Karyotype diversity and evolutionary trends in the Asian swamp eel Monopterus albus (Synbranchiformes, Synbranchidae): a case of chromosomal speciation?

Accumulation of Cd and Pb in sediments and Asian swamp eels (Monopterus albus) from downstream area of Cisadane River, Indonesia

This study examined the presence of two heavy metals (Cd and Pb) in the sediments and Asian swamp eels (Monopterus albus) in the downstream area of Cisadane River. The average concentrations of Cd and Pb in the sediments from all sampling locations were 0.594 ± 0.230 mg/kg and 34.677 ± 24.406 mg/kg, respectively. These concentrations were above the natural background concentration and the recommended value of interim sediment quality guidelines (ISQG), suggesting an enrichment process and potential ecological risk of studied metals to the ecosystem of Cisadane River. The increase in contamination within this region may be attributed to point sources such as landfill areas, as well as the industrial and agricultural land activities in surrounding area, and experienced an increasing level leading towards the estuary of Cisadane River. Meanwhile, the average concentrations of Cd and Pb in the eels from all sampling locations were 0.775 ± 0.528 μg/g and 28.940 ± 12.921 μg/g, respectively. This study also discovered that gill tissues contained higher levels of Cd and Pb than the digestive organ and flesh of Asian swamp eels. These concentrations were higher than Indonesian and international standards, suggesting a potential human health risk and therefore the needs of limitations in the consumption of the eels. Based on the human health risk assessment, the eels from the downstream of Cisadane River are still considered safe to be consumed as long as they comply with the specified maximum consumption limits.

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.

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.

Chromosomal rearrangements and the first indication of an ♀X1 X1 X2 X2 /♂X1 X2 Y sex chromosome system in Rineloricaria fishes (Teleostei: Siluriformes)

Rineloricaria is the most diverse genus within the freshwater fish subfamily Loricariinae, and it is widely distributed in the Neotropical region. Despite limited cytogenetic data, records from southern and south-eastern Brazil suggest a high rate of chromosomal rearrangements in this genus, mirrored in remarkable inter- and intraspecific karyotype variability. In the present work, we investigated the karyotype features of Rineloricaria teffeana, an endemic representative from northern Brazil, using both conventional and molecular cytogenetic techniques. We revealed different diploid chromosome numbers (2n) between sexes (33♂/34♀), which suggests the presence of an ♀X₁ X₁ X₂ X₂ /♂X₁ X₂ Y multiple sex chromosome system. The male-limited Y chromosome was the largest and the only biarmed element in the karyotype, implying Y-autosome fusion as the most probable mechanism behind its origination. C-banding revealed low amounts of constitutive heterochromatin, mostly confined to the (peri)centromeric regions of most chromosomes (including the X₂ and the Y) but also occupying the distal regions of a few chromosomal pairs. The chromosomal localization of the 18S ribosomal DNA (rDNA) clusters revealed a single site on chromosome pair 4, which was adjacent to the 5S rDNA cluster. Additional 5S rDNA loci were present on the autosome pair 8, X₁ chromosome, and in the presumed fusion point on the Y chromosome. The probe for telomeric repeat motif (TTAGGG)_n revealed signals of variable intensities at the ends of all chromosomes except for the Y chromosome, where no detectable signals were evidenced. Male-to-female comparative genomic hybridization revealed no sex-specific or sex-biased repetitive DNA accumulations, suggesting a presumably low level of neo-Y chromosome differentiation. We provide evidence that rDNA sites might have played a role in the formation of this putative multiple sex chromosome system and that chromosome fusions originate through different mechanisms among different Rineloricaria species.

