Cytogenetic Mapping of the Retroelements <i>Rex1, Rex3</i> and <i>Rex6</i> among Cichlid Fish: New Insights on the Chromosomal Distribution of Transposable Elements

Abstract

To enhance our understanding of the organization of the genome and chromosome evolution of cichlid fish species, we have isolated and physically mapped onto the chromosomes the transposable elements (TEs) Rex1, Rex3 and Rex6, which are conserved in teleost fish, in the chromosomes of African and South American cichlid species. The physical mapping of different Rex elements showed that they are primarily compartmentalized in the pericentromeric heterochromatic regions, although dispersed or clustered signals in euchromatic regions were also observed. The presence of TEs in heterochromatin can be correlated with their role in the structure and organization of heterochromatic areas (such as centromeres) or with the lower selective pressure that act on these gene-poor regions. The Rex elements were also concentrated in the largest chromosome pair of the Nile tilapia, Oreochromis niloticus. This chromosome pair is supposed to have originated by fusions, demonstrating the possible involvement of TEs with chromosome rearrangements. Besides general patterns of chromosomal distribution, comparative analysis suggests that Rex elements could differ in their chromosomal distribution among different fish groups or species and that intrinsic aspects of the genomes could influence the spread, accumulation or elimination of TEs.

Transposable elements as a potential source for understanding the fish genome

“…The analysis of different Rex elements revealed that they are compartimentalized in pericentromeric heterochromatic regions, suggesting that these elements are part of the structure and organization of heterochromatic areas. 34,35 Similar data on the organization of Rex elements was observed in Perciformes, representatives of the Antartic fish as in the species Notothenia coriiceps. 33 In the family Cichlidae, more specifically in the genome of the Nile Tilapia Oreochromis niloticus, besides the Rex elements 35 several other TEs, such as the LINE CiLINE2, 36 non-LTR On2318 and Tc1-like On239, 37 SINE ROn-1, 38,39 and ROn-2 39 have been encountered.…”

“…34,35 Similar data on the organization of Rex elements was observed in Perciformes, representatives of the Antartic fish as in the species Notothenia coriiceps. 33 In the family Cichlidae, more specifically in the genome of the Nile Tilapia Oreochromis niloticus, besides the Rex elements 35 several other TEs, such as the LINE CiLINE2, 36 non-LTR On2318 and Tc1-like On239, 37 SINE ROn-1, 38,39 and ROn-2 39 have been encountered. In O. niloticus, these TEs are found generally dispersed in the genome and may also be related to sex differentiation within this species.…”

“…37,40 Another interesting fact is that besides being present in the heterochromatic regions, Rex elements are generally concentrated on the first chromosome pair, which appears entirely marked with such elements. 35 In this species, the first chromosome pair seems to correspond to the sex chromosomes, 41,42 which possibly originated from fusion processes. 43 The determined localization of Rex elements in this chromosome pair could indicate that these TEs might be involved in such chromosome rearrangements.…”

Construction and Characterization of a Repetitive DNA Library in Parodontidae (Actinopterygii: Characiformes): A Genomic and Evolutionary Approach to the Degeneration of the W Sex Chromosome

“…4C). A. hasemani contains terminal sites on pairs 2, Z, 5,8,12,13,18,20,24,26,27 and at the terminal region on the short arm of the W chromosome (Fig. 4D).…”

“…However, reassociation kinetics based on C o t-1 DNA and the microdissection of chromosomes submitted to C-banding and the subsequent amplification of heterochromatic sequences using degenerate oligonucleotide-primed polymerase chain reaction (PCR) have been used successfully. [9][10][11][12][13][14] The Parodontidae is a Neotropical fish group that is divided into three genera: Parodon, Apareiodon, and Saccodon. 15 Cytogenetic studies reported a conserved diploid number of 54 chromosomes in Parodontidae with distinctive karyotype formulae and numerical/position variation of 18S and 5S rDNA sites.…”