Evolutionary insights into the genomic organization of major ribosomal DNA in ant chromosomes.

G A Teixeira,F Petitclerc,J Orivel,H J A C De Aguiar,D M Lopes,L A C Barros

doi:10.1111/imb.12699

Abstract

The major rDNA genes are composed of tandem repeats and are part of the nucleolus organizing regions (NORs). They are highly conserved and therefore useful in understanding the evolutionary patterns of chromosomal locations. The evolutionary dynamics of the karyotype may affect the organization of rDNA genes within chromosomes. In this study, we physically mapped 18S rDNA genes in 13 Neotropical ant species from four subfamilies using fluorescence in situ hybridization. Furthermore, a survey of published rDNA cytogenetic data for 50 additional species was performed, which allowed us to detect the evolutionary patterns of these genes in ant chromosomes. Species from the Neotropical, Palearctic, and Australian regions, comprising a total of 63 species from 19 genera within six subfamilies, were analysed. Most of the species (48 out of 63) had rDNA genes restricted to a single chromosome pair in their intrachromosomal regions. The position of rDNA genes within the chromosomes appears to hinder their dispersal throughout the genome, as translocations and ectopic recombination are uncommon in intrachromosomal regions because they can generate meiotic abnormalities. Therefore, rDNA genes restricted to a single chromosome pair seem to be a plesiomorphic feature in ants, while multiple rDNA sites, observed in distinct subfamilies, may have independent origins in different genera.

Highlights

Eukaryotic genomes have repetitive tandem sequences such as in the major ribosomal RNA genes (45S = 18S + 5.8S + 28S), denominated rDNA, which contain highly conserved genic sequences and are useful as molecular genetic markers, allowing comparisons across distant taxa
We investigated whether the number and location of the ribosomal gene clusters followed a specific pattern or were randomly distributed in order to understand the genomic organization and evolutionary dynamics of these genes in ants
All of the studied ant species presented only a single 18S rDNA site that was colocalized with GC-rich regions (CMA3+) (Table 2), while AT-rich regions (DAPI+) were not detected in any species

Summary

Introduction

The number of studies mapping these genes has increased (Mariano et al, 2008; Santos et al, 2016; Micolino et al, 2019a; Teixeira et al, 2020) and other repetitive sequences, such as telomeres (Meyne et al, 1995; Pereira et al, 2018; Micolino et al, 2020; Castro et al, 2020), satellite DNA (Lorite et al, 2004; Huang et al, 2016), 5S ribosomal genes (Aguiar et al., 2017), and microsatellites (Barros et al, 2018; Micolino et al, 2019b), have been mapped in the chromosomes using the FISH technique. We reviewed previous molecular cytogenetic data related to rDNA gene clusters (45S, 18S, or 28S) in ants Using these data, we investigated whether the number and location of the ribosomal gene clusters followed a specific pattern or were randomly distributed in order to understand the genomic organization and evolutionary dynamics of these genes in ants

Results

Discussion

Experimental Procedures