Small Body, Large Chromosomes: Centric Fusions Shaped the Karyotype of the Amazonian Miniature Fish Nannostomus anduzei (Characiformes, Lebiasinidae)

Miniature refers to species with extraordinarily small adult body size when adult and can be found within all major metazoan groups. It is considered that miniature species have experienced severe alteration of numerous morphological traits during evolution. For a variety of reasons, including severe labor concerns during collecting, chromosomal acquisition, and taxonomic issues, miniature fishes are neglected and understudied. Since some available studies indicate possible relationship between diploid chromosome number (2n) and body size in fishes, we aimed to study one of the smallest Neotropical fish Nannostomus anduzei (Teleostei, Characiformes, Lebiasinidae), using both conventional (Giemsa staining, C-banding) and molecular cytogenetic methods (FISH mapping of rDNAs, microsatellites, and telomeric sequences). Our research revealed that N. anduzei possesses one of the lowest diploid chromosome numbers (2n = 22) among teleost fishes, and its karyotype is entirely composed of large metacentric chromosomes. All chromosomes, except for pair number 11, showed an 18S rDNA signal in the pericentromeric region. 5S rDNA signals were detected in the pericentromeric regions of chromosome pair number 1 and 6, displaying synteny to 18S rDNA signals. Interstitial telomeric sites (ITS) were identified in the centromeric region of pairs 6 and 8, indicating that centric fusions played a significant role in karyotype evolution of studied species. Our study provides further evidence supporting the trend of diploid chromosome number reduction along with miniaturization of adult body size in fishes.

Integrated chromatin accessibility and DNA methylation analysis to reveal the critical epigenetic modification and regulatory mechanism in gonadal differentiation of the sequentially hermaphroditic fish, Monopterus albus

BACKGROUND

Monopterus albus is a hermaphroditic and economically farmed fish that undergoes sex reversal from ovary to testis via ovotestis during gonadal development. The epigenetic changes that are associated with gonadal development in this species remain unclear.

METHODS

We produced DNA methylome, transcriptome, and chromatin accessibility maps of the key stages of gonad development: ovary, ovotestis, and testis. The expression of the key candidate genes was detected using qRT-PCR and in situ hybridization and the methylation levels were analysed using bisulphite sequencing PCR. Promoter activity and regulation were assessed using dual-luciferase reporter assays.

RESULTS

Gonadal development exhibits highly dynamic transcriptomic, DNA methylation, and chromatin accessibility changes. We found that DNA methylation status, especially of the transcription start site, was significantly negatively correlated with gene expression while chromatin accessibility exhibited no correlation with gene expression during gonadal development. The epigenetic signatures revealed many novel regulatory elements and genes involved in sex reversal, which were validated. DNA methylation detection and site mutation of plastin-2 promoter, as a candidate gene, revealed that DNA methylation could impact the binding of transcription factor dmrt1 and foxl2 through methylation and demethylation to regulate plastin-2 expression during gonadal development.

CONCLUSIONS

These data provide novel insights into epigenetic modification and help elucidate the potential molecular mechanism by which dynamic modification of DNA methylation plays a crucial role in gonadal development.

The ontogenic gonadal transcriptomes provide insights into sex change in the ricefield eel Monopterus albus

BACKGROUND

The ricefield eel is a freshwater protogynous hermaphrodite fish and has become an important aquaculture species in China. The sex change of ricefield eel is impeding its aquaculture practice, particularly the large-scale artificial breeding. Many studies including transcriptomes of mixed gonadal samples from different individuals have been aimed to elucidate mechanisms underlying the sex change. However, the key physiological factors involved in the initiation of sex change remain to be identified.  RESULTS: The present study performed transcriptomic analysis on gonadal samples of different sexual stages obtained through biopsy from the same fish undergoing sex change. A total of 539,764,816 high-quality reads were generated from twelve cDNA libraries of gonadal tissues at female (F), early intersexual (EI), mid-intersexual (MI), and late intersexual (LI) stages of three individual sex-changing fish. Pairwise comparisons between EI and F, MI and EI, and LI and MI identified 886, 319, and 10,767 differentially expressed genes (DEGs), respectively. Realtime quantitative PCR analysis of 12 representative DEGs showed similar expression profiles to those inferred from transcriptome data, suggesting the reliability of RNA-seq data for gene expression analysis. The expression of apoeb, csl2, and enpp2 was dramatically increased and peaked at EI while that of cyp19a1a, wnt4a, fgf16, and foxl2a significantly downregulated from F to EI and remained at very low levels during subsequent development until LI, which suggests that apoeb, csl2, enpp2, cyp19a1a, wnt4a, fgf16, and foxl2a may be closely associated with the initiation of sex change of ricefield eels.

