Repetitive DNAs and chromosome evolution in Megaleporinus obtusidens and M. reinhardti (Characiformes: Anostomidae).

The high dynamism of repetitive DNAs is a major driver of chromosome evolution. In particular, the accumulation of repetitive DNA sequences has been reported as part of the differentiation of sex-specific chromosomes. In turn, the fish species of the genus Megaleporinus are a monophyletic clade in which the presence of differentiated ZZ/ZW sex chromosomes represents a synapomorphic condition, thus serving as a suitable model to evaluate the dynamic evolution of repetitive DNA classes. Therefore, transposable elements (TEs) and in tandem repeats were isolated and located on chromosomes of Megaleporinus obtusidens and M. reinhardti to infer their role in chromosome differentiation with emphasis on sex chromosome systems. Despite the conserved karyotype features of both species, the location of repetitive sequences - Rex 1, Rex 3, (TTAGGG)n, (GATA)n, (GA)n, (CA)n, and (A)n - varied both intra and interspecifically, being mainly accumulated in Z and W chromosomes. The physical mapping of repetitive sequences confirmed the remarkable dynamics of repetitive DNA classes on sex chromosomes that might have promoted chromosome diversification and reproductive isolation in Megaleporinus species.

U2 and U4 snDNA Comparative Chromosomal Mapping in the Neotropical Fish Genera Apareiodon and Parodon (Characiformes: Parodontidae).

Small nuclear DNA (snDNA) are valuable cytogenetic markers for comparative studies in chromosome evolution because different distribution patterns were found among species. Parodontidae, a Neotropical fish family, is known to have female heterogametic sex chromosome systems in some species. The U2 and U4 snDNA sites have been found to be involved in Z and W chromosome differentiation in Apareiodon sp., Apareiodon affinis, and Parodon hilarii. However, few studies have evaluated snDNA sites as propulsors of chromosome diversification among closely related fish species. In this study, we investigated the distribution of U2 and U4 snDNA clusters in the chromosomes of 10 populations/species belonging to Apareiodon and Parodon, aiming to identify chromosomal homeologies or diversification. In situ localization data revealed a submetacentric pair carrying the U2 snDNA site among the populations/species analyzed. Furthermore, all studied species demonstrated homeology in the location of U4 snDNA cluster in the proximal region of metacentric pair 1, besides an additional signal showing up with a divergence in Apareiodon. Comparative chromosomal mapping of U4 snDNA also helped to reinforce the proposal of the ZZ/ZW1W2 sex chromosome system origin in an A. affinis population. According to cytogenetic data, the study corroborates the diversification in Parodontidae paired species with uncertain taxonomy.

Integrating morphological, molecular and cytogenetic data for F2 sea turtle hybrids diagnosis revealed balanced chromosomal sets.

Hybridization could be considered part of the evolutionary history of many species. The hybridization among sea turtle species on the Brazilian coast is atypical and occurs where nesting areas and reproductive seasons overlap. Integrated analysis of morphology and genetics is still scarce, and there is no evidence of the parental chromosome set distribution in sea turtle interspecific hybrids. In this study, chromosome markers previously established for pure sea turtle species were combined with morphological and molecular analyses aiming to recognize genetic composition and chromosome sets in possible interspecific hybrids initially identified by mixed morphology. The data showed that one hybrid could be an F2 individual among Caretta caretta × Eretmochelys imbricata × Chelonia mydas, and another is resulting from backcross between C. caretta × Lepidochelys olivacea. Native alleles of different parental lineages were reported in the hybrids, and, despite this, it was verified that the hybrid chromosome sets were still balanced. Thus, how sea turtle hybridism can affect genetic features in the long term is a concern, as the implications of the crossing-over in hybrid chromosomal sets and the effects on genetic function are still unpredictable.

Homeology of sex chromosomes in Amazonian Harttia armored catfishes supports the X-fission hypothesis for the X1X2Y sex chromosome system origin.

