Ferretti Ana B S M, Ruiz-Ruano Francisco J, Milani Diogo, Loreto Vilma, Martí Dardo A, Ramos Erica, Martins Cesar, Cabral-de-Mello Diogo C
Departamento de Biologia, UNESP - Univ Estadual Paulista, Instituto de Biociências/IB, Rio Claro, São Paulo, Brazil.
Departamento de Genética, UGR - Univ de Granada, Facultad de Ciencias, Granada, Spain.
Chromosoma. 2019 Jun;128(2):165-175. doi: 10.1007/s00412-019-00706-8. Epub 2019 May 20.
To better understand the structure and variability of the 45S rDNA cistron and its evolutionary dynamics in grasshoppers, we performed a detailed analysis combining classical and molecular cytogenetic data with whole-genome sequencing in Abracris flavolienata, which shows extraordinary variability in the chromosomal distribution for this element. We found astonishing variability in the number and size of rDNA clusters at intra- and inter-population levels. Interestingly, FISH using distinct parts of 45S rDNA cistron (18S rDNA, 28S rDNA, and ITS1) as probes revealed a distinct number of clusters, suggesting independent mobility and amplification of the 45S rDNA components. This hypothesis is consistent with the higher genomic coverage of almost the entire cistron of 45S rDNA observed in A. flavolineata compared to other grasshoppers, besides coverage variability along the 45S rDNA cistron in the species. In addition, these differences in coverage for distinct components of the 45S rDNA cistron indicate emergence of pseudogenes evidenced by existence of truncated sequences, demonstrating the rDNA dynamics in the species. Although the chromosomal distribution of 18S rDNA was highly variable, the chromosomes 1, 3, 6, and 9 harbored rDNA clusters in all individuals with the occurrence of NOR activity in pair 9, suggesting ancestry or selective pressures to prevent pseudogenization of rDNA sequences in this chromosome pair. Additionally, small NORs and cryptic rDNA loci were observed. Finally, there was no evidence of enrichment and association of transposable elements, at least, inside or nearby rDNA cistron. These findings broaden our knowledge of rDNA dynamics, revealing an independent movement and amplification of segments of 45S rDNA cistron, which in A. flavolineata could be attributed to ectopic recombination.
为了更好地理解45S核糖体DNA顺反子在蚱蜢中的结构、变异性及其进化动态，我们结合经典和分子细胞遗传学数据以及黄缘阿扁蜢（Abracris flavolienata）的全基因组测序进行了详细分析，该物种在该元件的染色体分布上表现出非凡的变异性。我们发现在种群内和种群间水平上，核糖体DNA簇的数量和大小存在惊人的变异性。有趣的是，使用45S核糖体DNA顺反子的不同部分（18S核糖体DNA、28S核糖体DNA和ITS1）作为探针进行荧光原位杂交，揭示了不同数量的簇，这表明45S核糖体DNA组分具有独立的移动性和扩增。这一假设与在黄缘阿扁蜢中观察到的45S核糖体DNA几乎整个顺反子的基因组覆盖率高于其他蚱蜢一致，此外该物种中45S核糖体DNA顺反子的覆盖率也存在变异性。此外，45S核糖体DNA顺反子不同组分的覆盖率差异表明存在假基因，这由截短序列的存在所证明，展示了该物种中核糖体DNA的动态变化。尽管18S核糖体DNA的染色体分布高度可变，但1号、3号、6号和9号染色体在所有个体中都含有核糖体DNA簇，且9号染色体对存在核仁组织区（NOR）活性，这表明存在祖先遗传或选择性压力以防止该染色体对中核糖体DNA序列的假基因化。此外，还观察到了小的核仁组织区和隐蔽的核糖体DNA位点。最后，至少在核糖体DNA顺反子内部或附近，没有证据表明转座元件存在富集和关联。这些发现拓宽了我们对核糖体DNA动态变化的认识，揭示了45S核糖体DNA顺反子片段的独立移动和扩增，在黄缘阿扁蜢中这可能归因于异位重组。
