Institut de Systématique, Évolution, Biodiversité, ISYEB-UMR 7205-CNRS, MNHN, EPHE, Sorbonne Université, 57 rue Cuvier CP50 F, 75005 Paris, France.
Genes (Basel). 2023 Jan 5;14(1):150. doi: 10.3390/genes14010150.
Amongst the 460 karyotypes of Polyphagan Coleoptera that we studied, 50 (10.8%) were carriers of an X autosome rearrangement. In addition to mitotic metaphase analysis, the correct diagnosis was performed on meiotic cells, principally at the pachytene stage. The percentages of these inter-chromosomal rearrangements, principally fusions, varied in relation to the total diploid number of chromosomes: high (51%) below 19, null at 19, low (2.7%) at 20 (the ancestral and modal number), and slightly increasing from 7.1% to 16.7% from 22 to above 30. The involvement of the X in chromosome fusions appears to be more than seven-fold higher than expected for the average of the autosomes. Examples of karyotypes with X autosome rearrangements are shown, including insertion of the whole X in the autosome (ins(A;X)), which has never been reported before in animals. End-to-end fusions (Robertsonian translocations, terminal rearrangements, and pseudo-dicentrics) are the most frequent types of X autosome rearrangements. As in the 34 species with a 19,X formula, there was no trace of the Y chromosome in the 50 karyotypes with an X autosome rearrangement, which demonstrates the dispensability of this chromosome. In most instances, C-banded heterochromatin was present at the X autosome junction, which suggests that it insulates the gonosome from the autosome portions, whose genes are subjected to different levels of expression. Finally, it is proposed that the very preferential involvement of the X in inter-chromosome rearrangements is explained by: (1) the frequent acrocentric morphology of the X, thus the terminal position of constitutive heterochromatin, which can insulate the attached gonosomal and autosomal components; (2) the dispensability of the Y chromosome, which considerably minimizes the deleterious consequences of the heterozygous status in male meiosis, (3) following the rapid loss of the useless Y chromosome, the correct segregation of the X autosome-autosome trivalent, which ipso facto is ensured by a chiasma in its autosomal portion.
在我们研究的 460 个多目 Coleoptera 染色体组型中，有 50 个（10.8%）携带 X 染色体与常染色体的重排。除了有丝分裂中期分析外，我们还在减数分裂细胞中进行了正确的诊断，主要是在粗线期。这些染色体间重排，主要是融合，与总染色体的二倍体数有关：低于 19 的染色体数量较高（51%），19 的染色体数量为零，20（祖先和典型数量）的染色体数量较低（2.7%），从 22 到 30 以上的染色体数量逐渐增加到 16.7%。X 染色体参与染色体融合的比例似乎比预期的常染色体平均值高七倍以上。展示了具有 X 染色体与常染色体重排的染色体组型的例子，包括整个 X 染色体插入常染色体（ins(A;X)），这在动物中以前从未报道过。端到端融合（罗伯逊易位、末端重排和假双着丝粒）是 X 染色体与常染色体重排的最常见类型。与具有 19,X 公式的 34 个物种一样，在 50 个具有 X 染色体与常染色体重排的染色体组型中没有 Y 染色体的痕迹，这证明了该染色体的非必需性。在大多数情况下，X 染色体与常染色体交界处存在 C 带异染色质，这表明它将性染色体与常染色体部分隔离开来，性染色体部分的基因受到不同水平的表达。最后，有人提出，X 染色体在染色体间重排中非常优先的参与可以解释为：（1）X 染色体的近端着丝粒形态频繁，因此构成异染色质的末端位置可以隔离附着的性染色体和常染色体成分；（2）Y 染色体的非必需性，这大大减少了雄性减数分裂中杂合状态的有害后果，（3）在无用的 Y 染色体迅速丢失后，X 染色体与常染色体的三价体正确分离，这事实上是通过其常染色体部分的交叉确保的。