The Neotropical monophyletic catfish genus Harttia represents an excellent model to study karyotype and sex chromosome evolution in teleosts. Its species split into three phylogenetic clades distributed along the Brazilian territory and they differ widely in karyotype traits, including the presence of standard or multiple sex chromosome systems in some members. Here, we investigate the chromosomal rearrangements and associated synteny blocks involved in the origin of a multiple X1X2Y sex chromosome system present in three out of six sampled Amazonian-clade species. Using 5S and 18S ribosomal DNA fluorescence in situ hybridization and whole chromosome painting with probes corresponding to X1 and X2 chromosomes of X1X2Y system from H. punctata, we confirm previous assumptions that X1X2Y sex chromosome systems of H. punctata, H. duriventris and H. villasboas represent the same linkage groups which also form the putative XY sex chromosomes of H. rondoni. The shared homeology between X1X2Y sex chromosomes suggests they might have originated once in the common ancestor of these closely related species. A joint arrangement of mapped H. punctata X1 and X2 sex chromosomes in early diverging species of different Harttia clades suggests that the X1X2Y sex chromosome system may have formed through an X chromosome fission rather than previously proposed Y-autosome fusion.

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

Ancistrus is a highly diverse neotropical fish genus that exhibits extensive chromosomal variability, encompassing karyotypic morphology, diploid chromosome number (2n = 34-54), and the evolution of various types of sex chromosome systems. Robertsonian rearrangements related to unstable chromosomal sites are here described. Here, the karyotypes of two Ancistrus species were comparatively analyzed using classical cytogenetic techniques, in addition to isolation, cloning, sequencing, molecular characterization, and fluorescence in situ hybridization of repetitive sequences (i.e., 18S and 5S rDNA; U1, U2, and U5 snDNA; and telomere sequences). The species analyzed here have different karyotypes: Ancistrus sp. 1 (2n = 38, XX/XY) and Ancistrus cirrhosus (2n = 34, no heteromorphic sex chromosomes). Comparative mapping showed different organizations for the analyzed repetitive sequences: 18S and U1 sequences occurred in a single site in all populations of the analyzed species, while 5S and U2 sequences could occur in single or multiple sites. A sequencing analysis confirmed the identities of the U1, U2, and U5 snDNA sequences. Additionally, a syntenic condition for U2-U5 snDNA was found in Ancistrus. In a comparative analysis, the sequences of rDNA and U snDNA showed inter- and intraspecific chromosomal diversification. The occurrence of Robertsonian rearrangements and other dispersal mechanisms of repetitive sequences are discussed.

Turnover of multiple sex chromosomes in Harttia catfish (Siluriformes, Loricariidae): a glimpse from whole chromosome painting.

The remarkable fish biodiversity encompasses also great sex chromosome variability. Harttia catfish belong to Neotropical models for karyotype and sex chromosome research. Some species possess one of the three male-heterogametic sex chromosome systems, XY, X1X2Y or XY1Y2, while other members of the genus have yet uncharacterized modes of sex determination. Particularly the XY1Y2 multiple sex chromosome system shows a relatively low incidence among vertebrates, and it has not been yet thoroughly investigated. Previous research suggested two independent X-autosome fusions in Harttia which led to the emergence of XY1Y2 sex chromosome system in three of its species. In this study, we investigated evolutionary trajectories of synteny blocks involved in this XY1Y2 system by probing six Harttia species with whole chromosome painting (WCP) probes derived from the X (HCA-X) and the chromosome 9 (HCA-9) of H. carvalhoi. We found that both painting probes hybridize to two distinct chromosome pairs in Amazonian species, whereas the HCA-9 probe paints three chromosome pairs in H. guianensis, endemic to Guyanese drainages. These findings demonstrate distinct evolutionary fates of mapped synteny blocks and thereby elevated karyotype dynamics in Harttia among the three evolutionary clades.

Comparative cytogenetics among Boana species (Anura, Hylidae): focus on evolutionary variability of repetitive DNA.

