Research Article |
Corresponding author: Patricia Pasquali Parise-Maltempi ( parise@rc.unesp.br ) Academic editor: Inna Kuznetsova
© 2015 Diovani Piscor, Patricia Pasquali Parise-Maltempi.
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:
Piscor D, Parise-Maltempi PP (2015) First description of B chromosomes in the Hyphessobrycon (Characiformes, Characidae) genus: a hypothesis for the extra element of Hyphessobrycon eques Steindachner, 1882. Comparative Cytogenetics 9(3): 325-333. https://doi.org/10.3897/CompCytogen.v9i3.5224
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The Hyphessobrycon are allocated in the incertae sedis group of the Characidae family, one of the genera with more species of the group. The chromosomes of some species of Hyphessobrycon are known, and the diploid number most common for genus is 2n = 50 chromosomes. The aims of this study were to examine the karyotype macrostructure in the Hyphessobrycon eques Steindachner, 1882, and show a new origin hypothesis for B chromosomes. The diploid number observed for H. eques was 2n = 52 chromosomes, and a karyotype formulae of 12m + 18sm + 8st + 14a, with FN (fundamental number) = 90 for both sexes. Only two females showed one B chromosome. The heterochromatin was observed mainly on centromeric regions, and in the long arm of the B chromosome. In this paper, the relationship of the B chromosome of H. eques with an occasional chromosome rearrangement was discussed.
Karyotype, supernumerary chromosomes, C-banding, heteromorphism, chromosome evolution
The Hyphessobrycon are allocated in the incertae sedis group of the Characidade family (
The chromosomal data of the Hyphessobrycon genus are restricted primarily to the knowledge of the diploid number. Literature data showed that the diploid number vary between 2n = 42 and 52 chromosomes, being 2n = 50 chromosomes the most frequently observed number for the genus, i.e. Hyphessobrycon scholzei Ahl, 1937 (
The B chromosomes have been described in many neotropical fish groups (see, for example,
Whereas the diversity of events described in an attempt to explain the origin and function of B chromosomes, the present study aims to demonstrate the probable origin of B chromosome in Hyphessobrycon eques through the study of heterochromatin, and describe for the first time the presence of an extra element in the Hyphessobrycon genus.
The H. eques (seven males and four females) specimens were obtained from Ribeirão Claro river (22°21'36"S, 47°30'42"W) in the state of São Paulo (SP), Brazil. The individuals were anesthetized with benzocaine (5%) and then used for cytogenetic analysis. The individuals were fixed in formaldehyde 10% and then in ethanol 70%, and placed in the ichthyological collection from Departamento de Biologia do Instituto de Biociências da UNESP, campus de Rio Claro. The chromosomes were obtained as described by
The H. eques specimens had 2n = 52 chromosomes, and the karyotype contained 12 metacentric, 18 submetacentric, 8 subtelocentric, and 14 acrocentric chromosomes (12m + 18sm + 8st + 14a), yielding a FN of 90 for both sexes (Figure
Giemsa stained chromosomes of H. eques. A Karyotype without B chromosome B Karyotype with B chromosome. Inset show the B chromosome. Bar = 10 µm.
Mitotic metaphase chromosomes. A Giemsa stained B C-banding. The arrow indicates the B chromosome and the arrowhead indicates the secondary constriction. Inset show the pair 19 C-banded of an individual without B chromosome. Bar = 10 µm.
Cytogenetic data and presence of B chromosomes in the Hyphessobrycon genus.
Species | 2n | Karyotype formulae | Presence of Bs | References |
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H. minor | 52 | 14m+20sm+16st | – |
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H. scholzei | 50 | 8m+20sm+8st+14a | – |
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H. flammeus | 52 | 18m/sm+32st+2a | – |
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H. herbertaxelrodi | 52 | 10m/sm+42st/a | – |
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H. reticulatus | 50 | 20m+14sm+16st/a | – |
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H. bifasciatus | 50 | 16m+10sm+12st+12a | – |
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H. aff. santae | 50 | 12m+10sm+10st+18a | – |
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H. reticulatus | 50 | 14m+20sm+16st | – |
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H. reticulatus | 50 | - | – |
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H. griemi | 48 | - | – |
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H. anisitsi | 50 | 6m+16sm+12st+16a | – |
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H. anisitsi | 50 | 18m+10sm+6st+16a | – |
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H. luetkenii | 50 | 6m+8sm+36a | – |
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H. eques | 52 | 14m+16sm+4st+18a | – |
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H. eques | 52 | 12m+18sm+8st+14a | 0–1♀/0♂ | Present study |
A summary diagram indicating a possible origin mechanism of the B chromosome in H. eques by heterochromatin blocks is shown in Figure
The heterochromatin was observed mainly in the centromeric regions on chromosomes of H. eques in this present paper. On the other hand,
An interesting feature observed by C-band technique in the H. eques was a heteromorphic block of heterochromatin always presents on short arm (pair 19) in all specimens (with and without B chromosomes), which another population of H. eques studied by
This study reported for the first time the presence of B chromosomes in the Hyphessobrycon genus. According to
Different postulations have been formulated to explain the independent evolution of B chromosomes in the genome of organisms that possess them.
One hypothesis proposed to explain the presence and function of the B chromosomes is the isochromosome (
Nevertheless, the presence of one B chromosome in females may be less likely due to the sex chromosome system in the H. eques (even if only one sex) than involved with possible chromosomal break. However, we cannot rule out the possibility that this occasional chromosome break, from now on, may have resulted in the maintenance of this element in the females and drive to differentiation of a sex chromosome system for H. eques.
The authors are grateful to Coordenadoria de Aperfeiçoamento de Ensino Superior (CAPES) for the financial support.