Abstract
ABSTRACTEukaryotic genomes exhibit substantial accumulation of repetitive DNA sequences. These sequences can participate in chromosomal reorganization events and undergo molecular cooption to interfere with the function and evolution of genomes. In turtles, repetitive DNA sequences appear to be accumulated at probable break points and may participate in events such as non-homologous recombination and chromosomal rearrangements. In this study, repeated sequences of 5S rDNA, U2 snRNA and Tc1/Mariner transposons were amplified from the genomes of the turtles, Podocnemis expansa and Podocnemis unifilis, and mapped by fluorescence in situ hybridization. Our data confirm the 2n=28 chromosomes for these species (the second lowest 2n in the order Testudines). We observe high conservation of the co-located 5S rDNA and U2 snRNA genes on a small chromosome pair (pair 13), and surmise that this represents the ancestral condition. Our analysis reveals a wide distribution of the Tc1/Mariner transposons and we discuss how the mobility of these transposons can act on karyotypic reorganization events (contributing to the 2n decrease of those species). Our data add new information for the order Testudines and provide important insights into the dynamics and organization of these sequences in the chelonian genomes.
Highlights
The wide variation in the size and organization of eukaryotic genomes is attributed principally to the accumulation of repetitive DNAs (Feschotte and Pritham, 2007; Kordis, 2009)
Podocnemis expansa had a fundamental number (FN) of 54 and a karyotype formula of 24m / sm + 2st + 2a, while P. unifilis presented with FN=52 and a karyotype formula of 22m / sm + 2st + 4a
We demonstrate that the gene locus number for the 5S rDNA is highly conserved in P. expansa and P. unifilis
Summary
The wide variation in the size and organization of eukaryotic genomes is attributed principally to the accumulation of repetitive DNAs (Feschotte and Pritham, 2007; Kordis, 2009). Due to its conserved character, the 5S rDNA has been widely used as a marker in molecular cytogenetics for the characterization of various species. The 5S rDNA has been reported to co-localize with other multigenes, such as histones genes and small nuclear RNAs (snRNAs) of the U family (Novotná et al 2011; Piscor et al, 2018); these associations seem to indicate an old and linked organization of such sequences in the genomes of the relevant species (Cabral-de-Mello et al, 2011)
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