Boana comprises a diverse genus of Neotropical treefrogs, currently rearranged into seven taxonomic species groups. Although cytogenetic studies have demonstrated diversity in its representatives, the chromosomal mapping of repetitive DNA sequences is still scarce. In this study, Boana albopunctata, Boana faber, and Boana prasina were subjected to in situ localization of different repetitive DNA units to evaluate trends of chromosomal evolution in this genus. Boana faber and B. prasina had 2n=24 chromosomes, while B. albopunctata has 2n=22 and an intra-individual variation related to the presence/absence of one B chromosome. The location of 45S rDNA sites was different in the analyzed karyotypes, corroborating with what was found in the distinct phylogenetic groups of Boana. We presented the first description of 5S rDNA in a Boana species, which showed markings resulting from transposition/translocation mechanisms. In situ localization of microsatellite loci proved to be a helpful marker for karyotype comparison in Boana, commonly with cis accumulation in the heterochromatin. On the other hand, genomic dispersion of microsatellites may be associated with hitchhiking effects during the spreading of transposable elements. The obtained results corroborated the independent diversification of these lineages of species from three distinct phylogenetic groups of Boana.

Telomere organization and the interstitial telomeric sites involvement in insects and vertebrates chromosome evolution.

Telomere has a central role in chromosomal stability events. Chromosome ends organized in telomere-loop prevent activation of DNA damage response (DDR) mechanisms, thus keeping the chromosome structure organized. On the other hand, free chromosome ends, dysfunctional telomeres, and interstitial telomeric sequences (ITS) can trigger chromosome rearrangements. Here, the telomere organization, function, and maintenance mechanisms, in addition to ITS types and their involvement in chromosome changes, were revisited. Despite a general (TTAGGG)n sequence being present in vertebrate telomeres, insects show more diversification of their telomere motif. The relation between ITS and chromosome rearrangements was observed in insects and vertebrates, demonstrating different types of genome organization and distribution. Some ITS cannot be considered relicts of chromosome rearrangements because probable they were inserted during a double-strand break repair mechanism. On the other hand, the involvement of telomere sequences participating or triggering chromosome rearrangements or organizing satellite DNA components in several species groups is evident. The genomic assembling advances and applying other methodologies over ITS, and their flanking regions, can help to understand the telomere participation in the chromosomal evolution in species groups with highly diversified karyotypes.

Comparative Cytogenetic and Sequence Analysis of U Small Nuclear RNA Genes in Three Ancistrus Species (Siluriformes: Loricariidae).

Ancistrus presents a wide karyotypic diversity, resulting from numeric and structural chromosomal rearrangements. It has been proposed that some genome-specific regions containing repetitive units could organize prone-to-break DNA sites in Loricariidae, triggering chromosomal rearrangements such as Robertsonian fusions (Rb fusions), centric fissions, translocations, and inversions. The tandemly repeats of the small nuclear RNAs (snRNAs) gene families are considered good cytogenetic markers for understanding chromosomal remodeling events among closely related species, but these snRNAs have been scarcely analyzed in Ancistrus. This study presented the nucleotide sequencing and comparative in situ location of U snRNA sequences from Ancistrus aguaboensis, Ancistrus cf. multispinis, and Ancistrus sp. (2n = 50, 52, and 50, respectively), aiming to provide information about snRNA clusters in the genome and chromosome evolution in Ancistrus. U snRNA nucleotide sequences of Ancistrus presented identity to orthologous copies and folded their secondary structures correctly. In situ localization and karyotyping of the three Ancistrus species revealed clustered copies of U2 and U5 snRNA gene families to a single chromosome site, one chromosome pair bearing U1 snRNA sequence, and one main locus of U4 snRNA sequence, besides scattered signals along the chromosomes. Previous studies related the participation of the rRNA gene families in centric fusion events, contributing to chromosome rearrangements and karyotype plasticity present in Loricariidae. In this study, homeologies in U snRNA loci chromosomal locations were detected, indicating the occurrence of conserved sites of these gene families in these three Ancistrus species with 2n = 50 or 52 chromosomes.

