Short Communication |
Corresponding author: Fabio Porto-Foresti ( fp.foresti@unesp.br ) Academic editor: Grazyna Furgala-Selezniow
© 2015 Manolo Penitente, Fausto Foresti, Fabio Porto-Foresti.
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:
Penitente M, Foresti F, Porto-Foresti F (2015) B chromosomes in the species Prochilodus argenteus (Characiformes, Prochilodontidae): morphologicalidentity and dispersion. Comparative Cytogenetics 9(1): 79-87. https://doi.org/10.3897/CompCytogen.v9i1.8587
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B chromosomes have attracted the attention of Neotropical fish cytogeneticists in recent years, both for their remarkable occurrence in this group and also because of the interest in studies of the genetic structure and role played in the genome of these organisms. The aim of this study was to report the first occurrence of supernumerary chromosomes in Prochilodus argenteus (Agassiz, 1829), this being the fifth carrier species among thirteen within the genus Prochilodus (Agassiz, 1829). The extra elements identified in this species are small sized heterochromatic chromosomes characterized by a low mitotic instability index, being very similar to other supernumerary chromosomes described in the species of the genus Prochilodus. Morphology, structure and dispersion of the supernumerary genomic elements which occur in species of this genus are discussed aiming to better understand aspects involved the origin of supernumerary chromosomes and the differentiation process and relationships among species of this family.
Curimbatá, cytogenetic, neotropical fish, supernumerary chromosomes
Among the Characiformes fish, representatives of the family Prochilodontidae are distinguished by their abundance in the environments in which they occur, by their wide distribution in South America and the high migratory capacity displayed by the species of this group of organisms (
Besides the conservative karyotype formulae found among representatives of the genus Prochilodus, variation in the chromosome number can be observed among species due to the occurrence of supernumerary chromosomes. Among seven species of this genus, the occurrence of B chromosomes has been identified and described in four species, present in Prochilodus lineatus Valenciennes, 1836 (
In this paper we describe for the first time the occurrence of supernumerary chromosomes in P. argenteus, identifying its similarity with the extra genomic elements characteristic of this group. The description of the occurrence of supernumerary chromosomes in this species, besides addressing structural and morphological aspects, has also broadened the knowledge of aspects involved in the differentiation, evolution and relationships among species of the genus Prochilodus.
In this study 29 individuals of the species P. argenteus (Table
B chromosome frequency and Mitotic Instability index (MI) of somatic cells in P. argenteus.
Specimen identification | Number of B per cell | MB | N | MI | |||
---|---|---|---|---|---|---|---|
0B | 1B | 2B | 3B | ||||
4170 | 30 | - | - | - | 0B | 30 | 0.000 |
4172 | 1 | 12 | - | - | 1B | 13 | 0.157 |
4173 | 30 | - | - | - | 0B | 30 | 0.000 |
4175 | 30 | - | - | - | 0B | 30 | 0.000 |
4176 | 3 | 27 | - | - | 1B | 30 | 0.200 |
4177 | 1 | 29 | - | - | 1B | 30 | 0.034 |
4178 | - | 1 | 29 | - | 2B | 30 | 0.036 |
4180 | 5 | 25 | - | - | 1B | 30 | 0.337 |
4181 | - | 30 | - | - | 1B | 30 | 0.000 |
4182 | - | 2 | 28 | - | 2B | 30 | 0.065 |
4183 | 2 | 12 | - | - | 1B | 14 | 0.294 |
4184 | - | 15 | - | - | 1B | 15 | 0.000 |
4188 | 30 | - | - | - | 0B | 30 | 0.000 |
4189 | 30 | - | - | - | 0B | 30 | 0.000 |
4190 | 1 | 15 | - | - | 1B | 16 | 0.133 |
4250 | 4 | 26 | - | - | 1B | 30 | 0.274 |
4251 | 30 | - | - | - | 0B | 30 | 0.000 |
4252 | - | 14 | - | - | 1B | 14 | 0.000 |
4253 | 30 | - | - | - | 0B | 30 | 0.000 |
4254 | 1 | 11 | - | - | 1B | 12 | 0.173 |
4256 | - | 2 | 15 | - | 2B | 17 | 0.111 |
4257 | 25 | - | - | - | 0B | 25 | 0.000 |
4258 | - | 1 | 13 | - | 2B | 14 | 0.072 |
4259 | 30 | - | - | - | 0B | 30 | 0.000 |
4260 | 2 | 28 | - | - | 1B | 30 | 0.136 |
4261 | - | - | 4 | 26 | 3B | 30 | 0.082 |
4262 | 3 | 15 | - | - | 1B | 18 | 0.337 |
4263 | 30 | - | - | - | 0B | 30 | 0.000 |
4264 | 1 | 13 | - | - | 1B | 14 | 0.152 |
XMI | 0,136 |
Chromosome preparations involved previous use of mitosis stimulation (
Chromosomal mapping of ribosomal genes was performed with the technique of fluorescent in situ hybridization (FISH) according to
Cytogenetic analysis performed on specimens of Prochilodus argenteus revealed the diploid number of 2n = 54 and fundamental number of 108 for this species, with a karyotype composed of meta and submetacentric chromosome types (Figure
Karyotype of P. argenteus (2n=54 chromosomes). In the inset, three B chromosomes and the NOR bearing pair.
Metaphase plates of P. argenteus showing a cell of individual without B chromosomes (a); in individual presenting one B chromosome (b); cell presenting two B chromosomes (c); and cell presenting tree B chromosomes indicated by arrows (d). Bar = 10 µm.
One of the first descriptions of B chromosomes in Neotropical fish refers to additional chromosomes found in P. lineatus by
The supernumerary genomic elements found in this species can be easily identified in metaphasic cells due to their small size when compared with the chromosomes of the standard complement, usually heterochromatic, but presenting diverse and complex patterns of heterochromatin distribution (Figure
C-banding in metaphases plates of P. argenteus showing the distribution of heterochromatin in B chromosomes. Different cells showing one B chromosome (a); two B chromosomes (b); and tree B chromosomes indicated by arrows (c). Bar = 10 µm.
Fluorescent in situ hybridization using probes of 5S (green) and 18S (red) rDNA in metaphasic cells of P. argenteus, carrying one B chromosome (a) and two B chromosomes (b), indicated by arrows. FISH marked segments are visualized only on the NOR bearing chromosomes. Bar = 10 µm.
An interesting feature of B chromosomes present in the carrier species of this genus is the identification as completely or partially heterochromatic microchromosomes, which exhibit expressive inter and intra-individual number variation (Table
The occurrence of homoplasy could be considered to explain the origin of morphologically similar genomic elements in species of the same biological group. Thus, the existence of specific chromosomes bearing structural elements capable of originating extra chromosomes in an ancestral form could act in an independent way and give rise to supernumerary chromosomes found today in some of these species. However, it can be also considered the idea of a common origin of these elements in all species of the genus from an ancestral carrier, followed by the loss in some species during the diversification process.
The rate of mitotic instability (MI) calculated from individuals with supernumerary revealed a mean value of 0.136 (Table
The great similarity found in morphology, size, heterochromatic nature and frequency of B chromosomes of P. argenteus with those of other species in the genus Prochilodus, described as microchromosomes and, in general, totally heterochromatic (
The authors are grateful to Suely Marlene Rodrigues from Projeto Peixe fish farm for providing fish samples used in the present work. Funds supporting this study were provided by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP).