Occurrence of multiple sexual chromosomes (XX/XY1Y2 and Z1Z1Z2Z2/Z1Z2W1W2) in catfishes of the genus Ancistrus (Siluriformes: Loricariidae) from the Amazon basin

Loricariid catfishes show a predominance of homomorphism in sex chromosomes, but cases of simple and multiple systems were also found. Here we describe two cases of multiple sex chromosome systems in loricariids from Brazilian Amazonia. Males of Ancistrus sp.1 "Balbina" have a modal number of 2n = 39 chromosomes, fundamental number (FN) of 78, and karyotypic formula of 27 m + 10 sm + 2 st; females have 2n = 38 chromosomes, FN = 76, and 26 m + 10 sm + 2 st. Ancistrus sp.2 "Barcelos" has 2n = 52 chromosomes for both sexes, FN = 80 for males and FN = 79 for females. Karyotypic formula is 12 m + 12 sm + 4 st + 24a for males and 11 m + 12 sm + 4st + 25a for females. The two species show different arrangements of constitutive heterochromatin blocks, which are coincident with NORs and absent in sex chromosomes. We suggest a XX/XY(1)Y(2) mechanism for Ancistrus sp.1 "Balbina", and a Z(1)Z(1)Z(2)Z(2)/Z(1)Z(2)W(1)W(2) mechanism for Ancistrus sp.2 "Barcelos". The XX/XY(1)Y(2) mechanism here reported is the second known occurrence of this type of multiple sex chromosomes for Loricariidae and the third for Neotropical fishes; the mechanism Z(1)Z(1)Z(2)Z(2)/Z(1)Z(2)W(1)W(2) represents the first record among fishes. The presence of different sex chromosome systems in Ancistrus indicates a probable independent origin and suggests that the differentiation of sex chromosomes is evolutionarily recent among species in this genus.     

A molecular assessment of species diversity in Tympanopleura and Ageneiosus catfishes (Auchenipteridae: Siluriformes)

In order to test the congruence of genetic data to the morphologically defined Neotropical catfish genera Tympanopleura and Ageneiosus and explore species diversity, we generated 17 DNA barcodes from five of six species of Tympanopleura and 12 of 13 species of Ageneiosus. To discriminate limits between species, an automatic barcode gap discovery (ABGD), a generalised mixed yule-coalescent model (GYMC) and fixed distance thresholds Kimura two-parameter (K2P; 3%) were used to discriminate putative species limits from the DNA barcodes. The ABGD, GMYC and K2P methods agreed by each generating 13 clusters: six in Tympanopleura (five nominal plus one undescribed species) and seven in Ageneiosus. These clusters corresponded broadly to the described species, except in the case of the Ageneiosus ucayalensis group (A. akamai, A. dentatus, A. intrusus, A. ucayalensis, A. uranophthalmus and A. vittatus). Haplotype sharing and low divergences may have prevented molecular methods from distinguishing these species. We hypothesise that this is the result of a recent radiation of a sympatric species group distributed throughout the Amazon Basin. One putative new species of Tympanopleura was also supported by the molecular data. These results taken together highlight the utility of molecular methods such as DNA barcoding in understanding patterns of diversification across large geographic areas and in recognising overlooked diversity.     

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.     

Insights into chromosomal evolution and sex determination of Pseudobagrus ussuriensis (Bagridae,Siluriformes) based on a chromosome-level genome

Pseudobagrus ussuriensis is an aquaculture catfish with significant sexual dimorphism. In this study, a chromosome-level genome with a size of 741.97 Mb was assembled for female P. ussuriensis. A total of 26 chromosome-level contigs covering 97.34% of the whole-genome assembly were obtained with an N50 of 28.53 Mb and an L50 of 11. A total of 24,075 protein-coding genes were identified, with 91.54% (22,039) genes being functionally annotated. Based on the genome assembly, four chromosome evolution clusters of catfishes were identified and the formation process of P. ussuriensis chromosomes was predicted. A total of 55 sex-related quantitative trait loci (QTLs) with a phenotypic variance explained value of 100% were located on chromosome 8 (chr08). The QTLs and other previously identified sex-specific markers were located in a sex-determining region of 16.83 Mb (from 6.90 to 23.73 Mb) on chr08, which was predicted as the X chromosome. The sex-determining region comprised 554 genes, with 135 of which being differently expressed between males and females/pseudofemales, and 16 candidate sex-determining genes were screened out. The results of this study provided a useful chromosome-level genome for genetic, genomic and evolutionary studies of P. ussuriensis, and also be useful for further studies on sex-determination mechanism analysis and sex-control breeding of this fish.     