CONCLUSIONS

Collectively, results of the present study confirmed that the down-regulation of female-related genes, such as cyp19a1a, wnt4a, fgf16, and foxl2a, is important for the sex change of ricefield eels. More importantly, some novel genes, including apoeb, csl2, and enpp2, were shown to be expressed with peak values at EI, which are potentially involved in the initiation of sex change. The present transcriptomic data may provide an important research resource for further unraveling the mechanisms underlying the sex change and testicular development in ricefield eels as well as other teleosts.

Asian Swamp eel Monopterus albus Population Structure and Genetic Diversity in China

The Asian swamp eel (Monopterus albus) is one of the most widely distributed freshwater fish in China. In this study, we identified the single nucleotide polymorphisms (SNPs) of M. albus from 19 wild populations in China using restriction-site associated DNA sequencing (RAD-seq), and used SNP markers to investigate the swamp eel the genetic diversity and population genetic structure. A total of 8941794 SNPs were identified. Phylogenetic and principal component analysis suggested that the 19 populations were clustered into four groups: The Jiaoling County (JL) and Poyang Lake (PYH)populations in Group Ⅰ; the Chengdu City (CD), Dali City (YN), Eli Village (EL), Dongting Lake (DTH), Huoqiu County (HQ), and Chaohu Lake (CH) populations in Group Ⅱ; the Puyang City (PY), Chongming Island (CM), Tai Lake (TH), Gaoyou Lake (GYH), Weishan Lake (WSH), Haimen City (HM), Hongze Lake (HZH), Baiyangdian Lake (BYD), Dagushan (DGS), and Pinghu City (PH) populations in group Ⅲ; and the Lingshui County (LS) populations in Group Ⅳ. All 19 populations may have evolved from four ancestors. The genetic diversity was relatively high in CM, GYH, and HM; and low in LS, EL, and JL. The LS, and CM populations had the highest and lowest differentiation from the other populations, respectively. These findings provide new insights for germplasm resources protection and artificial breeding of M. albus.

Chromosome-Scale, Haplotype-Resolved Genome Assembly of Non-Sex-Reversal Females of Swamp Eel Using High-Fidelity Long Reads and Hi-C Data

The Asian swamp eel (Monopterus albus) is an excellent model species for studying sex change and chromosome evolution. M. albus is also widely reared in East Asia and South-East Asia because of its great nutritional value. The low fecundity of this species (about 300 eggs per fish) greatly hinders fries production and breeding programs. Interestingly, about 3–5% of the eels could remain as females for 3 years and lay more than 3,000 eggs per fish, which are referred to as non-sex-reversal (NSR) females. Here, we presented a new chromosome-level genome assembly of such NSR females using Illumina, HiFi, and Hi-C sequencing technologies. The new assembly (Mal.V2_NSR) is 838.39 Mb in length, and the N50 of the contigs is 49.8 Mb. Compared with the previous assembly obtained using the continuous long-read sequencing technology (Mal.V1_CLR), we found a remarkable increase of continuity in the new assembly Mal.V2_NSR with a 20-times longer contig N50. Chromosomes 2 and 12 were assembled into a single contig, respectively. Meanwhile, two highly contiguous haplotype assemblies were also obtained, with contig N50 being 14.54 and 12.13 Mb, respectively. BUSCO and Merqury analyses indicate completeness and high accuracy of these three assemblies. A comparative genomic analysis revealed substantial structural variations (SVs) between Mal.V2_NSR and Mal.V1_CLR and two phased haplotype assemblies, as well as whole chromosome fusion events when compared with the zig-zag eel. Additionally, our newly obtained assembly provides a genomic view of sex-related genes and a complete landscape of the MHC genes. Therefore, these high-quality genome assemblies would provide great help for future breeding works of the swamp eel, and it is a valuable new reference for genetic and genomic studies of this species.

