Research Article |
Corresponding author: Luísa Antônia Campos Barros ( luufv@yahoo.com.br ) Academic editor: Rodolpho Menezes
© 2021 Gisele Amaro Teixeira, Luísa Antônia Campos Barros, Hilton Jeferson Alves Cardoso de Aguiar, Denilce Meneses Lopes.
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:
Amaro Teixeira G, Antônia Campos Barros L, Jeferson Alves Cardoso de Aguiar H, Meneses Lopes D (2021) Distribution of GC-rich heterochromatin and ribosomal genes in three fungus-farming ants (Myrmicinae, Attini, Attina): insights on chromosomal evolution. Comparative Cytogenetics 15(4): 413-428. https://doi.org/10.3897/compcytogen.v15.i4.73769
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Cytogenetic studies on fungus-farming ants have shown remarkable karyotype diversity, suggesting different chromosomal rearrangements involved in karyotype evolution in some genera. A notable cytogenetic characteristic in this ant group is the presence of GC-rich heterochromatin in the karyotypes of some ancient and derivative species. It was hypothesized that this GC-rich heterochromatin may have a common origin in fungus-farming ants, and the increase in species studied is important for understanding this question. In addition, many genera within the subtribe Attina have few or no cytogenetically studied species; therefore, the processes that shaped their chromosomal evolution remain obscure. Thus, in this study, we karyotyped, through classical and molecular cytogenetic techniques, the fungus-farming ants Cyphomyrmex transversus Emery, 1894, Sericomyrmex maravalhas Ješovnik et Schultz, 2017, and Mycetomoellerius relictus (Borgmeier, 1934), to provide insights into the chromosomal evolution in these genera and to investigate the presence the GC-rich heterochromatin in these species. Cyphomyrmex transversus (2n = 18, 10m + 2sm + 6a) and S. maravalhas (2n = 48, 28m + 20sm) showed karyotypes distinct from other species from their genera. Mycetomoellerius relictus (2n = 20, 20m) presented the same karyotype as the colonies previously studied. Notably, C. transversus presented the lowest chromosomal number for the genus and a distinct karyotype from the other two previously observed for this species, showing the existence of a possible species complex and the need for its taxonomic revision. Chromosomal banding data revealed GC-rich heterochromatin in all three species, which increased the number of genera with this characteristic, supporting the hypothesis of a common origin of GC-rich heterochromatin in Attina. Although a single chromosomal pair carries rDNA genes in all studied species, the positions of these rDNA clusters varied. The rDNA genes were located in the intrachromosomal region in C. transversus and M. relictus, and in the terminal region of S. maravalhas. The combination of our molecular cytogenetic data and observations from previous studies corroborates that a single rDNA site located in the intrachromosomal region is a plesiomorphic condition in Attina. In addition, cytogenetic data obtained suggest centric fission events in Sericomyrmex Mayr, 1865, and the occurrence of inversions as the origin of the location of the ribosomal genes in M. relictus and S. maravalhas. This study provides new insights into the chromosomal evolution of fungus-farming ants.
Biodiversity, chromatin, chromosomal rearrangements, Formicidae, karyotype evolution, molecular cytogenetics
Fungus-farming ants, included in the subtribe Attina (sensu
Several molecular phylogenetic studies have been conducted in Attina to address the relationships between genera and species (
Some Attina genera have been extensively revised (
Cytogenetic data are available for 56 taxa of fungus-farming ants with representatives from 12 genera (reviewed by
According to available cytogenetic data, different chromosomal rearrangements have been proposed to explain karyotype evolution in some Attina genera. The occurrence of centric fissions, according to Minimum Interaction Theory (MIT) (
There are different possible mechanisms involved in the karyotype evolution of Attina genera, highlighting the need to increase the number of studied species for more robust inferences (
Colonies of C. transversus, M. relictus, and S. maravalhas were collected in Viçosa, in the Minas Gerais state, Brazil (-20.757041, -42.873516) (Table
Species of fungus-farming ants cytogenetically analyzed in the present study collected in Viçosa, Minas Gerais, Brazil. Species, total number of colonies and individuals; diploid chromosome numbers; diploid karyotype formulae, presence of GC-rich heterochromatin, and idiogram showing the location of 18S rDNA genes in the karyotype.
Mitotic metaphases were obtained from cerebral ganglia of larvae after meconium elimination accordingly to
The heterochromatin distribution pattern was observed by C-banding technique according to
The ribosomal 18S gene clusters were detected by FISH, following the protocol of
Chromosomes from ten metaphases of each taxon were measured in order to determine the chromosomal morphology. For C-banding, fluorochrome staining, and FISH techniques, at least 30 metaphases of each taxon were analyzed. The metaphases were photographed using an epifluorescent microscope Olympus BX60 attached to an image system QColor Olympus with the filters WB (450–480 nm), WU (330–385 nm), and WG (510–550 nm) for the fluorochromes CMA3, DAPI, and rhodamine, respectively.
The chromosome numbers and karyotypic formulae observed in the three fungus-farming ant species were as follows: 2n = 18 (10m + 2sm + 6a) in C. transversus (Fig.
Karyotypes of fungus-farming ants a Cyphomyrmex transversus (2n = 18, 10m + 2sm + 6a) b, c Mycetomoellerius relictus (2n = 20, 20m and n = 10, 10m), and d Sericomyrmex maravalhas (2n = 48, 28m + 20sm). Scale bars: 5 µm.