New species of the Pseudancistrus barbatus group (Siluriformes,Loricariidae) with comments on its biogeography and dispersal routes

A new species of Pseudancistrus is described from the Tapajós Basin, and assigned to the P. barbatus group by having hypertrophied odontodes along the snout and lacking evertible cheek plates. The new species is distinguished from other species in that group (P. barbatus, P. corantijniensis, P. depressus and P. nigrescens) by its pattern of spots, length and color of snout odontodes, greater head depth, cleithral width, anal-fin spine length, peduncle depth and internares width. Molecular phylogenetic results corroborate placement of the new species in the Pseudancistrus barbatus group which is otherwise distributed in the Xingu Basin and rivers draining the Guyana Shield into the Atlantic Ocean. Topology tests strongly reject alternative hypotheses supporting close relationships with Guyanancistrus, Lithoxancistrus or the species Pseudancistrus pectegenitor, P. sidereus and P. genisetiger. Additionally, we propose two hypotheses on the distribution of the new species in the rio Tapajós, a Brazilian Shield drainage. The first one proposes that ancestral stock of the P. barbatus group was widely distributed throughout rivers draining the Guyana and Brazilian shields, and the species P. zawadzkii and Pseudancistrus sp. L17 are in the limit of the distribution for the group in Tapajós and Xingu rivers. The second hypothesis proposes that ancestral stock of the P. barbatus group was restricted to Guyana Shield rivers, and that headwater capture events permitted several dispersal routs through Guyana and Amazon rivers, permitted that the ancestral lineages of Pseudancistrus sp. L17 and P. zawadzkii reached the rivers of Amazon basin.     

Morphological and functional diversity of the mandible in suckermouth armored catfishes (Siluriformes: Loricariidae)

We examined the mandibles of 377 individuals representing 25 species, 12 genera, 5 tribes, and 2 subfamilies of the Loricariidae, a species‐rich radiation of detritivorous–herbivorous neotropical freshwater fishes distinguished by having a ventral oral disk and jaws specialized for surface attachment and benthic feeding. Loricariid mandibles are transversely oriented and bilaterally independent, each rotating predominantly around its long axis, although rotational axes likely vary with mandibular geometry. On each mandible, we measured three traditional and three novel morphological parameters chosen primarily for their functional relevance. Five parameters were linear distances and three of these were analogous to traditional teleost in‐ and out‐levers for mandibular adduction. The sixth parameter was insertion area of the combined adductor mandibulae muscle (AM_area), which correlated with adductor mandibulae volume across a subset of taxa and is interpreted as being proportional to maximum force deliverable to the mandible. Multivariate analysis revealed distributions of phylogenetically diagnosed taxonomic groupings in mandibular morphospace that are consistent with an evolutionary pattern of basal niche conservatism giving rise to multiple adaptive radiations within nested clades. Correspondence between mandibular geometry and function was explored using a 3D model of spatial relationships among measured parameters, potential forces, and axes of rotation. By combining the model with known loricariid jaw kinematics, we developed explicit hypotheses for how individual parameters might relate to each other during kinesis. We hypothesize that the ratio [AM_area/tooth row length²] predicts interspecific variation in the magnitude of force entering the mandible per unit of substrate contacted during feeding. Other newly proposed metrics are hypothesized to predict variation in aspects of mandibular mechanical advantage that may be specific to Loricariidae and perhaps shared with other herbivorous and detritivorous fishes. 2011. © 2011Wiley Periodicals, Inc.     

Karyotypic diversity of the armoured catfish genus Hypostomus (Siluriformes: Loricariidae) in the context of its occurrence and distribution

This study taxonomically reviewed the specimens studied by Artoni & Bertollo (1996) and assimilated species of Hypostomus into three groups according to their cytogenetic characteristics, vagility and occurrence environments.     

