Chromosomal Diversification in Ancistrus Species (Siluriformes: Loricariidae) Inferred From Repetitive Sequence Analysis

Abstract

The Ancistrus genus has extensive chromosomal diversity among species, including heteromorphic sex chromosomes occurrence. However, studies have been shown that chromosomal diversity may still be underestimated. Repetitive sequences represent a large part of eukaryotic genomes, associated with mechanisms of karyotypic diversification, including sex chromosomes evolution. This study analyzed the karyotype diversification of two Ancistrus species (Ancistrus sp. 1 and Ancistrus sp. 2) from the Amazon region by classical and molecular chromosomal markers. Conventional chromosome bands and fluorescence in situ hybridization using probes 18S and 5S rDNA, besides (CA)n, (CG)n, (GA)n, (CAC)n, (CAG)n, (CAT)n, (GAA)n, (GAC)n, (TAA)n, and (TTAGGG)n in tandem repeats were determined on the karyotypes. Ancistrus sp. 1 and Ancistrus sp. 2 presented karyotypes with 2n = 38 (20 m + 14sm+4st, XX/XY) and 2n = 34 (20 m + 14sm, without heteromorphic sex chromosomes), respectively. Robertsonian rearrangements can explain the diploid number difference. C-bands occurred in pericentromeric regions in some chromosomes, and a single 18S rDNA locus occurred in both species. The 5S rDNA showed variation in the number of loci between species karyotypes, suggesting the occurrence of unstable sites and rearrangements associated with these sequences in Ancistrus. The microsatellite mapping evidenced distinct patterns of organization between the two analyzed species, occurring mainly in the sex chromosomes in Ancistrus sp. 1, and in the centromeric and pericentromeric regions of chromosomes m/sm in Ancistrus sp. 2. These data shows the extensive chromosomal diversity of repetitive sequences in Ancistrus, which were involved in Robertsonian rearrangements and sex chromosomes differentiation.

Chromosomal Diversification in Pseudacanthicus Species (Loricariidae, Hypostominae) Revealed by Comparative Mapping of Repetitive Sequences

“…In Loricariidae, there is evidence of extensive karyotype diversity, with an emphasis on the variation in 2n (ranging from 2n = 34 to 96), with a supposed ancestral karyotype of 2n = 54 [ 11 , 32 ]. In the subfamily Hypostominae, many of the variations in diploid number are observed in species of the genera Ancistrus , tribe Ancistrini (2n = 34 to 54) [ 33 ] and Hypostomus , tribe Hypostomini (2n = 64 to 84) [ 34 , 35 ]. However, the karyotypes known so far for species of the clades Acanthicus , Hemiancistrus and Peckoltia and basal lineages of the tribe Hypostomini (e.g., genus Pterygoplichthys ) do not show any variation in diploid number (2n = 52) [ 9 , 10 , 13 ].…”

Mechanisms of Karyotypic Diversification in Ancistrus (Siluriformes, Loricariidae): Inferences from Repetitive Sequence Analysis

“…Currently, there are seventy-six valid species, along with numerous lineages not formally identified, that are related to Ancistrus [ 1 , 2 , 3 ]. This group presents large karyotypic diversity, mainly with extensive variation in the diploid number (2n = 34–54 chromosomes) and karyotypic morphologies; additionally, the occurrence of different types of sex chromosome systems (e.g., single: XX/XY, XX/X0, and ZZ/ZW; and multiple: XX/XY 1 Y 2 , ZZ/ZW 1 W 2 , and Z 1 Z 1 Z 2 Z 2 /Z 1 Z 2 W 1 W 2 ) represents another important aspect of chromosomal evolution in Ancistrus [ 4 , 5 , 6 , 7 , 8 , 9 , 10 ].…”

“…In Hypostominae, most species have karyotypes with 2n = 52, this 2n being considered an ancestral condition for the Ancistrini tribe [ 11 ]. Interestingly, most of the Ancistrus species analyzed have karyotypes with 2n < 52, suggesting a tendency towards 2n reduction due to the occurrence of double-strand breaks (DSB) and DNA fusions throughout the karyotypic evolution of this group of fish [ 9 , 12 , 13 ].…”

“…In fish, especially in Loricariidae species, several studies have associated the occurrence of chromosomal reorganization events with unstable chromosomal sites that are rich in different groups of repetitive sequences [ 9 , 12 , 14 , 15 , 16 ]. Repetitive sequences represent the majority of eukaryotic genomes and have numerous physiological and structural functions, and can be grouped in tandem repeats (e.g., gene families) or distributed in dispersed groups throughout the genome (e.g., transposable elements) [ 17 , 18 ].…”

“…Gene families (e.g., ribosomal genes: rDNA; and small nuclear DNA: snDNA) are classes of repetitive DNA useful for the study of chromosomal evolution in fish, showing different patterns of organization and distribution in the genomes of related species [ 22 , 23 , 24 , 25 , 26 , 27 ]. In Loricariidae, a syntenic and single-site organization between the 18S and 5S rDNA genes represents a plesiomorphic condition [ 11 ], while the synteny break and occurrence of multiple sites of these genes is frequent in Ancistrini, showing mechanisms of chromosomal differentiation involving this group of sequences [ 9 , 13 , 26 ]. However, information about the chromosomal organization of U snDNA sequences is limited to a few species of Loricariidae [ 26 , 28 , 29 , 30 ].…”