Enriched tandem repeats in chromosomal fusion points of Rineloricaria latirostris (Boulenger, 1900) (Siluriformes: Loricariidae).

Cytogenetic data showed the enrichment of repetitive DNAs in chromosomal rearrangement points between closely related species in armored catfishes. Still, few studies integrated cytogenetic and genomic data aiming to identify their prone-to-break DNA sites. Here, we aimed to obtain the repetitive fraction in Rineloricaria latirostris to recognize the microsatellite and homopolymers flanking the regions previously described as chromosomal fusion points. The results indicated that repetitive DNAs in R. latirostris are predominantly DNA transposons, and considering the microsatellite and homopolymers, A/T-rich expansions were the most abundant. The in situ localization demonstrated the A/T-rich repetitive sequences were scattered on the chromosomes, while A/G-rich microsatellite units were accumulated in some regions. The DNA transposon hAT, the 5S rDNA, and 45S rDNA (previously identified in Robertsonian fusion points in R. latirostris) were clusterized with some microsatellites, especially (CA)n, (GA)n, and poly-A, which were also enriched in regions of chromosomal fusions. Our findings demonstrated that repetitive sequences such as rDNAs, hAT transposons, and microsatellite units flank probable evolutionary breakpoint regions in R. latirostris. However, due to the sequence unit homologies in different chromosomal sites, these repeat DNAs only may facilitate chromosome fusion events in R. latirostris rather than working as a double-strand breakpoint site.

Evolutionary breakpoint regions and chromosomal remodeling in Harttia (Siluriformes: Loricariidae) species diversification.

The Neotropical armored catfish genus Harttia presents a wide variation of chromosomal rearrangements among its representatives. Studies indicate that translocation and Robertsonian rearrangements have triggered the karyotype evolution in the genus, including differentiation of sex chromosome systems. However, few studies used powerful tools, such as comparative whole chromosome painting, to clarify this highly diversified scenario. Here, we isolated probes from the X1 (a 5S rDNA carrier) and the X2 (a 45S rDNA carrier) chromosomes of Harttia punctata, which displays an X1X1X2X2/X1X2Y multiple sex chromosome system. Those probes were applied in other Harttia species to evidence homeologous chromosome blocks. The resulting data reinforce that translocation events played a role in the origin of the X1X2Y sex chromosome system in H. punctata. The repositioning of homologous chromosomal blocks carrying rDNA sites among ten Harttia species has also been demonstrated. Anchored to phylogenetic data it was possible to evidence some events of the karyotype diversification of the studied species and to prove an independent origin for the two types of multiple sex chromosomes, XX/XY1Y2 and X1X1X2X2/X1X2Y, that occur in Harttia species. The results point to evolutionary breakpoint regions in the genomes within or adjacent to rDNA sites that were widely reused in Harttia chromosome remodeling.

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

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 Rearrangements and Origin of the Multiple XX/XY1Y2 Sex Chromosome System in Harttia Species (Siluriformes: Loricariidae).

The Neotropical genus Harttia comprises species with extensive chromosomal remodeling and distinct sex chromosome systems (SCSs). So far, three different SCSs with male heterogamety have been characterized in the group. In some species, the presence of the XX/XY1Y2 SCS is associated with a decrease in diploid numbers and several chromosomal rearrangements, although a direct relation to sex chromosome differentiation has not been shown yet. Here, we aimed to investigate the differentiation processes that have led to the establishment of the rare XX/XY1Y2 SCS and track its evolutionary history among other Harttia species. For that, four whole chromosome painting probes derived from chromosome 1 of H. torrenticola (HTO-1), chromosomes 9 and X of H. carvalhoi (HCA-9 and HCA-X), and chromosome X from H. intermontana (HIN-X) were applied in nine Harttia species. Homeologous chromosome blocks were located in Harttia species and demonstrated that Robertsonian (Rb) fusions originated HTO-1, HCA-9, and HCA-X chromosomes, while Rb fissions explain Y1 and Y2 sex chromosomes. Specifically, in H. intermontana, HCA-X, HCA-9, and the NOR-bearing chromosome demonstrated that homeologous blocks were used in the HIN-X and metacentric pair 2 origins. Consequently, diploid numbers changed between the studied species. Overall, the data also reinforce the existence of unstable genomic sites promoting chromosomal differentiation and remodeling within the genus Harttia.