Molecular and morphometric data provide evidence of intraspecific variation in shape and pigmentation pattern in Otocinclus cocama (Siluriformes: Loricariidae) across major river drainages

Otocinclus cocama, a uniquely colored species of the loricariid catfish genus Otocinclus described solely from the type locality in the lower Ucayali River in northern Peru, is reported occurring in the Tigre River, a tributary to the Marañón River that drains a different section of the Andean Mountain range in the western Amazon. Both populations differ in the number of dark bars spanning the flanks of the body, and we investigated whether these morphotypes constitute distinct species. The body shapes of populations from the Tigre and Ucayali rivers were compared using geometric morphometrics. Although principal component analysis detected a broad overlap between populations, multivariate analysis of variance and linear driscriminat analysis revealed a subtle differentiation between the populations of the two hydrographic basins. Average body shape of the Ucayali River population tend to be slightly higher than that of the Tigre River, with the caudal peduncle stretched vertically in the Ucayali population. Multivariate regression of shape and centroid size revealed an allometric effect of 10.7% (p < 0.001), suggesting that the variation between Tigre and Ucayali populations was purely shape variation. Molecular data of coI, cytb, nd2, and 16S mitochondrial genes indicated a nucleotide diversity range from 0.001 to 0.003, and haplotypic diversity range from 0.600 ± 0.11 to 0.79 ± 0.07. The median-joining haplotype network for the concatenated matrix exhibited two divergent haplogroups related to the geographic area and separated by <10 mutational steps. The molecular species delimitation methods based on distance (automatic barcode gap discovery and assemble species by automatic partitioning) recovered two molecular lineages evolving independently, being one of the lineages formed by individuals from both populations. Tree-based methods (generalized mixed Yule coalescent and Bayesian implementation of the Poisson tree process) recovered similar topologies and supported single lineage recognition. Methods of molecular delimitation of species disclosed the high similarity between the two populations of Otocinclus cocama, further supported by the presence of old haplotypes common to both groups which could indicate that the populations still maintain gene flow. Although the morphological data reveal a subtle variation between both river basins, the molecular data suggest a weak population structuration based on hydrographic areas, but not different species lineages, therefore Otocinclus cocama is composed of a single lineage with two distinct morphotypes.     

Extensive chromosomal rearrangements and nuclear DNA content changes in the evolution of the armoured catfishes genus Corydoras (Pisces, Siluriformes, Callichthyidae)

Karyotypes and nuclear DNA content were studied in 11 species of the genus Corydoras from rivers in South America: C . sp. from Caripi river 2 n =60, C . cf. simulatus 2 n =62, C . simulatus 2 n =62, C . reticulatus 2 n =74, C . sp. from Galheiro river 2 n =84, C . aff. punctatus from Negro river 2 n =102, C . flaveolus 2 n =58, C . arcuatus 2 n =46, C . trilineatus 2 n =46, C . schwartzi 2 n =46, and C . metae 2 n =92. Extensive chromosome diversity and differences in DNA content were detected among species. The high variability in chromosome counts was not exclusively related to chromosomal structural rearrangements, but also to large changes in DNA content. Species could be grouped using their shared cytogenetic characteristics, suggesting that within the genus Corydoras different groups of species followed distinct evolutionary trends. Chromosomal rearrangements in Corydoras are, apparently, more frequent that morphological modifications, so cytogenetic data may be very useful for species delimitation and for the understanding of interrelationships among species.     

Cytogenetic analysis of three sea catfish species (Teleostei, Siluriformes, Ariidae) with the first report of Ag-NOR in this fish family

Despite their ecological and economical importance, fishes of the family Ariidae are still genetically and cytogenetically poorly studied. Among the 133 known species of ariids, only eight have been karyotyped. Cytogenetic analyses performed on Genidens barbus and Sciades herzbergii revealed that both species have 2n = 56 chromosomes and Cathorops aff. mapale has 2n = 52 chromosomes: Genidens barbus has 10 Metacentrics (M), 14 Submetacentrics (SM), 26 Subtelocentrics (ST), and 6 Acrocentrics (A), Sciades herzbergii has 14M, 20SM, 18ST and 4A, whereas Cathorops aff. mapale has 14M, 20SM, and 18ST. The nucleolus organizer regions (NORs) were found in a single chromosome pair on the short arm of a large-sized ST pair in Genidens barbus and on the short arm of a middle-size SM pair in Cathorops aff. mapale. Multiple NORs on the short arms of two large-sized ST pairs were found in Sciades herzbergii. The occurrence of diploid numbers ranging from 2n = 52 through 56 chromosomes and the presence of different karyotypic compositions, besides the number and position of NORs suggest that several numeric and structural chromosome rearrangements were fixed during the evolutionary history of this fish family.     

