Research Article |
Corresponding author: Carolina Labaroni ( carolinalabaroni@gmail.com ) Academic editor: Irina Bakloushinskaya
© 2016 Matías Malleret, Carolina Labaroni, Gabriela Verónica García, Juan Ferro, Dardo Andrea Marti, Cecilia Lanzone.
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:
Malleret MM, Labaroni CA, García GV, Ferro JM, Andrea Martí D, Lanzone C (2016) Chromosomal variation in Argentine populations of Akodon montensis Thomas, 1913 (Rodentia, Cricetidae, Sigmodontinae). Comparative Cytogenetics 10(1): 129-140. https://doi.org/10.3897/CompCytogen.v10i1.6420
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The genus Akodon Meyen, 1833 is one of the most species-rich among sigmodontine rodents and has great chromosome variability. Akodon montensis has a relatively broad distribution in South America, and Argentine populations are located in the southernmost region of its range. Brazilian populations have important chromosomal variability, but cytogenetic data from Argentina are scarce. We performed a chromosome characterization of natural populations of A. montensis using conventional staining, C-banding, Ag-NORs and base-specific fluorochromes. A total of 31 specimens from five localities of Misiones Province, in Argentina, were analyzed. The 2n=24 chromosomes was the most frequently observed karyotype. However, five individuals presented 25 chromosomes due to a supernumerary B-chromosome; and one individual had 2n=26 due to one B plus a trisomy for chromosome 11. Additionally, two XY females and two variants of the X chromosomes were found. C-positive centromeric bands occurred in all chromosomes; additional C-bands were observed in some autosomes, the X, Y and B chromosomes. Ag-NORs were observed in five autosomes, and the B chromosome was frequently marked. Fluorochrome banding was similar among karyotypes of the analyzed populations. Comparisons of cytogenetic data among populations of Argentina and Brazil showed the presence of high intraspecific variability in A. montensis and some differences among regions.
Rodents, karyotype variability, chromosome banding, heterochromatin, Ag-NORs
The genus Akodon Meyen, 1983, with about 41 species, is considered one of the most species-rich group within the subfamily Sigmodontinae. Its species are widely distributed in South America and inhabit a variety of habitats, among subtropical and tropical moist forest as well as desert regions (
Akodon montensis is an abundant species distributed in Argentina, Brazil and Paraguay, and has a great chromosomal variability (
In order to contribute to the knowledge of karyotypic variability in A. montensis we analyzed specimens from different localities of Misiones Province, Argentina, which is a part of the southernmost area of the range (
A total of 31 specimens of Akodon montensis (18 females and 13 males) were collected from five localities of Misiones Province, Argentina (Fig.
Map indicating a collection sites of Akodon montensis in the province of Misiones, Argentina analyzed in this work: A and B Iguazú C Parque Provincial Urugua-í D Puerto Esperanza E San Ignacio F Candelaria b different localities in Brazil and Argentina where A. montensis has been studied previously at cytogenetic level; Brazil: 1 Boracéia 2 Sumidouro 3 Nova Friburgo 4 Teresópolis 5 Petrópolis 6 S. J. do Barreiro 7 Taubaté 8 Caçapava 9 Salesópolis 10 Guararema 11 Itapetininga 12 Iguape 13 Quatro Barras 14 Tres barras; Argentina: 15 Misiones, Urugua-í.
All individuals of Akodon montensis had an autosome complement composed of nine pairs of large to medium size metacentric chromosomes, and two small-sized pairs, one acrocentric and one metacentric. The sex chromosome pair is XX/XY (Fig.
Mitotic chromosomes of A. montensis: a Giemsa stained karyotype of a male with 2n=24; FN=42 b the trisomy for pair 11 c Giemsa stained B chromosome d C-banded karyotype of a male, in the boxes pairs with telomeric C-bands are showed e C-band pattern of Xs-Xa sex chromosomes f Ag-NORs bearing chromosomes g, i karyotypes of a female with DAPI/ CMA3 fluorochrome staining respectively h, j DAPI/CMA3 fluorochrome pattern of sex chromosomes of a male. Bar = 10 µm.
Twenty-five individuals (fourteen females and eleven males) presented a karyotype with 2n=24 and FN=42 (Fig.
Sampling localities of Akodon montensis analyzed in this work. Geographical coordinates, N=number of individuals indicating females (F) and males (M), 2n=chromosome number, sex chromosomes morphology for the X (Xa=acrocentric, Xs=subtelocentric, the number of individuals with each genotype are indicated in bracket), and frequency of B chromosome in each locality (FB).