Robertsonian Fusion Site in Rineloricaria pentamaculata (Siluriformes: Loricariidae): Involvement of 5S Ribosomal DNA and Satellite Sequences.

Cytogenetic studies demonstrated that unstable chromosomal sites in armored catfishes (Loricariidae) triggered intense karyotypic diversification, mainly derived from Robertsonian rearrangements. In Loricariinae, the presence of ribosomal DNA (rDNA) clusters and their flanking repeated regions (such as microsatellites or partial transposable element sequences) was proposed to facilitate chromosomal rearrangements. Hence, this study aimed to characterize the numerical chromosomal polymorphism observed in Rineloricaria pentamaculata and evaluate the chromosomal rearrangements which originated diploid chromosome number (2n) variation, from 56 to 54. Our data indicate a centric fusion event between acrocentric chromosomes of pairs 15 and 18, bearing 5S rDNA sites on their short (p) arms. This chromosome fusion established the numerical polymorphism, decreasing the 2n from original 56 (karyomorph A) to 55 in karyomorph B and 54 in karyomorph C. Although vestiges of telomeric sequences were evidenced at the fusion point, no 5S rDNA was detected in this region. The acrocentric chromosomes involved in the origin of the fusion were enriched with (CA)n and (GA)n microsatellites. Repetitive sequences in the acrocentric chromosomes subtelomeres have facilitated the rearrangement. Our study thus reinforces the view on the important role of particular repetitive DNA classes in promoting chromosome fusions which frequently drive Rineloricaria karyotype evolution.

Population structuration and chromosomal features homogeneity in Parodon nasus (Characiformes: Parodontidae): A comparison between Lower and Upper Paraná River representatives

The ichthyofauna of the La Plata hydrographic basin is divided into Upper and Lower Paraná River systems due to the geographic isolation of the Sete Quedas waterfalls, currently flooded by the lake of the Itaipu dam. In Parodontidae, pairs of species, or groups of cryptic species were described between these systems. Although genetic isolation and speciation have already been proposed in other species in the group, Parodon nasus has been maintained as a valid species and distributed throughout the La Plata river basin. In this perspective, specimens of P. nasus from four different sampling sites in the Upper and Lower Paraná River systems were compared regarding the karyotypes, molecular analyzes of population biology and species delimitation to investigate their genetic and population isolation in the La Plata river basin. Despite a geographic barrier and the immense geographic distance separating the specimens sampled from the Lower Paraná River system compared to those from the Upper Paraná River, the data obtained showed P. nasus as a unique taxon. Thus, unlike other species of Parodontidae that showed diversification when comparing the groups residing in the Lower versus Upper Paraná River, P. nasus showed a population structure and a karyotypic homogeneity.(AU)

A ictiofauna do sistema hidrográfico La Plata é dividida em alto e baixo rio Paraná devido ao isolamento geográfico dos Saltos das Sete Quedas há 22 milhões de anos, atualmente inundado pelo lago da represa da Usina de Itaipu. Em Parodontidae, espécies pares ou grupos de espécies crípticas foram descritos entre esses sistemas. Contudo, embora o isolamento genético e especiação já tenham sido propostos em outras espécies do grupo, Parodon nasus tem sido mantido como espécie válida e distribuída em toda a bacia do rio La Plata. Nessa perspectiva, exemplares de P. nasus de quatro diferentes pontos de amostragem nos sistemas do alto e baixo rio Paraná foram comparados quanto ao arranjo dos cariótipos, análises moleculares de biologia populacional e delimitação de espécies, afim de investigar seu isolamento genético e populacional na bacia do rio La Plata. Apesar da barreira geográfica e imensa distância geográfica separando os exemplares amostrados no sistema baixo rio Paraná em comparação àqueles do alto rio Paraná, os dados obtidos demonstraram P. nasus como único táxon válido. Dessa forma, diferentemente de outras espécies de Parodontidae que demonstraram diversificação quando comparados grupos pares residentes no baixo e alto rio Paraná, P. nasus demonstrou estruturação populacional e homogeneidade cariotípica.(AU)