The erectile cheek-spine apparatus in the bristlenose catfish Ancistrus (Loricariidae, Siluriformes), and its relation to the formation of a secondary skull roof

In the South American catfish family Loricariidae, the opercle has been decoupled from the lower jaw, and has also lost its function in expiration. While many loricariid species have a small and slightly mobile opercle with reduced opercular musculature, within the hypostomine subfamily a novel opercular mechanism has developed that erects a tuft of enlarged odontodes anterior to the opercle. This defensive mechanism is examined in Ancistrus cf. triradiatus. The opercle has a prominent anterior process and the orientation of the reinforced articulation hinge to the hyomandibular bone has shifted. The opercular musculature is well developed, with a hypertrophied dilatator operculi that extends deep inside the skull roof bones and toward the midline, over the brain, but below the superficial skull roof. Hence the frontal, sphenotic, parieto-supraoccipital and compound pterotic bones consist of a dorsal, superficial part and a deeper part separating the brain from the muscle: two functional skull roofs are thus formed. The impact on the path of the cranial sensory canals is substantial, moving canals away from the skull surface. Hypertrophy of cranial muscles is known from many teleosts, but the invasion of such large muscles into the skull, which is drastically modified and literally hollowed out, has never been described before. These cranial modifications are greater in males than in females, related to the territorial behavior of the former, in which the erectile spines are usually used.     

Phylogenetic relationships of the suckermouth armoured catfishes (Loricariidae) with emphasis on the Hypostominae and the Ancistrinae

A phylogenetic analysis of nearly all genera of the Hypostominae and the Ancistrinae is provided based on osteology, external anatomy, and digestive tract anatomy. The results suggest that the Hypostominae is a paraphyletic assemblage. Delturus and Upsilodus form a monophyletic group sister to all other loricariids. Hemipsilichthys , Isbrueckerichthys , Kronichthys , and Pareiorhina form a monophyletic group with Neoplecostomus and the Hypoptopomatinae and are transferred to the Neoplecostominae. The remainder of the Hypostominae is made paraphyletic by the continuing recognition of the Ancistrinae. Ancistrinae is returned to the Hypostominae and recognized as a tribe, Ancistrini. In addition, four new tribes (Corymbophanini, Hypostomini, Pterygoplichthini, and Rhinelepini) are described. Hypostomus is also paraphyletic, the bulk of it forming a monophyletic clade with Aphanotorulus , Cochliodon , and Isorineloricaria . All of the potential monophyletic groups within Hypostomus grade into one another; therefore, Aphanotorulus , Cochliodon , and Isorineloricaria are placed in the synonymy of Hypostomus . Pterygoplichthys and Glyptoperichthys are also polyphyletic, and Liposarcus and Glyptoperichthys are recognized as synonyms of Pterygoplichthys . Sister to Pterygoplichthys is the Hemiancistrus annectens group (including Hypostomus panamensis ) which represents an undescribed genus. The phylogeny presented is compared with previous hypotheses.  © 2004 The Linnean Society of London, Zoological Journal of the Linnean Society , 2004, 141 , 1−80.     

Development of the chondrocranium in the suckermouth armored catfish Ancistrus cf. triradiatus (Loricariidae, Siluriformes)

The chondrocranium of the suckermouth armored catfish Ancistrus cf. triradiatus was studied. Its development is described based on specimens ranging from small prehatching stages with no cartilage visible, to larger posthatching stages where the chondrocranium is reducing. Cleared and stained specimens, as well as serial sections, revealed a cartilaginous skeleton with many features common for Siluriformes, yet several aspects of A. cf. triradiatus are not seen as such in other catfishes, or to a lesser extent. The skull is platybasic, but the acrochordal cartilage is very small and variably present, leaving the notochord protruding into the hypophyseal fenestra in the earlier stages. The ethmoid region is slender, with a rudimentary solum nasi. A lateral commissure and myodomes are present. The larger posterior myodome is roofed by a prootic bridge. The maxillary barbel is supported by a conspicuous cartilaginous rod from early prehatching stages. The ceratohyal has four prominent lateral processes. Infrapharyngobranchials I-II do not develop. During ontogeny, the skull lengthens, with an elongated ethmoid, pointing ventrally, and a long and bar-shaped hyosymplectic-pterygoquadrate plate. Meckel's cartilages point medially instead of rostrally.     