Decoding genome recombination and sex reversal

Over the past 440 years since the discovery of the medicinal value of swamp eels, much progress has been made in the study of their biology. The fish is emerging as an important model animal in sexual development, in addition to economic and pharmaceutical implications. Tracing genomic history that shapes speciation of the fish has led to discovery of the whole genome-wide chromosome fission/fusion events. Natural intersex differentiation is a compelling feature for sexual development research. Notably, identification of progenitors of germline stem cells that have bipotential to differentiate into either male or female germline stem cells provides new insight into sex reversal. Here, we review these advances that have propelled the field forwards and present unsolved issues that will guide future investigations to finally elucidate vertebrate sexual development using the new model.

The Complete Mitochondrial Genome of One Breeding Strain of Asian Swamp Eel (Monopterus albus, Zuiew 1793) Using PacBio and Illumina Sequencing Technologies and Phylogenetic Analysis in Synbranchiformes

Asian swamp eel (Monopterus albus, Zuiew 1793) is a commercially important fish due to its nutritional value in Eastern and Southeastern Asia. One local strain of M. albus distributed in the Jianghan Plain of China has been subjected to a selection breeding program because of its preferred body color and superiority of growth and fecundity. Some members of the genus Monopterus have been reclassified into other genera recently. These classifications require further phylogenetic analyses. In this study, the complete mitochondrial genomes of the breeds of M. albus were decoded using both PacBio and Illumina sequencing technologies, then phylogenetic analyses were carried out, including sampling of M. albus at five different sites and 14 species of Synbranchiformes with complete mitochondrial genomes. The total length of the mitogenome is 16,621 bp, which is one nucleotide shorter than that of four mitogenomes of M. albus sampled from four provinces in China, as well as one with an unknown sampling site. The gene content, gene order, and overall base compositions are almost identical to the five reported ones. The results of maximum likelihood (ML) and Bayesian inference analyses of the complete mitochondrial genome and 13 protein-coding genes (PCGs) were consistent. The phylogenetic trees indicated that the selecting breed formed the deepest branch in the clade of all Asian swamp eels, confirmed the phylogenetic relationships of four genera of the family Synbranchidae, also providing systematic phylogenetic relationships for the order Synbranchiformes. The divergence time analyses showed that all Asian swamp eels diverged about 0.49 million years ago (MYA) and their common ancestor split from other species about 45.96 MYA in the middle of the Miocene epoch. Altogether, the complete mitogenome of this breed of M. albus would serve as an important dataset for germplasm identification and breeding programs for this species, in addition to providing great help in identifying the phylogenetic relationships of the order Synbranchiformes.

Large-scale comparative analysis of cytogenetic markers across Lepidoptera

Fluorescence in situ hybridization (FISH) allows identification of particular chromosomes and their rearrangements. Using FISH with signal enhancement via antibody amplification and enzymatically catalysed reporter deposition, we evaluated applicability of universal cytogenetic markers, namely 18S and 5S rDNA genes, U1 and U2 snRNA genes, and histone H3 genes, in the study of the karyotype evolution in moths and butterflies. Major rDNA underwent rather erratic evolution, which does not always reflect chromosomal changes. In contrast, the hybridization pattern of histone H3 genes was well conserved, reflecting the stable organisation of lepidopteran genomes. Unlike 5S rDNA and U1 and U2 snRNA genes which we failed to detect, except for 5S rDNA in a few representatives of early diverging lepidopteran lineages. To explain the negative FISH results, we used quantitative PCR and Southern hybridization to estimate the copy number and organization of the studied genes in selected species. The results suggested that their detection was hampered by long spacers between the genes and/or their scattered distribution. Our results question homology of 5S rDNA and U1 and U2 snRNA loci in comparative studies. We recommend the use of histone H3 in studies of karyotype evolution.

