In the present study, we analyzed individuals of Hypostomus soniae (Loricariidae) collected from the Teles Pires River, southern Amazon basin, Brazil. Hypostomus soniae has a diploid chromosome number of 2n = 64 and a karyotype composed of 12 metacentric (m), 22 submetacentric (sm), 14 subtelocentric (st), and 16 acrocentric (a) chromosomes, with a structural difference between the chromosomes of the two sexes: the presence of a block of heterochromatin in sm pair No. 26, which appears to represent a putative initial stage of the differentiation of an XX/XY sex chromosome system. This chromosome, which had a heterochromatin block, and was designated proto-Y (pY), varied in the length of the long arm (q) in comparison with its homolog, resulting from the addition of constitutive heterochromatin. It is further distinguished by the presence of major ribosomal cistrons in a subterminal position of the long arm (q). The Nucleolus Organizer Region (NOR) had different phenotypes among the H. soniae individuals in terms of the number of Ag-NORs and 18S rDNA sites. The origin, distribution and maintenance of the chromosomal polymorphism found in H. soniae reinforced the hypothesis of the existence of a proto-Y chromosome, demonstrating the rise of an XX/XY sex chromosome system.

中文翻译：

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Hypostomus soniae Hollanda-Carvalho＆Weber，2004年从亚马逊河南部盆地的Teles Pires河的细胞遗传学特征：XX / XY性染色体系统的早期证据。

在本研究中，我们分析了从巴西南部亚马逊流域的Teles Pires河收集的海豚（Hypostomus soniae，Loricariidae）个体。Hypostomus soniae的二倍体染色体数为2n = 64，其核型由12个亚中心（m），22个亚中心（sm），14个亚中心（st）和16个acrocentric（a）染色体组成，两个染色体之间的结构差异两种性别：第26 sm对中存在异染色质块，这似乎代表了XX / XY性染色体系统分化的推定初始阶段。该染色体具有异染色质阻滞，被命名为proto-Y（pY），由于添加了组成型异染色质，与同系物相比，长臂（q）的长度有所不同。它的进一步特点是在长臂的亚末端位置存在大量核糖体顺反子（q）。就Ag-NORs和18S rDNA位点的数目而言，核仁组织者区域（NOR）在沙门氏菌个体之间具有不同的表型。在沙门氏菌中发现的染色体多态性的起源，分布和维持，增强了原Y染色体存在的假说，表明了XX / XY性染色体系统的兴起。