Evolutionary and biogeographic history of the subfamily Neoplecostominae (Siluriformes: Loricariidae)

Freshwater fish evolution has been shaped by changes in the earth's surface involving changes in the courses of rivers and fluctuations in sea level. The main objective of this study is to improve our knowledge of the evolution of loricariids, a numerous and adaptive group of freshwater catfish species, and the role of geological changes in their evolution. We use a number of different phylogenetic methods to test the relationships among 52 representative taxa within the Neoplecostominae using 4676 bps of mitochondrial and nuclear DNA. Our analysis revealed that the subfamily Neoplecostominae is monophyletic, including Pseudotocinclus, with three lineages recognized. The first lineage is composed of part of Pareiorhina rudolphi, P. cf. rudolphi, and Pseudotocinclus; the second is composed of Isbrueckerichthys, Pareiorhaphis, Kronichthys, and the species Neoplecostomus ribeirensis; and the third is composed of Pareiorhina carrancas, P. cf. carrancas, Pareiorhina sp. 1, a new genus, and all the species of the genus Neoplecostomus, except N. ribeirensis. The relaxed molecular clock calibration provides a temporal framework for the evolution of the group, which we use for a likelihood‐based historical biogeographic analysis to test relevant hypotheses on the formation of southeast Brazil. We hypothesize that headwater capture events and marine regressions have shaped the patterns of distribution within the subfamily Neoplecostominae throughout the distinct basins of southeast Brazil.     

Molecular systematic and historical biogeography of the armored Neotropical catfishes Hypoptopomatinae and Neoplecostominae (Siluriformes: Loricariidae)

The Neotropics possess the greatest freshwater fish diversity of the world, rendering the study of their evolutionary history extremely challenging. Loricariidae catfishes are one of the most diverse components of the Neotropical ichthyofauna and despite a long history of classification, major issues still need elucidation. Based on a nuclear gene, we present a robust phylogeny of two former loricariid subfamilies: Hypoptopomatinae and Neoplecostominae. Our results show that Neoplecostominae is nested within Hypoptopomatinae, and is the sister group to the former Otothyrini tribe. According to our results, supplemented by morphological observations, we erect two new subfamilies, the Otothyrinae and a new Hypoptopomatinae, and modify the Neoplecostominae by including the genus Pseudotocinclus. The uncovered evolutionary relationships allow a detailed analysis of their historical biogeography. We tested two Dispersal-Extinction-Cladogenesis models for inferring the distribution range evolution of the new subfamilies, and show that the model having no constrains performs better than a model constraining long-range dispersal. The Maximum Likelihood reconstructions of ancestral ranges showed a marked division between the Amazonian origin of the Hypoptopomatinae and the eastern coastal Brazil+Upper Paraná origin of the Neoplecostominae and Otothyrinae. Markedly few instances of dispersal across the border separating the Amazon basin and the Paraná-Paraguay+eastern coastal Brazil+Uruguay were reconstructed. This result is in clear contrast with the historical biogeography of many Neotropical fishes, including other Loricariidae. Part of the dispersal limitation may be explained by divergent ecological specialization: lowland rivers versus mountain streams habitats. Moreover, because most species of the new subfamilies are small, we hypothesize that body size-related effects might limit their dispersal, like predation and energetic cost to migration. Finally, morphological and anatomical features are presented that limit or, to the contrary, enhance dispersal capability in these small and fascinating catfishes.     

A new species of Chaetostoma (Siluriformes: Loricariidae) from the Orinoco basin with comments on Amazonian species of the genus in Colombia

Chaetostoma chimu sp. nov. is described from 119 specimens collected in the Andean foothills of the Orinoco River in Colombia. The new species can be distinguished from congeners by a combination of characters, including shape of cheek odontodes, presence of a supraoccipital excrescence, sexual dimorphism characters and colour pattern. In addition, this species can be distinguished from the sympatric and geographically close congeners of the Orinoco that have curved cheek odontodes (i.e., Chaetostoma anale, Chaetostoma formosae and Chaetostoma joropo), by presenting a generally larger orbital diameter and interorbital distance, details of sexual dimorphism and colour pattern. The type locality of specimens collected by Kjell von Sneidern in the mid-twentieth century in the Colombian Amazon is also clarified. Chaetostoma alternifasciatum and Chaetostoma vagum are proposed as junior synonyms of C. anale. An identification key for the species of Chaetostoma from cis-Andean drainages in Colombia is provided. The new species is herein categorized as Vulnerable (Vu) B1a (i, iii), following the IUCN criteria.     