Comparative study of four Mystus species (Bagridae, Siluriformes) from Thailand: insights into their karyotypic diversity

Karyotypes of four catfishes of the genus Mystus Scopoli, 1777 (family Bagridae), M. atrifasciatus Fowler, 1937, M. mysticetus Roberts, 1992, M. singaringan (Bleeker, 1846) and M. wolffii (Bleeker, 1851), were analysed by conventional and Ag-NOR banding as well as fluorescence in situ hybridization (FISH) techniques. Microsatellite d(GC)₁₅, d(CAA)₁₀, d(CAT)₁₀ and d(GAA)₁₀ repeat probes were applied in FISH. The obtained data revealed that the four studied species have different chromosome complements. The diploid chromosome numbers (2n) and the fundamental numbers (NF) range between 52 and 102, 54 and 104, 56 and 98, or 58 and 108 in M. mysticetus, M. atrifasciatus, M. singaringan or M. wolffii, respectively. Karyotype formulae of M. mysticetus, M. atrifasciatus, M. singaringan and M. wolffii are 24m+26sm+4a, 26m+24sm+2a, 24m+18sm+14a and 30m+22sm+6a, respectively. A single pair of NORs was identified adjacent to the telomeres of the short arm of chromosome pairs 3 (metacentric) in M. atrifasciatus, 20 (submetacentric) in M. mysticetus, 15 (submetacentric) in M. singaringan, and 5 (metacentric) in M. wolffii. The d(GC)₁₅, d(CAA)₁₀, d(CAT)₁₀ and d(GAA)₁₀ repeats were abundantly distributed in species-specific patterns. Overall, we present a comparison of cytogenetic and molecular cytogenetic patterns of four species from genus Mystus providing insights into their karyotype diversity in the genus.

Comparative chromosomal mapping of microsatellite repeats reveals divergent patterns of accumulation in 12 Siluridae (Teleostei: Siluriformes) species

The freshwater family Siluridae occurs in Eurasia and is especially speciose in South and Southeast Asia, representing an important aquaculture and fishery targets. However, despite the restricted cytogenetic data, a high diploid number variation (from 2n=40 to 92) characterizes this fish group. Considering the large genomic divergence among its species, silurid genomes have experienced an enormous diversification throughout their evolutionary history. Here, we aim to investigate the chromosomal distribution of several microsatellite repeats in 12 Siluridae species and infer about their possible roles in the karyotype evolution that occurred in this group. Our results indicate divergent patterns of microsatellite distribution and accumulation among the analyzed species. Indeed, they are especially present in significant chromosome locations, such as the centromeric and telomeric regions, precisely the ones associated with several kinds of chromosomal rearrangements. Our data provide pieces of evidence that repetitive DNAs played a direct role in fostering the chromosomal differentiation and biodiversity in this fish family.

Evolutionary Dynamics of the Repetitive DNA in the Karyotypes of Pipa carvalhoi and Xenopus tropicalis (Anura, Pipidae)

The large amphibian genomes contain numerous repetitive DNA components that have played an important role in the karyotypic diversification of this vertebrate group. Hypotheses based on the presumable primitive karyotype (2n = 20) of the anurans of the family Pipidae suggest that they have evolved principally through intrachromosomal rearrangements. Pipa is the only South American pipid, while all the other genera are found in Africa. The divergence of the South American lineages from the African ones occurred at least 136 million years ago and is thought to have had a strong biogeographic component. Here, we tested the potential of the repetitive DNA to enable a better understanding of the differentiation of the karyotype among the family Pipidae and to expand our capacity to interpret the chromosomal evolution in this frog family. Our results indicate a long history of conservation in the chromosome bearing the H3 histone locus, corroborating inferences on the chromosomal homologies between the species in pairs 6, 8, and 9. The chromosomal distribution of the microsatellite motifs also provides useful markers for comparative genomics at the chromosome level between Pipa carvalhoi and Xenopus tropicalis, contributing new insights into the evolution of the karyotypes of these species. We detected similar patterns in the distribution and abundance of the microsatellite arrangements, which reflect the shared organization in the terminal/subterminal region of the chromosomes between these two species. By contrast, the microsatellite probes detected a differential arrangement of the repetitive DNA among the chromosomes of the two species, allowing longitudinal differentiation of pairs that are identical in size and morphology, such as pairs 1, 2, 4, and 5. We also found evidence of the distinctive composition of the repetitive motifs of the centromeric region between the species analyzed in the present study, with a clear enrichment of the (CA) and (GA) microsatellite motifs in P. carvalhoi. Finally, microsatellite enrichment in the pericentromeric region of chromosome pairs 6, 8, and 9 in the P. carvalhoi karyotype, together with interstitial telomeric sequences (ITS), validate the hypothesis that pericentromeric inversions occurred during the chromosomal evolution of P. carvalhoi and reinforce the role of the repetitive DNA in the remodeling of the karyotype architecture of the Pipidae.

