Physical mapping of repetitive DNA suggests 2n reduction in Amazon turtles Podocnemis (Testudines: Podocnemididae)

Abstract

Cytogenetic studies show that there is great karyotypic diversity in order Testudines (2n = 26–68), and that this may be mainly attributed to the presence/absence of microchromosomes. Members of the Podocnemididae family have the smallest diploid numbers of this order (2n = 26–28), which may be a derived condition of the group. Diverse studies suggest that repetitive-DNA-rich sites generally act as hotspots for double-strand breaks and chromosomal reorganization. In this context, we used fluorescent in situ hybridization (FISH) to map telomeric sequences (TTAGGG)n, 45S rDNA, and the genes encoding histones H1 and H3 in two species of genus Podocnemis. We also observed conservation of the 45S rDNA and H1 histone sequences (probable case of conserved synteny), but multiple conserved and non-conserved clusters of H3 genes, which colocalized with the interstitial telomeric sequences in the Podocnemis genome. Our results suggest that fusions have occurred between macro and microchromosomes or between microchromosomes, leading to the observed reduction in diploid number in the family Podocnemididae.

Centric Fusions behind the Karyotype Evolution of Neotropical Nannostomus Pencilfishes (Characiforme, Lebiasinidae): First Insights from a Molecular Cytogenetic Perspective

“…From the cytogenetic standpoint only, certain repetitive DNA markers, including 5S and 18S rDNA, have been formerly found to be involved in the formation of centric fusions (e.g., [39][40][41]). In the case of rDNAs, this may be possibly linked with the susceptibility of these tandemly repeated clusters to double-stranded DNA breaks, perhaps resulting from (1) a frequent rRNA transcription and thus break-prone R-loop emergence, (2) intermingling of NOR (Nuclear Organizer Region)-bearing chromosomes in the interphase nucleus, or (3) possible association of rDNA-bearing sites during the meiotic prophase I [42][43][44][45][46][47][48].…”

Evolutionary insights in Amazonian turtles (Testudines, Podocnemididae): co-location of 5S rDNA and U2 snRNA and wide distribution of Tc1/Mariner

“…Our results corroborate previous reports that P. expansa and P. unifilis (Montiel et al, 2016;Noronha et al, 2016) have a diploid number of 2n=28 chromosomes. Molecular cytogenetics studies indicate that the smaller diploid numbers of Podocnemis represent a derived condition; multiple fusions involving microchromosomes appear to be responsible for the reduction of the diploid number (Montiel et al, 2016;Cavalcante et al, 2018). Previously, Cavalcante et al (2018) demonstrated evidence of possible chromosomal fusions in these species.…”

“…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). Studies suggest that sites rich in repetitive sequences can be critical points for double-strand breaks, non-homologous recombination, and chromosomal reorganization in several organisms (Cazaux et al, 2011;Barros et al 2017;Cavalcante et al, 2018). Moreover, the high mobility of certain sequences (as transposable elements, or TEs) can enable them to interrupt the coding sequences of endogenous genes and modify their expression (Kemp and Longworth, 2015;Yin et al, 2018), or be co-opted for the regulation of host genes and thereby interfere with genome function and evolution (McCullers and Steiniger, 2017;Guichard et al, 2018).…”

“…The order Testudines is considered one of the oldest lineages among existing vertebrates (Ferri, 2002). Cytogenetic studies have revealed wide karyotypic variation among their representatives (2n=26-68), which is attributed mainly to the number of microchromosomes (Montiel et al, 2016;Cavalcante et al, 2018). Species of genus Podocnemis (Pleurodira, Podocnemididae) present the second smallest diploid number in the order (2n=28) (Noronha et al, 2016;Cavalcante et al, 2018).…”

“…Cytogenomic studies indicate a derived condition for Podocnemis and suggest that multiple fusions involving microchromosomes may have been responsible for the reduction of 2n in this genus (Montiel et al, 2016;Cavalcante et al, 2018).…”