Department of Human Genetics, University of Würzburg Biocenter, 97074 Würzburg, Germany.
Developmental Biochemistry, University of Würzburg Biocenter, 97074 Würzburg, Germany.
Cells. 2022 Mar 25;11(7):1118. doi: 10.3390/cells11071118.
The conspicuous colour sexual dimorphism of guppies has made them paradigmatic study objects for sex-linked traits and sex chromosome evolution. Both the X- and Y-chromosomes of the common guppy () are genetically active and homomorphic, with a large homologous part and a small sex specific region. This feature is considered to emulate the initial stage of sex chromosome evolution. A similar situation has been documented in the related Endler's and Oropuche guppies () indicating a common origin of the Y in this group. A recent molecular study in the swamp guppy () reported a low SNP density on the Y, indicating Y-chromosome deterioration. We performed a series of cytological studies on to show that the Y-chromosome is quite small compared to the X and has accumulated a high content of heterochromatin. Furthermore, the Y-chromosome stands out in displaying CpG clusters around the centromeric region. These cytological findings evidently illustrate that the Y-chromosome in is indeed highly degenerated. Immunostaining for SYCP3 and MLH1 in pachytene meiocytes revealed that a substantial part of the Y remains associated with the X. A specific MLH1 hotspot site was persistently marked at the distal end of the associated XY structure. These results unveil a landmark of a recombining pseudoautosomal region on the otherwise strongly degenerated Y chromosome of . Hormone treatments of females revealed that, unexpectedly, no sexually antagonistic color gene is Y-linked in . All these differences to the group of guppies indicate that the trajectories associated with the evolution of sex chromosomes are not in parallel.
孔雀鱼明显的性别二态性使它们成为与性连锁特征和性染色体进化相关的典型研究对象。普通孔雀鱼（Poecilia reticulata）的 X 染色体和 Y 染色体在遗传上都是活跃的且同源的，具有很大的同源部分和很小的性别特异性区域。这种特征被认为模拟了性染色体进化的初始阶段。在相关的埃氏孔雀鱼（Poecilia wingei）和奥罗普切氏孔雀鱼（Poecilia oropuche）中也记录到了类似的情况，表明这一组的 Y 染色体具有共同的起源。最近在沼泽孔雀鱼（Poecilia palembangensis）中的一项分子研究报告称，Y 染色体上的 SNP 密度较低，表明 Y 染色体退化。我们对进行了一系列细胞学研究，结果表明，与 X 染色体相比，Y 染色体非常小，并且积累了大量的异染色质。此外，Y 染色体在靠近着丝粒区域显示出 CpG 簇，这一点尤为突出。这些细胞学发现清楚地表明，的确高度退化了。在粗线期减数分裂细胞中用 SYCP3 和 MLH1 进行免疫染色表明，Y 染色体的很大一部分仍然与 X 染色体相连。在与 X 染色体相连的 XY 结构的远端，始终有一个 MLH1 热点标记。这些结果揭示了一个重组假常染色体区域的标志性特征，该特征存在于强烈退化的 Y 染色体中。对雌性进行激素处理的结果出人意料地表明，没有一个性拮抗颜色基因是 Y 连锁的。所有这些与孔雀鱼组的差异表明，与性染色体进化相关的轨迹并不平行。