Shape variation in surface and cave populations of the armoured catfishes Ancistrus(Siluriformes: Loricariidae) from the São Domingos karst area, upper Tocantins River, Brazil

Body shape of four distinct populations of Ancistrus cryptophthalmus from different caves in the São Domingos karst area and the local epigean (surface) Ancistrus species were compared using geometric morphometrics. Angélica and Bezerra catfishes were closer to each other and successively to the studied sample of epigean catfishes than to Passa Três and São Vicente I catfishes. Greater similarity between catfishes from the Angélica‐Bezerra cave system to epigean catfishes than to other cave catfishes may represent a plesiomorphic feature. On the other hand, the present analyses allowed the distinction between Passa Três and São Vicente I catfishes, which inhabited the same cave system. Topographic isolation due to the presence of several waterfalls interposed between the stream reaches where these catfishes live may explain such morphological differentiation. Also, the presence of a waterfall downstream of the reach where the Angélica sample was obtained may have contributed to the partial isolation of these catfishes from those living in Bezerra Cave. Differences in population sizes probably influenced the degrees of morphological differentiation observed among the cave populations. The mosaic distribution of character states indicated that the four populations of A. cryptophthalmus have evolved rather independently, but the continuous variation suggests a recent connection between these populations, perhaps with a limited gene flow. Considering other features such as reduction of eyes and pigmentation, and changes in body shape, a separate species status for the epigean and hypogean Ancistrus species from the São Domingos karst area has been maintained.     

Population differentiation and speciation in the genus Characidium (Characiformes: Crenuchidae): effects of reproductive and chromosomal barriers

Both time and low gene flow are the key factors by which different biological species arise. The divergence process among lineages and the development of pre‐ or postzygotic isolation occur when gene flow events are lacking. The separation among species of the genus Characidium was analysed in relation to the geomorphological mechanisms in river courses, events of captured adjacent upland drainages in south‐eastern Brazil, and sex chromosome differences. The ZZ/ZW sex chromosomes of Characidium vary in size, morphology, degree of heterochromatinization, and presence/absence of ribosomal DNA. The goal of this study was to understand the mechanism of sex chromosome differentiation, its close association with the geological history of cladogenetic events among drainages, and reproductive isolation leading to Characidium speciation. The W‐specific probe from Characidium gomesi generated a highlighted signal on the entire W chromosome of C. gomesi, Characidium heirmostigmata, Characidium pterostictum, and Characidium sp., instead of karyotypes of three Characidium aff. zebra populations, which showed scattered signals. An evolutionary and biogeographic landscape arose by analysis of ribosomal DNA site location and differentiation of the sex chromosomes, which established mechanisms of reproductive isolation leading to meiotic barriers, keeping the biological unit distinct even if the contact among species was restored. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 541–553.     

Heteromorphic sex chromosomes of lizardfish (Synodontidae): focus on the ZZ-ZW1W2 system in Trachinocephalus myops

The karyotype and DNA content of four lizardfish species (family Synodontidae), that is, Saurida elongata, Synodus ulae, Synodus hoshinonis and Trachinocephalus myops, were analyzed. The karyotype of T. myops significantly differed from that of the other three species having diploid chromosome number of 48 with mainly acrocentric chromosomes and the ZZ-ZW sex chromosome system. The chromosome number of male T. myops was 2n=26, while that of female T. myops was 2n=27. The karyotype consisted of 11 pairs of metacentrics, one pair of acrocentrics and, in addition, two large metacentrics in the male and a single large metacentric, a distinctly small subtelocentric and a microchromosome in the female. C-banding demonstrated that in the female the subtelocentric chromosome and the microchromosome were heterochromatic. The karyotype of T. myops was thought to be derived from a 48 chromosome type synodontid fish through the involvement of Robertsonian rearrangement; the rearrangement of the sex chromosomes proceeded during karyotype evolution. Among the chromosomes, the large metacentrics were determined to be neo-Z (a fusion of the original Z and an autosome), the microchromosomes the W₁ (originally W), and the subtelocentric chromosomes the W₂ (derived from an autosome pair). The miniaturization of W₁ and W₂ chromosomes and their heterochromatinization suggested that sex chromosomes in this species have been already highly differentiated. The findings on DNA content implied that the karyotype of T. myops evolved by centric fusion events without loss in DNA amount.