Comparative Cytogenetics Analysis Among Peckoltia Species (Siluriformes, Loricariidae): Insights on Karyotype Evolution and Biogeography in the Amazon Region.

Peckoltia is widely distributed genus in the Amazon and Orinoco basins and the Guiana Shield, containing 18 valid species, and distinct morphotypes still needing description in the scientific literature due to its great taxonomic complexity. This study performed a comparative chromosomal analysis of two undescribed Peckoltia species (Peckoltia sp. 3 Jarumã and Peckoltia sp. 4 Caripetuba) from the Brazilian Amazon using conventional chromosome bands methods and in situ localization of the repetitive DNA (5S and 18S rRNA and U1 snRNA genes and telomeric sequences). Both species presented 2n = 52 but differed in their karyotype formula, probably due to inversions or translocations. The nucleolus organizer regions (NORs) showed distal location on a probably homeologous submetacentric pair in both species, besides an extra signal in a subtelocentric chromosome in Peckoltia sp. 4 Caripetuba. Heterochromatin occurred in large blocks, with different distributions in the species. The mapping of the 18S and 5S rDNA, and U1 snDNA showed differences in locations and number of sites. No interstitial telomeric sites were detected using the (TTAGGG)n probes. Despite 2n conservationism in Peckoltia species, the results showed variation in karyotype formulas, chromosomal bands, and locations of repetitive sites, demonstrating great chromosomal diversity. A proposal for Peckoltia karyotype evolution was inferred in this study based on the diversity of location and number of chromosomal markers analyzed. A comparative analysis with other Peckoltia karyotypes described in the literature, their biogeography patterns, and molecular phylogeny led to the hypothesis that the derived karyotype was raised in the left bank of the Amazon River.

COVID-19: The question of genetic diversity and therapeutic intervention approaches.

Coronavirus disease 2019 (COVID-19), caused by the Severe Acute Respiratory Syndrome Coronavirus type 2 (SARS-CoV-2), is the largest pandemic in modern history with very high infection rates and considerable mortality. The disease, which emerged in China's Wuhan province, had its first reported case on December 29, 2019, and spread rapidly worldwide. On March 11, 2020, the World Health Organization (WHO) declared the COVID-19 outbreak a pandemic and global health emergency. Since the outbreak, efforts to develop COVID-19 vaccines, engineer new drugs, and evaluate existing ones for drug repurposing have been intensively undertaken to find ways to control this pandemic. COVID-19 therapeutic strategies aim to impair molecular pathways involved in the virus entrance and replication or interfere in the patients' overreaction and immunopathology. Moreover, nanotechnology could be an approach to boost the activity of new drugs. Several COVID-19 vaccine candidates have received emergency-use or full authorization in one or more countries, and others are being developed and tested. This review assesses the different strategies currently proposed to control COVID-19 and the issues or limitations imposed on some approaches by the human and viral genetic variability.

Heterochromatin and microsatellites detection in karyotypes of four sea turtle species: Interspecific chromosomal differences.