Heterochromatin was observed in the centromeric/pericentromeric regions of all chromosomes besides short arms of acrocentric chromosomes in C. transversus (Fig.
Heterochromatic patterns after C-banding technique in the karyotypes of the studied fungus-farming ants a Cyphomyrmex transversus (2n = 18) b Mycetomoellerius relictus (2n = 20), and c Sericomyrmex maravalhas (2n = 48). Dark blocks indicate heterochromatin in the centromeric/pericentromeric regions and short arms of the chromosomes. Scale bars: 5 µm.
GC-rich chromatin patterns using Chromomycin A3 fluorochrome on metaphases of the studied fungus-farming ants a Cyphomyrmex transversus (2n = 18) b Mycetomoellerius relictus (n = 10), and c Sericomyrmex maravalhas (2n = 48). The GC-rich bands in the centromeric/pericentromeric regions and short arms of the chromosomes are colocalized with heterochromatic blocks. Scale bars: 5 µm.
The three species showed a single pair of chromosomes bearing rDNA clusters. The 18S ribosomal gene clusters were mapped in the pericentromeric region of the short arm of the 2nd metacentric pair in C. transversus (Fig.
The association of cytogenetic and molecular data provided insights into the karyotype evolution of the three genera of fungus-farming ants in this study. In Sericomyrmex, the molecular phylogeny proposed by
Sericomyrmex maravalhas (scrobifer clade) has a basal position to Sericomyrmex amabilis Wheeler, 1925 (amabilis clade) (
The molecular phylogeny of Mycetomoellerius, proposed by
Summary of available cytogenetic data in the literature and this study for the genera of fungus-farming ants Cyphomyrmex, Sericomyrmex, and Mycetomoellerius. Species, localities, chromosome numbers: diploid (2n)/haploid (n), diploid karyotype formulae, and references. The terminology used for karyotype formulae is in accordance to the published data.
Species | Localities | 2n/(n) | Karyotype formulae | References |
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Cyphomyrmex | ||||
C. costatus | Panama | 20 | 20M |
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C. cornutus | French Guiana | 22 | 10M + 12SM |
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C. rimosus | Panama | 32 | 28M + 4A |
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C. transversus | French Guiana | 24/(12) | 14m + 6sm + 4a |
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C. transversus | SP - Brazil | 42 | 42A |
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C. transversus | MG - Brazil | 18 | 10m + 2sm + 6a | Present study |
Cyphomyrmex sp. § | MG - Brazil | 32 | 14M + 18A |
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Sericomyrmex | ||||
S. amabilis | Panama | 50 | 50M |
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S. maravalhas | MG - Brazil | 48 | 28m + 20sm | Present study |
Sericomyrmex sp. | MG - Brazil | 50/(25) | 44m + 6sm |
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Mycetomoellerius | ||||
M. urichii* | MG - Brazil | 18 | 16m + 2sm |
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M. holmgreni | MG - Brazil | 20 | 20m |
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M. iheringi | SC - Brazil | 20 | 18M + 2SM |
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M. relictus | MG - Brazil | 20/(10) | 20m |
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Mycetomoellerius sp.† | MG - Brazil | 22 | 18m + 4sm |
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The cytogenetic data obtained in this study for C. transversus (2n = 18) showed the lowest chromosome number for this genus. This karyotype is different from the other two previously studied karyotypes in French Guiana (2n = 24) and Brazil (2n = 42) (
Regarding heterochromatin constitution, the three species of the present study showed GC-rich heterochromatin, as evidenced by the colocalization of the heterochromatic and CMA3+ bands. These data were first reported in Sericomyrmex and Cyphomyrmex. Other fungus-farming ants showed the same heterochromatic composition such as M. goeldii (Paleoattina) (
The physical mapping of rDNA genes showed a single chromosome pair bearing these genes for the three species in this study. This pattern is similar to that observed for other fungus-farming ants, which is suggested to be a plesiomorphic characteristic in Formicidae (reviewed by
However, in S. maravalhas, the rDNA clusters were mapped in the terminal region of the heterochromatic short arm of the 7th metacentric pair (see Figs
Diagram of origin of terminal rDNA clusters in metacentric chromosome from Sericomyrmex maravalhas, considering its ancestor with intrachromosomal rDNA clusters. Black bars: chromosomal breaks; Fis: centric fission; Inv: pericentric inversion; Green blocks: GC-rich regions; Red blocks: 18S ribosomal clusters.
In addition, a difference in the location of rDNA clusters was observed between M. relictus in this study and M. holmgreni (
In this study, the distribution of 18S ribosomal genes and GC-rich heterochromatin in Sericomyrmex and Cyphomyrmex, which were reported for the first time, suggest the origin of this heterochromatin in the common ancestor of Attina. The karyotype observed in C. tranversus shows the lowest chromosomal number for the genus, and chromosomal variability among populations of the species highlights the need for taxonomic revision of this species using an integrative approach. Although Sericomyrmex spp. are morphologically complex (
We are grateful to Dr. Jacques H. C. Delabie for species identification. GAT thanks the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for the scholarship granted. This study was supported by Programa de Auxílio ao Pesquisador PAPESQ/UNIFAP.
Gisele Amaro Teixeira https://orcid.org/0000-0002-7106-5798
Luísa Antônia Campos Barros https://orcid.org/0000-0002-1501-4734
Hilton Jeferson Alves Cardoso de Aguiar https://orcid.org/0000-0001-7738-1460
Denilce Meneses Lopes https://orcid.org/0000-0001-7209-4411