Evolution and dosage compensation of nucleolar organizing regions (NORs) mediated by mobile elements in turtles with female (ZZ/ZW) but not with male (XX/XY) heterogamety

• Published in: Journal of evolutionary biology 2022-12, Vol.35 (12), p.1709-1720
• Main Authors: Montiel, Eugenia E., Badenhorst, Daleen, Lee, LingSze, Valenzuela, Nicole
• Format: Article
• Language: English
• Subjects: 18S/28S repetitive DNA; adaptive evolution of sex chromosome dosage compensation; Aging; Animals; Apalone; Biosynthesis; Cellular stress response; Chromosomes; Compensation; Copy number; DNA, Ribosomal; Dosage; Dosage compensation; Dosage Compensation, Genetic; Epigenetics; Evolution; Evolution, Molecular; Female; Females; Fluorescence; Fluorescence in situ hybridization; Gene clusters; gene regulation silencing; Genomes; In Situ Hybridization, Fluorescence; Male; Males; nucleolar organizing region; Nucleoli; Protein biosynthesis; R2 retrotransposon retroelement transposable element; Recombination; ribosomal RNA genes (rRNA, rDNA); rRNA; Sex; Sex chromosomes; Sex Chromosomes - genetics; silver‐staining AgNOR; Structure-function relationships; temperature‐dependent (TSD) and ZZ/ZW XX/XY genotypic sex determination (GSD); turtle reptile vertebrates; Turtles - genetics
• ISSN: 1010-061X; 1420-9101
• DOI: 10.1111/jeb.14064

Understanding the evolution and regulation of nucleolar organizing regions (NORs) is important to elucidate genome structure and function. This is because ribosomal gene (rDNA) copy number and activity mediate protein biosynthesis, stress response, ageing, disease, dosage compensation and genome stability. Here, we found contrasting dosage compensation of sex‐linked NORs in turtles with male and female heterogamety. Most taxa examined exhibit homomorphic rRNA gene clusters in a single autosome pair (determined by 28S rDNA fluorescence in situ hybridization), whereas NORs are sex‐linked in Apalone spinifera, Pelodiscus sinensis and Staurotypus triporcatus. Full‐dosage compensation upregulates the male X‐NOR (determined via silver staining—AgNOR) in Staurotypus (who lacks Y‐NOR) compared with female X‐AgNORs. In softshell Apalone and Pelodiscus, who share homologous ZZ/ZW micro‐chromosomes, their enlarged W‐NOR is partially active (due to 28S rDNA invasion by R2 retroelements), whereas their smaller Z‐NOR is silent in females but active in both male‐Zs (presumably because the W‐NOR meets cellular demands and excessive NOR activity is costly). We hypothesize that R2 disruption favoured W enlargement to add intact 28S‐units, perhaps facilitated by reduced recombination during sex chromosome evolution. The molecular basis of the potentially adaptive female Z‐silencing is likely intricate and perhaps epigenetic, as non‐ribosomal Z genes are active in Apalone females. Yet, Emydura maquarii exhibit identical heteromorphism in their autosomal NOR (R2 invaded 28S‐units and the small‐autosome NOR is silent), suggesting that the softshell turtle pattern can evolve independent of sex chromosome evolution. Our study illuminates the complex sex chromosome evolution and dosage compensation of non‐model systems that challenges classic paradigms. Nucleolar organizing regions (NORs) that are sex‐linked in turtles are dimorphic in size and activity. These sexually dimorphic NORs are compensated by simple upregulation in hemizygous male‐X of Staurotypus, as in Drosophila. In female softshell turtles (Apalone and Pelodiscus), NOR dossage compensation occurs by partial W‐NOR and full Z‐NOR silencing, resembling an autosomal‐NOR pattern in unrelated turtles (Emydura).