The wide variation in size and content of eukaryotic genomes is mainly attributed to the accumulation of repetitive DNA sequences, like microsatellites, which are tandemly repeated DNA sequences. Sea turtles share a diploid number (2n) of 56, however recent molecular cytogenetic data have shown that karyotype conservatism is not a rule in the group. In this study, the heterochromatin distribution and the chromosomal location of microsatellites (CA)n, (GA)n, (CAG)n, (GATA)n, (GAA)n, (CGC)n and (GACA)n in Chelonia mydas, Caretta caretta, Eretmochelys imbricata and Lepidochelys olivacea were comparatively investigated. The obtained data showed that just the (CA)n, (GA)n, (CAG)n and (GATA)n microsatellites were located on sea turtle chromosomes, preferentially in heterochromatic regions of the microchromosomes (mc). Variations in the location of heterochromatin and microsatellites sites, especially in some pericentromeric regions of macrochromosomes, corroborate to proposal of centromere repositioning occurrence in Cheloniidae species. Furthermore, the results obtained with the location of microsatellites corroborate with the temperature sex determination mechanism proposal and the absence of heteromorphic sex chromosomes in sea turtles. The findings are useful for understanding part of the karyotypic diversification observed in sea turtles, especially those that explain the diversification of Carettini from Chelonini species.

Highly Rearranged Karyotypes and Multiple Sex Chromosome Systems in Armored Catfishes from the Genus Harttia (Teleostei, Siluriformes).

Harttia comprises an armored catfish genus endemic to the Neotropical region, including 27 valid species with low dispersion rates that are restricted to small distribution areas. Cytogenetics data point to a wide chromosomal diversity in this genus due to changes that occurred in isolated populations, with chromosomal fusions and fissions explaining the 2n number variation. In addition, different multiple sex chromosome systems and rDNA loci location are also found in some species. However, several Harttia species and populations remain to be investigated. In this study, Harttia intermontana and two still undescribed species, morphologically identified as Harttia sp. 1 and Harttia sp. 2, were cytogenetically analyzed. Harttia intermontana has 2n = 52 and 2n = 53 chromosomes, while Harttia sp. 1 has 2n = 56 and 2n = 57 chromosomes in females and males, respectively, thus highlighting the occurrence of an XX/XY1Y2 multiple sex chromosome system in both species. Harttia sp. 2 presents 2n = 62 chromosomes for both females and males, with fission events explaining its karyotype diversification. Chromosomal locations of the rDNA sites were also quite different among species, reinforcing that extensive rearrangements had occurred in their karyotype evolution. Comparative genomic hybridization (CGH) experiments among some Harttia species evidenced a shared content of the XY1Y2 sex chromosomes in three of them, thus pointing towards their common origin. Therefore, the comparative analysis among all Harttia species cytogenetically studied thus far allowed us to provide an evolutionary scenario related to the speciation process of this fish group.

Karyotypic Diversification in Two Megaleporinus Species (Characiformes, Anostomidae) Inferred from In Situ Localization of Repetitive DNA Sequences.

Anostomidae species have conserved diploid numbers (2n = 54), although comparative cytogenetic studies have demonstrated chromosomal rearrangements occurrence among them, especially in repetitive DNA rich regions. The location and distribution of ribosomal DNA (rDNA) and small nuclear RNAs (snRNAs) multigene families are highly dynamic in the genomes of several organisms. In this study, we in situ located the rDNA and snRNA sites in two populations of Megaleporinus obtusidens and a sample of Megaleporinus reinhardti to infer their chromosomal changes in the evolutionary lineages. Both species of Megaleporinus shared 2n = 54 chromosomes with the presence of ZZ/ZW sex chromosome system, but they diverged in relationship to the location of 5S and 45S rDNAs as well as the distribution of snRNAs sites. The characterization of the analyzed sequences revealed the presence of complete rDNA and snRNAs sequences as well as snRNAs containing transposable elements (TEs) and microsatellite repeats. After chromosomal mapping, the sequences encompassing TEs proved to be dispersed through autosomes and accumulated on sex chromosomes. The data demonstrate that intra- and interspecific chromosomal changes occurred involving the multigene family's sites in Megaleporinus karyotypes. Furthermore, we detected TE-like sequences in the differentiation of sex chromosome systems in M. obtusidens and M. reinhardti.