Whole genome-wide chromosome fusion and new gene birth in the Monopterus albus genome

Background

Teleost fishes account for over half of extant vertebrate species. A core question in biology is how genomic changes drive phenotypic diversity that relates to the origin of teleost fishes.

Results

Here, we used comparative genomic analyses with chromosome assemblies of diverse lineages of vertebrates and reconstructed an ancestral vertebrate genome, which revealed phylogenomic trajectories in vertebrates. We found that the whole-genome-wide chromosome fission/fusions took place in the Monopterus albus lineage after the 3-round whole-genome duplication. Four times of genomic fission/fusions events resulted in the whole genome-wide chromosome fusions in the genomic history of the lineage. In addition, abundant recently evolved new genes for reproduction emerged in the Monopterus albus after separated from medaka. Notably, we described evolutionary trajectories of conserved blocks related to sex determination genes in teleosts.

Conclusions

These data pave the way for a better understanding of genomic evolution in extant teleosts.

Cytogenetics of the small-sized fish, Copeina guttata (Characiformes, Lebiasinidae): Novel insights into the karyotype differentiation of the family

Lebiasinidae is a small fish family composed by miniature to small-sized fishes with few cytogenetic data (most of them limited to descriptions of diploid chromosome numbers), thus preventing any evolutionary comparative studies at the chromosomal level. In the present study, we are providing, the first cytogenetic data for the red spotted tetra, Copeina guttata, including the standard karyotype, C-banding, repetitive DNA mapping by fluorescence in situ hybridization (FISH) and comparative genomic hybridization (CGH), providing chromosomal patterns and novel insights into the karyotype differentiation of the family. Males and females share diploid chromosome number 2n = 42 and karyotype composed of 2 metacentric (m), 4 submetacentric (sm) and 36 subtelocentric to acrocentric (st-a) chromosomes. Blocks of constitutive heterochromatin were observed in the centromeric and interstitial regions of several chromosomes, in addition to a remarkably large distal block, heteromorphic in size, which fully corresponded with the 18S rDNA sites in the fourth chromosomal pair. This overlap was confirmed by 5S/18S rDNA dual-color FISH. On the other hand, 5S rDNA clusters were situated in the long and short arms of the 2nd and 15th pairs, respectively. No sex-linked karyotype differences were revealed by male/female CGH experiments. The genomic probes from other two lebiasinid species, Lebiasina melanoguttata and Pyrrhulina brevis, showed positive hybridization signals only in the NOR region in the genome of C. guttata. We demonstrated that karyotype diversification in lebiasinids was accompanied by a series of structural and numeric chromosome rearrangements of different types, including particularly fusions and fissions.

