Data from: A new approach to chromosomal evolution in the giant water bug (Heteroptera: Belostomatidae)

The genus Belostoma, known colloquially as ‘giant water bugs’, presents striking cytogenetic diversity and extensive chromosome variability. Notwithstanding, its karyotype evolution is not well understood. We analyzed eight species of Belostoma (77 samples). The meiotic analysis revealed 2n=14+XY for B. horvathi and B. candidulum; 2n=22+XY for B. cummings; 2n=26+X1X2Y for B. dentatum, B. elongatum, and B. discretum; and 2n=26+X1X2X3Y for B. testacopallidum and B. dilatatum. All species showed holokinetic chromosomes. Based on heterochromatin distribution patterns and 18S rDNA, the species of the genus Belostoma were separated into four groups. The analysis of C0t-1 DNA showed that the repetitive DNA, partly composed of microsatellite DNA, was absent on the Y chromosome. FISH using a microdissected X chromosome in species with simple sex system presents uniform hybridization in the nuclear region corresponding to the X chromosome. Species with multiple systems revealed discrete markings. The present data in conjunction with the existing literature led us to propose a new evolutionary hypothesis for the group, with an ancestral karyotype with a low diploid number, simple sex determination system, and Nucleolus Organizer Regions (NORs) on the sex chromosomes. That karyotype would have originated other karyotypes through agmatoploidy, simploidy, heterochromatinization, and movement of the 18S rDNA.Belostoma属（Belostoma）俗称“巨型水虫”，其细胞遗传学多样性显著，染色体变异程度极高。尽管如此，该类群的核型（karyotype）进化机制仍未得到充分阐明。本研究对Belostoma属的8个物种共计77份样本开展了分析。减数分裂分析（meiotic analysis）结果显示：B. horvathi与B. candidulum的核型为2n=14+XY；B. cummings为2n=22+XY；B. dentatum、B. elongatum及B. discretum为2n=26+X1X2Y；B. testacopallidum与B. dilatatum为2n=26+X1X2X3Y。所有供试物种均具有全着丝粒染色体（holokinetic chromosomes）。基于异染色质分布模式与18S核糖体DNA（18S rDNA）的分析结果，Belostoma属物种可被划分为4个类群。对C0t-1 DNA的分析表明，由微卫星DNA（microsatellite DNA）部分构成的重复DNA序列在Y染色体上缺失。针对具有简单性染色体系统的物种开展的显微切割X染色体荧光原位杂交（fluorescence in situ hybridization, FISH）分析显示，杂交信号均匀分布于对应X染色体的核区域内；而具有多性染色体系统的物种则呈现出离散的杂交信号位点。结合本研究数据与现有文献资料，我们为该类群提出了全新的进化假说：其祖先核型具有较低的二倍体染色体数、简单的性染色体决定系统，且性染色体上携带有核仁组织区（Nucleolus Organizer Regions, NORs）。该祖先核型可通过断裂多倍化（agmatoploidy）、简化多倍化（simploidy）、异染色质化以及18S rDNA的移动，衍生出其他各类核型。