Physical chromosomal mapping of major ribosomal genes in 15 ant species with a review of hypotheses regarding evolution of the number and position of NORs in ants

Recently, hypotheses regarding the evolutionary patterns of ribosomal genes in ant chromosomes have been under discussion. One of these hypotheses proposes a relationship between chromosomal location and the number of rDNA sites, suggesting that terminal locations facilitate the dispersion of rDNA clusters through ectopic recombination during meiosis, while intrachromosomal locations restrict them to a single chromosome pair. Another hypothesis suggests that the multiplication of rDNA sites could be associated with an increase in the chromosome number in Hymenoptera due to chromosomal fissions. In this study, we physically mapped rDNA sites in 15 new ant species and also reviewed data on rDNA available since the revision by Teixeira et al. (2021a). Our objectives were to investigate whether the new data confirm the relationship between chromosomal location and the number of rDNA sites, and whether the increase in the chromosome number is significant in the dispersion of rDNA clusters in ant karyotypes. Combining our new data with all information on ant cytogenetics published after 2021, 40 new species and nine new genera were assembled. Most species exhibited intrachromosomal rDNA sites on a single chromosome pair, while three species showed these genes in terminal regions of multiple chromosome pairs. On one hand, the hypothesis that the chromosomal location of rDNA clusters may facilitate the dispersion of rDNA sites in the ant genome, as previously discussed, was strengthened, but, on the other hand, the hypothesis of chromosomal fission as the main mechanism for dispersion of ribosomal genes in ants is likely to be refuted. Furthermore, in certain genera, the location of rDNA sites remained similar among the species studied, whereas in others, the distribution of these genes showed significant variation between species, suggesting a more dynamic chromosomal evolution.近期，有关蚁类染色体中核糖体基因（ribosomal genes）进化模式的假说正受到广泛讨论。其中一项假说提出，染色体位置与核糖体DNA（rDNA）位点数量存在关联：末端位置可在减数分裂过程中通过异位重组（ectopic recombination）促进rDNA簇的扩散，而染色体内位置则将该位点限制在单一染色体对上。另一项假说则认为，由于染色体裂变（chromosomal fissions），膜翅目昆虫的rDNA位点增殖可能与染色体数目增加相关。本研究对15个新蚁类物种的rDNA位点进行了物理定位，并对Teixeira等人（2021a）的修订工作以来已公开的rDNA相关数据进行了综述整理。本研究的核心目标为验证新数据是否能够证实染色体位置与rDNA位点数量间的关联，以及染色体数目增加是否对蚁类核型（karyotypes）中rDNA簇的扩散存在显著影响。将本研究的新数据与2021年后发表的所有蚁类细胞遗传学（cytogenetics）研究信息整合后，共汇总得到40个新物种与9个新属的相关数据。多数受试物种的rDNA位点仅分布于单一染色体对的染色体内区域，而另有3个物种的该基因分布于多条染色体对的末端区域。一方面，此前提出的"rDNA簇的染色体位置可促进蚁类基因组中rDNA位点扩散"的假说得到了进一步支撑；另一方面，以染色体裂变作为蚁类核糖体基因扩散主要机制的假说则大概率被推翻。此外，在所研究的类群中，部分属内物种的rDNA位点位置保持相对稳定，而另一些属内物种的该基因分布则存在显著差异，这表明蚁类的染色体进化具有较高的动态性。