Corresponding author: Fábio Hiroshi Takagui ( firstname.lastname@example.org )
Academic editor: Irina Bakloushinskaya
© 2017 Fábio Hiroshi Takagui, Ana Lucia Dias, José Luís Olivan Birindelli, Ana Claudia Swarça, Renata da Rosa, Roberto Laridondo Lui, Alberto Sergio Fenocchio, Lucia Giuliano-Caetano.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation: Takagui FH, Dias AL, Birindelli JLO, Swarça AC, Rosa R, Lui RL, Fenocchio AS, Giuliano-Caetano L (2017) First report of B chromosomes in three neotropical thorny catfishes (Siluriformes, Doradidae). Comparative Cytogenetics 11(1): 55-64. https://doi.org/10.3897/CompCytogen.v11i1.10496
The family Doradidae (Siluriformes) is an important group of fishes endemic to freshwater ecosystems in South America. Some cytogenetic studies have been conducted focused on the group; however, there are no reports on the occurrence of B chromosomes for the family. In this paper the chromosomal characteristics of Platydoras armatulus (Valenciennes, 1840), Pterodoras granulosus (Valenciennes, 1821) and Ossancora punctata (Kner, 1855) were investigated through classical cytogenetics approaches. The conventional staining reveals 2n=58 in Platydoras armatulus and Pterodoras granulosus, however with distinct karyotypic formulae, possibly originated by pericentric inversions. In Ossancora punctata a derivate karyotype was described with 2n=66 and predominance of acrocentric chromosomes. The C banding pattern was resolutive in discriminating the three species, being considered an important cytotaxonomic marker. All species showed B chromosomes totally heterochromatic with non-Mendelian segregation during meiosis and low frequencies in mitotic cells. The probably origin of these additional elements was through fragmentations of chromosomes of the standard complement, which occurred recently and independently in these three species. The diploid number observed in O. punctata is an evidence of centric fusions and up to the moment it is the highest diploid number reported for Doradidae.
Centric fusion, chromosomal rearrangements, diploid number, neotropical fish, pericentric inversions, supernumerary chromosome
Cytogenetic studies in Doradidae are scarce and restricted to nine species. Eight of these bear 58 chromosomes and single nucleolus organizer regions (NORs) in terminal positions (
Supernumerary chromosomes have already been reported in several neotropical Siluriformes families, however up to now they have not been observed in Doradidae (
In most organisms, the B chromosomes are dispensable elements, as their presence is not associated with phenotypic alterations. However, there are exceptions, as described in Nectria haematococca Samuels & Rossman, 1999 where the Bs possess resistance genes which grant a better pathogenicity (
In the present study, the karyotypic structure of Platydoras armatulus, Pterodoras granulosus and Ossancora punctata was investigated in mitotic and meiotic cells. This comparative analysis to provide a better understanding of the karyotype diversification in Doradidae, reporting for the first time the occurrence of B chromosomes and discussing the probably origins of this feature in this family.
In the present study, cytogenetic analyses were performed on 9 females and 8 males of Platydoras armatulus; 3 females and 6 males of Ossancora punctata, all collected in the Miranda river, in Corumbá, Mato Grosso do Sul, in the Brazilian Pantanal (19°31'25"S 57°02'26"W). Additionally, 5 females and 4 males of Pterodoras granulosus also collected in the Paraná river, in Pauliceia, São Paulo, Brazil (21°06'10.26"S 51°47'14.1"W), were analyzed. The collection of specimens was authorized by ICMBio (Instituto Chico Mendes de Conservação da Biodiversidade). After processing and subsequent fixation of the material, all specimens were deposited in the Museu de Zoologia da Universidade Estadual de Londrina (data available via SpeciesLink).
Before euthanasia (48 hours), the specimens received an intraperitoneal injection of 2 ml of Broncho-vaxom (bacterial lysate) to trigger an inflammatory process and hence increase the number of kidney cells in mitotic division (
All specimens analyzed exhibited 58 chromosomes (22m + 14sm + 18st + 4a). Eleven samples showed cells carrying from 1-3 B chromosomes (Fig.
