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.

负子蝽属（Belostoma）俗称“巨型水蝽（giant water bugs）”，具有显著的细胞遗传学多样性与广泛的染色体变异特征。然而，学界对其核型演化的认知仍较为有限。本研究对该属的8个物种共计77份样本开展了分析。减数分裂分析结果显示：霍氏负子蝽（B. horvathi）与纯白负子蝽（B. candidulum）的染色体数目为2n=14+XY；卡明负子蝽（B. cummings）为2n=22+XY；齿突负子蝽（B. dentatum）、修长负子蝽（B. elongatum）与离散负子蝽（B. discretum）为2n=26+X1X2Y；淡壳负子蝽（B. testacopallidum）与扩张负子蝽（B. dilatatum）则为2n=26+X1X2X3Y。所有受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 rDNA的分析结果，该属物种可被划分为4个类群。对C0t-1 DNA的分析表明，部分由微卫星DNA（microsatellite DNA）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，利用显微切割获取的X染色体开展荧光原位杂交（Fluorescence in situ hybridization, FISH）实验，结果显示杂交信号均匀分布于对应X染色体的核区域；而具有多重性染色体系统的物种则呈现出离散的杂交信号标记。结合本研究数据与已有文献资料，我们为该类群提出了全新的演化假说：其祖先核型具有较低的二倍体染色体数目、简单的性别决定系统，且核仁组织区（Nucleolus Organizer Regions, NORs）位于性染色体上。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的移位演化出其他核型。