Bressa M J, Papeschi A G, Vítková M, Kubícková S, Fuková I, Pigozzi M I, Marec F
Laboratory of Cytogenetics and Evolution, Faculty of Exact and Natural Sciences, University of Buenos Aires, Buenos Aires, Argentina. mjbressa @ ege.fcen.uba.ar
Cytogenet Genome Res. 2009;125(4):292-305. doi: 10.1159/000235936. Epub 2009 Oct 27.
The neo-X and neo-Y sex chromosomes of Dysdercus albofasciatus represent a unique model for the study of early stages of sex chromosome evolution since they retained the ability to pair and recombine, in contrast to sex chromosomes in most Heteroptera. Here we examined structure, molecular differentiation, and meiotic behaviour of the D. albofasciatus neo-sex chromosomes. Two related species with the ancestral X0 system, D. chaquensis and D. ruficollis, were used for a comparison. In D. albofasciatus, 2 nucleolar organizer regions (NORs) were identified on the neo-X chromosome using fluorescence in situ hybridization (FISH) with an rDNA probe, whereas a single NOR was found on an autosomal pair in the other 2 species. Genomic in situ hybridization (GISH) differentiated a part of the original X in the neo-X chromosome but not the neo-Y chromosome. The same segment of the neo-X chromosome was identified by Zoo-FISH with a chromosome painting probe derived from the X chromosome of D. ruficollis, indicating that this part is conserved between the species. Immunostaining against the cohesin subunit SMC3 revealed that only terminal regions of the D. albofasciatus neo-Xneo-Y bivalent pair and form a synaptonemal complex, which is in keeping with the occurrence of terminal chiasmata, whereas the interstitial region forms a large loop indicating the absence of homology. These results support the hypothesis that the neo-X chromosome evolved by insertion of the original X chromosome into 1 NOR-bearing autosome in an ancestor carrying the X0 system. As a consequence, the homologue of this NOR-autosome became the neo-Y chromosome. A subsequent inversion followed by transposition of the NOR located on the neo-Y onto the neo-X chromosome resulted in the present neo-sex chromosome system in D. albofasciatus.
白纹红蝽的新X和新Y性染色体代表了一个研究性染色体进化早期阶段的独特模型，因为与大多数异翅目昆虫的性染色体不同，它们保留了配对和重组的能力。在此，我们研究了白纹红蝽新性染色体的结构、分子分化和减数分裂行为。我们使用了具有原始X0系统的两个近缘物种——查氏红蝽和红颈红蝽进行比较。在白纹红蝽中，使用rDNA探针通过荧光原位杂交（FISH）在新X染色体上鉴定出2个核仁组织区（NORs），而在其他两个物种的一对常染色体上发现了单个NOR。基因组原位杂交（GISH）区分了新X染色体中原始X的一部分，但未区分新Y染色体。用来自红颈红蝽X染色体的染色体涂染探针进行动物荧光原位杂交（Zoo-FISH），鉴定出了新X染色体的相同片段，这表明该部分在物种间是保守的。针对黏连蛋白亚基SMC3的免疫染色显示，白纹红蝽新X-新Y二价体对中只有末端区域形成联会复合体，这与末端交叉的出现一致，而中间区域形成一个大环，表明不存在同源性。这些结果支持了这样一种假说，即新X染色体是通过将原始X染色体插入携带X0系统的祖先的一条带有NOR的常染色体中进化而来的。因此，这条NOR-常染色体的同源染色体成为了新Y染色体。随后发生的一次倒位，接着是位于新Y上的NOR转座到新X染色体上，从而形成了白纹红蝽目前的新性染色体系统。
