Hale D W, Hedin M C, Smith S A, Sudman P D, Greenbaum I F
Department of Biology, Texas A & M University, College Station 77843.
Cytogenet Cell Genet. 1991;56(1):48-56. doi: 10.1159/000133045.
The pairing behavior of the sex chromosomes in male and female individuals representing seven species of Peromyscus was analyzed by electron microscopy of silver-stained zygotene and pachytene configurations. Six species possess submetacentric or metacentric X chromosomes with heterochromatic short arms. Sex-chromosome pairing in these species is initiated during early pachynema at an interstitial position on the X and Y axes. Homologous synapsis then progresses in a unidirectional fashion towards the telomeres of the X short arm and the corresponding arm of the heterochromatic Y chromosome. The distinctive pattern of synaptic initiation allowed a late-synapsing bivalent in fetal oocytes to be tentatively identified as that of the X chromosomes. In contrast to the other species, Peromyscus megalops possesses an acrocentric X chromosome and a very small Y chromosome. Sex-chromosome pairing in this species is initiated at the proximal telomeric region during late zygonema, and then proceeds interstitially towards the distal end of the Y chromosome. These observations suggest that the presence of X short-arm heterochromatin and corresponding Y heterochromatin interferes with late-zygotene alignment of the pairing initiation sites, thereby delaying XY synaptic initiation until early pachynema. The pairing initiation sites are conserved in the vicinity of the X and Y centromeres in Peromyscus, and consequently the addition of heterochromatin during sex-chromosome evolution essentially displaces these sites to an interstitial position.
通过对银染偶线期和粗线期构型进行电子显微镜观察，分析了代表七种鹿鼠属物种的雄性和雌性个体中性染色体的配对行为。六个物种拥有亚中着丝粒或中着丝粒X染色体，其短臂为异染色质。在这些物种中，性染色体配对在粗线期早期于X和Y轴的间质位置开始。同源联会随后以单向方式朝着X短臂的端粒和异染色质Y染色体的相应臂进行。独特的联会起始模式使得能够初步将胎儿卵母细胞中较晚联会的二价体鉴定为X染色体的二价体。与其他物种不同，巨耳鹿鼠拥有近端着丝粒X染色体和非常小的Y染色体。该物种的性染色体配对在偶线期晚期于近端端粒区域开始，然后向Y染色体远端进行间质联会。这些观察结果表明，X短臂异染色质和相应的Y异染色质的存在会干扰配对起始位点在晚偶线期的排列，从而将XY联会起始推迟到粗线期早期。在鹿鼠属中，配对起始位点在X和Y着丝粒附近保守，因此在性染色体进化过程中异染色质的添加实际上将这些位点转移到了间质位置。