Adaptive Radiation from a Chromosomal Perspective: Evidence of Chromosome Set Stability in Cichlid Fishes (Cichlidae: Teleostei) from the Barombi Mbo Lake, Cameroon

Cichlid fishes are the subject of scientific interest because of their rapid adaptive radiation, resulting in extensive ecological and taxonomic diversity. In this study, we examined 11 morphologically distinct cichlid species endemic to Barombi Mbo, the largest crater lake in western Cameroon, namely Konia eisentrauti, Konia dikume, Myaka myaka, Pungu maclareni, Sarotherodon steinbachi, Sarotherodon lohbergeri, Sarotherodon linnellii, Sarotherodon caroli, Stomatepia mariae, Stomatepia pindu, and Stomatepia mongo. These species supposedly evolved via sympatric ecological speciation from a common ancestor, which colonized the lake no earlier than one million years ago. Here we present the first comparative cytogenetic analysis of cichlid species from Barombi Mbo Lake using both conventional (Giemsa staining, C-banding, and CMA₃/DAPI staining) and molecular (fluorescence in situ hybridization with telomeric, 5S, and 28S rDNA probes) methods. We observed stability on both macro and micro-chromosomal levels. The diploid chromosome number was 2n = 44, and the karyotype was invariably composed of three pairs of meta/submetacentric and 19 pairs of subtelo/acrocentric chromosomes in all analysed species, with the same numbers of rDNA clusters and distribution of heterochromatin. The results suggest the evolutionary stability of chromosomal set; therefore, the large-scale chromosomal rearrangements seem to be unlikely associated with the sympatric speciation in Barombi Mbo.

Extensive Chromosomal Reorganization in Apistogramma Fishes (Cichlidae, Cichlinae) Fits the Complex Evolutionary Diversification of the Genus

Neotropical cichlid fishes are one of the most diversified and evolutionarily successful species assemblages. Extremely similar forms and intraspecific polychromatism present challenges for the taxonomy of some of these groups. Several species complexes have a largely unknown origin and unresolved evolutionary processes. Dwarf cichlids of the genus Apistogramma, comprising more than a hundred species, exhibit intricate taxonomic and biogeographic patterns, with both allopatric and sympatric distributions. However, karyotype evolution and the role of chromosomal changes in Apistogramma are still unknown. In the present study, nine South American Apistogramma species were analyzed using conventional cytogenetic methods and the mapping of repetitive DNA sequences [18S rDNA, 5S rDNA, and (TTAGGG)n] by fluorescence in situ hybridization (FISH). Our results showed that Apistogramma has unique cytogenetic characteristics in relation to closely related groups, such as a reduced 2n and a large number of bi-armed chromosomes. Interspecific patterns revealed a scenario of remarkable karyotypic changes, including a reduction of 2n, the occurrence of B-chromosomes and evolutionary dynamic of rDNA tandem repeats. In addition to the well-known pre-zygotic reproductive isolation, the karyotype reorganization in the genus suggests that chromosomal changes could act as postzygotic barriers in areas where Apistogramma congeners overlap.

Chromosomal Evolution and Evolutionary Relationships of Lebiasina Species (Characiformes, Lebiasinidae)

We present the first cytogenetic data for Lebiasina bimaculata and L. melanoguttata with the aim of (1) investigating evolutionary events within Lebiasina and their relationships with other Lebiasinidae genera and (2) checking the evolutionary relationships between Lebiasinidae and Ctenoluciidae. Both species have a diploid number 2n = 36 with similar karyotypes and microsatellite distribution patterns but present contrasting C-positive heterochromatin and CMA3+ banding patterns. The remarkable interstitial series of C-positive heterochromatin occurring in L. melanoguttata is absent in L. bimaculata. Accordingly, L. bimaculata shows the ribosomal DNA sites as the only GC-rich (CMA3+) regions, while L. melanoguttata shows evidence of a clear intercalated CMA3+ banding pattern. In addition, the multiple 5S and 18S rDNA sites in L. melanogutatta contrast with single sites present in L. bimaculata. Comparative genomic hybridization (CGH) experiments also revealed a high level of genomic differentiation between both species. A polymorphic state of a conspicuous C-positive, CMA3+, and (CGG)n band was found only to occur in L. bimaculata females, and its possible relationship with a nascent sex chromosome system is discussed. Whole chromosome painting (WCP) and CGH experiments indicate that the Lebiasina species examined and Boulengerella maculata share similar chromosomal sequences, thus supporting the relatedness between them and the evolutionary relationships between the Lebiasinidae and Ctenoluciidae families.