Karyotypes after conventional staining and C-banding – Platydoras armatulus a Giemsa staining reveals 2n=58 (22m+14sm+18st+4a) and 1-3 B chromosomes b C-banding pattern characterized by the many heterochromatin blocks in different positions, including in B chromosomes. Pterodoras granulosus c Giemsa staining also reveals 2n=58 but with distinct karyotypic formulae: 16m+16sm+14st+12a and 1 B chromosome d a few heterochromatin blocks was evidenced after C banding, observe the B chromosome totally heterochromatic. Ossancora punctata e After Giemsa staining it was observed 2n=66 (12m+8sm+6st+40a), the high number of subtelocentric and acrocentric chromosomes is a remarkable feature of this specie f C-banding reveals heterochromatin regions in terminal position and in B chromosomes.
Meiotic cells in different phases with B chromosomes evidenced after C-banding - Platydoras armatulus a spermatogonial metaphase with 58 chromosomes and 2 B chromosomes b late pachytene with bivalents in advanced condensation stage, note two B chromosomes forming univalents without homologies of standard complement c metaphase I with 27 bivalents and two B chromosomes. Pterodoras granulosus d spermatogonial metaphase composed by 58 chromosomes and one B chromosome e late pachytene, the isolated univalent probably correspond to B chromosome f diplotene/diakinesis with 27 bivalents and one B chromosome, note the high number of chiasms. Ossancora punctata g spermatogonial metaphase with 66 chromosomes and one B chromosome h metaphase I reveals heterochromatic B chromosome and 33 bivalents i anaphase I, observe the late segregation of B chromosomes.
Frequencies of supernumerary chromosomes in Platydoras armatulus, Pterodoras granulosus and Ossancora punctata. ♀= female; ♂= male.
|Species/Samples||Sex||Number of B/cell||Total of cells||Cells with B|
Conventional staining with Giemsa revealed 58 chromosomes, with a karyotype formula 16m + 16sm + 14st + 12a. In six specimens, one acrocentric supernumerary chromosome was observed (Fig.
The studied specimens presented 66 chromosomes and a karyotype formula of 12m + 8sm + 6st + 40a. Among all nine specimens analyzed, seven exhibited B chromosomes (Fig.
Phylogenetic analysis based on morphological and molecular data supports the monophyly of Doradidae that, together with Auchenipteridae, constitutes the superorder Doradoidea (
Notwithstanding, not all doradid species have 2n = 58, as is the case of Trachydoras paraguayensis with 2n = 56 chromosomes (
The dispersion of heterochromatic regions is a high variable in Doradidae. Pterodoras granulosus exhibited few blocks, similarly described for Platydoras costatus Linnaeus, 1758 (
Cytogenetic studies in Neotropical Siluriformes revealed the occurrence of B chromosomes in more than 25 species, including representatives of the families Heptapteridae, Callichthyidae, Pimelodidae, Pseudopimelodidae, Auchenipteridae, Tricomycteridae and Loricariidae (
In Neotropical fish, the mechanisms responsible for the origin and evolution of B chromosomes remain unclear, as several theories were proposed (
This study contributed with relevant information to the better understanding of the karyotype variability in Doradidae. In this family, the 2n=58 is considered a primitive condition, such that the chromosomal diversification is based primarily on pericentric inversions and at lower frequency fissions and fusions. Additionally, the mitotic and meiotic analysis revealed at the first time in Doradidae the occurrence of B chromosomes, which originated recently from fragmentations in chromosomes of standard complement. Additional studies such as the isolation and molecular characterization of these chromosomes can be resolutive in confirming its origin and evolution.
The authors are grateful to the Universidade Estadual de Londrina (UEL), Centro de Ciências Biológicas (CCB), Departamento de Biologia Geral for providing the laboratory infrastructure to carry out this work; CAPES and Fundação Araucária (JLB,641/2014) for their financial support; and ICMBio (Instituto Chico Mendes de Conservação da Biodiversidade), for permitting the collection of biological material.