这可能解释了性染色体出现与转座元件富集之间的相关性。
我们对白泻根中一个与雄性相关的标记BdY1侧翼的Y染色体2321bp进行了测序，该标记先前已有报道。在这个应该抑制重组的区域内，我们观察到一个嵌套在类Copia转座元件中的单独LTR。我们还在潜在的类Copia转座元件的共有序列中发现了其他可能是旁系同源的单独LTR。
鉴于单独LTR是通过重组事件产生的，值得注意的是，我们在一个应该抑制重组的基因组区域中发现了一个。虽然单独LTR可能在重组被抑制之前就已出现，从而产生了与雄性相关的标记BdY1，但我们之前对白泻根的研究表明，BdY1可能并非在所有种群的Y染色体重组抑制区域中。因此，BdY1附近单独LTR的存在与在性染色体进化早期反转录转座子积累与重组抑制之间观察到的相关性相符。这些发现进一步表明，白泻根的同型性染色体是进化上较年轻的，白泻根是第一个推断出遗传XY性别决定的生物体，为其他植物性染色体的比较研究提供了参考信息。