Locality (Lat/Long) |
N | 2n | Sex Chromosome types | FB | |
---|---|---|---|---|---|
24 | 25 (24+B) | ||||
Iguazú (25°42.08'S; 54°20.68'W) |
10F | 8 | 2 | XaXa(7) | 0.13 |
XsXa (2) | |||||
XsY (1)** | |||||
6M | 6 | - | XaY (6) | ||
San Ignacio (27°16.88'S; 55°34.72'W) |
2F | 1 | 1 | XsXa (1) XaXa (1) |
0.25 |
2M | 2 | - | XaY (2) | ||
Puerto Esperanza (25°59.23'S; 54°38.85'W) |
4F | 3 | 1 | XsXa (3) XsXs (1) |
0.25 |
Urugua-í (25°51.33'S; 54°10.02'W) |
1F | 1 | - | XaXa (1) | 0.20 |
4M | 3 | 1 | XaY (4) | ||
Candelaria (27°22.79'S; 55°38.54'W) |
1F | 1 | - | XaY (1)** | 0.50 |
1M | - | 1* | XaY (1) | ||
Total | 31 | 25 | 6 | - | 0.19 |
The Y chromosome was small acrocentric. The X was a medium-sized chromosome and showed two morphological variants: acrocentric (Xa) observed on both sexes, and subtelocentric (Xs) detected only for females (Figs
Variants of sex chromosomes in the females of A. montensis with Giemsa staining: a homozygous acrocentrics b acrocentric and subtelocentric c homozygous subtelocentrics d heterogametic sex chromosomes. Bar = 10 µm.
Positive C-band (C+) were found in the pericentromeric region of pairs 1 to 11, and at the telomeres of pairs 3, 6, 8 and 10 (Fig.
B chromosome of A. montensis: a Giemsa staining b C-banding, pericentromeric and interstitial C-bands c silver nitrate staining with Ag-NORs in both telomeric ends (C1) and single in the end of the short arm (C2) d, e DAPI/ CMA3 fluorochrome stained respectively. Bar = 10 µm.
Ag-NORs were evident in the distal position of pairs 2, 4, 6, 7 and 10 (Fig.
The banding pattern with DAPI/CMA3 was similar in all specimens and varied among chromosomes (Fig.
Meiotic cells of a male with 2n=24 showed 11 autosomal bivalents during diakinesis and one sex bivalent, which was recognized by its differential pyknosis, size and shape. From 30 studied cells in the specimen with trisomy with a B chromosome (2n=26), the three chromosomes 11 were observed either a trivalent (14/30) or as one bivalent plus a univalent (16/30) (Fig.
Diakinesis cells of an individual with trisomy and one B chromosome: a 10 autosomal bivalents, plus a trivalent (III) of chromosome 11, the sex pair XY and the supernumerary chromosome as univalent (B) b Note the presence of pair 11 as one bivalent (II) plus a univalent (I) and X chromosome dissociated of Y. Bar = 10 µm.
The studied populations of Akodon montensis from Brazil showed high chromosome variability (
Constitutive heterochromatin (CH) is in mammals, and particularly in rodents, an important source of karyotype variability (
Patterns of fluorescent bands DAPI/CMA3 are comparable to G- and R-banding respectively (
The XX/XY sex chromosome system is the most common among mammals, being males heterogametic and females homogametic. However, certain species depart from this pattern (
In Brazil and Argentina, two morphologies for the X chromosome were observed: acrocentric and subtelocentric (
Sex chromosomes of several rodents showed variation in the amount and distribution of heterochromatin (
B chromosomes (Bs) appear as supernumerary elements to the standard chromosome complement and are highly variables (
In A. montensis the B chromosome showed NORs at the end of both arms, which are also coincident with the location of rDNA detected by fluorescent in situ hybridization (
Variation in the frequency of B chromosomes is common among populations (
Finally, in the present paper we report for the first time a trisomy of chromosome 11 in a single individual. In A. cursor also were observed an individual with trisomy for chromosome 7 (
In conclusion, chromosome data for Akodon montensis showed high variability in all studied populations throughout its geographic range. However, additional data are needed to understand the dynamic of the multiple chromosome polymorphism observed in this species of sigmodontine rodents.
We thank to Marcelo Cavicchia and to several members of the LGE for the cooperation in the fieldwork. We are grateful to the reviewers for a critical revision of an early version of this paper. We appreciate the support of the Ministerio de Ecología y Recursos Naturales Renovables de la Provincia de Misiones, and Administración de Parques Nacionales Argentina, Delegación Regional NEA, Parque Nacional Iguazú. This study was partially funded by Agencia SECYT PICT 2010/1095 and Consejo Nacional de Investigaciones Científicas y Técnicas CONICET PIP 198 grants.
Pattern of Ag-NOR´s distribution in Akodon montensis from Argentina analyzed in this work.
Data type: Chromosome pairs with positive signals after silver staining.
Explanation note: 1 HOM = only one homologue was marked, 2 HOM = both homologues were marked. 2 Telo = both ends of the B chromosome were marked; 1 Telo = only one end of the B chromosome was marked, being “p” when the short arm was marked and “q” when the long arm was marked. Individuals from 1 to 10 had no supernumerary chromosome and from 11 to 14 had the B chromosome. Total = total number of Ag-NOR marks in each cell; Total without B = total number of Ag-NOR marks excluding that of the B chromosome.