Cytogenetic data on the threatened leafcutter ant Atta robusta Borgmeier, 1939 (Formicidae: Myrmicinae: Attini)

Abstract

The karyotype of the threatened ant species Atta robusta is described so as to establish the evolutionary relationships of this taxon with other leafcutter ants. Standard Giemsa staining, C-banding, NOR banding, fluorochromes CMA3/DAPI, Hsc-FA technique and Fluorescence in situ Hybridization (FISH) using 18S rDNA probe were conducted on a population from Aracruz, state of Espírito Santo, Brazil, allowing for comparisons with data available on Atta and other fungus-growing ant species. The diploid chromosome number observed for A. robusta was 2n=22, and the karyotypic formula was 18m+2sm+2st. Heterochromatic blocks were observed in the centromeric region of most chromosomes, where one pair of metacentric chromosomes is characterized by a GC-rich heterochromatic band in the interstitial region of its long arm. The detection of 18S rDNA using FISH confirmed the presence of single NOR for A. robusta. This is the first report of rDNA 18S detection using FISH for leafcutter ants. The cytogenetic results of this study confirm the information available for Atta and allow us to confirm the conserved chromosome number, morphology and banding pattern within the genus for the taxa studied to date, which included species from three out of the four groups of Atta indicated by molecular data. The accumulation of cytogenetic data on fungus-growing ants enhances the understanding of the genomic evolutionary patterns of Atta, since it belongs to a group of recent origin between the most well studied ants. Cytogenetic data does not indicate restrictions in relocation or reintroduction in areas where populations were extinct due to the conserved karyotype. This study allows for cytogenetic comparison of A. robusta with other ants of Atta, emphasizing the importance of chromosomal information for species conservation.

Cytogenetic data on six leafcutter ants of the genus Acromyrmex Mayr, 1865 (Hymenoptera, Formicidae, Myrmicinae): insights into chromosome evolution and taxonomic implications

“…In these species, 2n = 22 and a karyotypic formula of 2n = 18m+2sm+2st were found. Similar banding patterns were also observed in different species (Barros et al 2014a, Murakami et al 1998, Barros et al 2015) which belonged to the three of four species groups defined on the basis of molecular data (Bacci et al 2009). Cytogenetic data on Acromyrmex are also restricted but available for some taxa collected in Brazil: Acromyrmex (Acromyrmex) crassispinus (Forel, 1909); Acromyrmex (Acromyrmex) subterraneus molestans Santschi, 1925; Acromyrmex (Acromyrmex) subterraneus subterraneus Forel, 1893 (Fadini and Pompolo 1996); and in Uruguay: Acromyrmex (Acromyrmex) ambiguus Emery, 1888; Acromyrmex (Acromyrmex) hispidus Santschi, 1925; and Acromyrmex (Moellerius) heyeri (Forel, 1899) (Goñi et al 1983).…”

“…All the species had the same chromosome number, 2n = 38. However, Acromyrmex (Moellerius) striatus (Roger, 1863) has recently shown 2n = 22, with a karyotypic formula of 2n = 20m+2sm (Cristiano et al 2013), the same chromosome number found in all Atta species studied to date (Barros et al 2014a, Fadini and Pompolo 1996, Murakami et al 1998, Barros et al 2015). Since Acromyrmex striatus belongs to the well-supported clade which is quite distinct from other members of the genus Acromyrmex , it is suggested that this species is a sister group of all other leafcutter ants, which split before the divergence between Acromyrmex and Atta (Cristiano et al 2013).…”

“…Nowadays, ant cytogenetics is a rapidly developing research field (Delabie et al 2012). Cytogenetic data on fungus-growing ants with information for at least the chromosome number and morphology are available at present for 38 taxa (reviewed in Barros et al 2011, Cristiano et al 2013, Barros et al 2013, 2014a, 2014b, Cardoso et al 2014, Barros et al 2015), corresponding to about 10% of described species (Brandão et al 2011). In some ant genera, e.g.…”

“…in Mycetarotes Emery, 1913 and Cyphomyrmex Mayr, 1862, chromosome numbers are variable at the species level (reviewed in Barros et al 2011). However, species within the genera Atta (Barros et al 2014a, Fadini and Pompolo 1996, Murakami et al 1998, Barros et al 2015) and Acromyrmex (Fadini and Pompolo 1996, Goñi et al 1983) have the same chromosome numbers, 2n = 22 and 2n = 38, respectively, and similar chromosome morphology.…”

“…Namely, these data are available for five Atta species (Barros et al 2014a, Fadini and Pompolo 1996, Murakami et al 1998, Barros et al 2015). In these species, 2n = 22 and a karyotypic formula of 2n = 18m+2sm+2st were found.…”