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How dynamic could be the 45S rDNA cistron? An intriguing variability in a grasshopper species revealed by integration of chromosomal and genomic data

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Abstract

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.

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Acknowledgements

The authors are grateful to Dr. Frantisek Marec for critical reading of an earlier version of the manuscript, to the anonymous reviewers for valuable suggestions and to Dr. Edison Zefa for providing samples from Murici/AL.

Funding

This study was partially supported by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (process number 2015/16661-1 and 2014/11763-8), Coordenadoria de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). DCCM received a research productivity fellowship from the Conselho Nacional de Desenvolvimento Científico e Tecnológico-CNPq (process number 305300/2017-2), and Ana Beatriz Stein Machado Ferretti received a scientific initiation scholarship from PIBIC-UNESP, CNPq.

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Correspondence to Diogo C. Cabral-de-Mello.

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Supplementary Material 1

Map showing the geographical location for each population studied. (JPG 245 kb)

Supplementary Material 2

Contigs containing sequences associated with the 45S rDNA, annotation of associated sequence, and frequency of association. (DOCX 70 kb)

Supplementary Material 3

Difference in size for classification of large (L) and small (S) sites of 18S rDNA in chromosomes of Abracris flavolineata, (a) meiotic chromosomes (metaphase I) and (b) mitotic chromosomes from embryo. The large signals (L) occurred in the pericentromeric region extending to the short chromosomal arm, while the small signals (S) appeared as small dots in the short arms near to the cetromeric region. (JPG 657 kb)

Supplementary Material 4

Occurrence of large (L) and small (S) 18S rDNA clusters and their frequency in individual chromosomes of Abracris flavolineata for the six populations studied. Asterisks indicate heteromorphism (monosomicity). (DOCX 43 kb)

Supplementary Material 5

FISH mapping using the probe ITS1–2 in individuals from six populations of Abracris flavolineata studied. (JPG 2147 kb)

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Ferretti, A.B.S.M., Ruiz-Ruano, F.J., Milani, D. et al. How dynamic could be the 45S rDNA cistron? An intriguing variability in a grasshopper species revealed by integration of chromosomal and genomic data. Chromosoma 128, 165–175 (2019). https://doi.org/10.1007/s00412-019-00706-8

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  • DOI: https://doi.org/10.1007/s00412-019-00706-8

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