Phillips R B, Noakes M A, Morasch M, Felip A, Thorgaard G H
School of Biological Sciences, Washington State University, Vancouver, WA 98686, USA.
Cytogenet Genome Res. 2004;105(1):122-5. doi: 10.1159/000078018.
Many but not all rainbow trout strains have morphologically distinguishable sex chromosomes. In these strains, the short arm of the X has multiple copies of 5S rDNA and a bright DAPI band near the centromere, both of which are missing from the Y chromosome, which has a very small short arm. We examined the presence of these markers using fluorescence in situ hybridization (FISH) in four different YY clonal lines derived from different strains and compared the results with sexed fish of the Donaldson strain with the normal X/Y heteromorphism. The Y chromosome in two of the YY clonal lines (Arlee and Swanson) is indistinguishable from the X chromosome and it is positive for 5S rDNA and the DAPI bright band. On the other hand, both 5S rDNA sequences and the DAPI band were not found on the Y chromosome in Hot Creek and Clearwater which have the normal Y. Thus the presence of these two cytogenetic markers may account for the size difference between the short arm of the X and Y chromosome found in most rainbow trout strains. In fishes the expression of one type of 5S rRNA is restricted to oocytes and previous work suggests that although XX males are fairly common, XY females are rare, implying a selective disadvantage for XY females. A hypothesis is presented to explain why this sex chromosome heteromorphism is not closely linked to the SEX locus, which is found on the long arm of the Y chromosome in rainbow trout.
许多但并非所有虹鳟鱼品系都有形态上可区分的性染色体。在这些品系中，X染色体的短臂有多个5S rDNA拷贝，且在着丝粒附近有一条明亮的DAPI带，而Y染色体上则没有这些特征，Y染色体的短臂非常小。我们使用荧光原位杂交（FISH）技术检测了来自不同品系的四个不同YY克隆系中这些标记的存在情况，并将结果与具有正常X/Y异型性的唐纳森品系的性成熟鱼进行了比较。在两个YY克隆系（阿利和斯旺森）中，Y染色体与X染色体无法区分，并且对5S rDNA和DAPI亮带呈阳性反应。另一方面，在具有正常Y染色体的霍特溪和清水克隆系中，Y染色体上未发现5S rDNA序列和DAPI带。因此，这两个细胞遗传学标记的存在可能解释了大多数虹鳟鱼品系中X和Y染色体短臂之间的大小差异。在鱼类中，一种类型的5S rRNA的表达仅限于卵母细胞，先前的研究表明，虽然XX雄性相当常见，但XY雌性很少见，这意味着XY雌性具有选择性劣势。本文提出了一个假设，以解释为什么这种性染色体异型性与性位点没有紧密联系，性位点位于虹鳟鱼Y染色体的长臂上。
