Cytogenetic and Molecular Analyses Reveal a Divergence between Acromyrmex striatus (Roger, 1863) and Other Congeneric Species: Taxonomic Implications

Abstract

The leafcutter ants, which consist of Acromyrmex and Atta genera, are restricted to the New World and they are considered the main herbivores in the neotropics. Cytogenetic studies of leafcutter ants are available for five species of Atta and 14 species of Acromyrmex, both including subspecies. These two ant genera have a constant karyotype with a diploid number of 22 and 38 chromosomes, respectively. The most distinct Acromyrmex species from Brazil is A. striatus, which is restricted to the southern states of Santa Catarina and Rio Grande do Sul. Several cytogenetic and phylogenetic studies have been conducted with ants, but the karyotypic characterization and phylogenetic position of this species relative to leafcutter ants remains unknown. In this study, we report a diploid number of 22 chromosomes for A. striatus. The phylogenetic relationship between A. striatus and other leafcutter ants was estimated based on the four nuclear genes. A. striatus shared the same chromosome number as Atta species and the majority of metacentric chromosomes. Nuclear data generated a phylogenetic tree with a well-supported cluster, where A. striatus formed a different clade from other Acromyrmex spp. This combination of cytogenetic and molecular approaches provided interesting insights into the phylogenetic position of A. striatus among the leafcutter ants and the tribe Attini.

Banding Patterns of Three Leafcutter Ant Species of the GenusAtta(Formicidae: Myrmicinae) and Chromosomal Inferences

“…The chromosome morphology was shown to be constant in the different Atta species, unlike the observations made for the species of Acromyrmex (Barros 2010;Cristiano et al 2013). The genera Atta and Acromyrmex are considered phylogenetically close and recognized for the constancy of their karyotype, with respect to both chromosome number and similarity in morphology in most of the species studied (revised in Barros et al 2011;Cristiano et al 2013). This characteristic has already been observed in the different ant groups, like those of the subgenera Myrmothrix and Myrmamblys in genus Camponotus (Formicinae) (Mariano et al 2003) and genus Pheidole (Myrmicinae) (revised in Lorite & Palomeque 2010).…”

“…In the genus Acromyrmex, Acromyrmex striatus (2n = 22) differs the most from the other already known species with respect to both morphological traits and chromosome number. Its karyotype bears closer similarity to that of the species of the genus Atta (2n = 22) studied earlier, and associated with the molecular data it provided clear insights into a distinct phylogenetic position of this species among the leafcutter ants (Cristiano et al 2013).…”

“…This characteristic has already been observed in the different ant groups, like those of the subgenera Myrmothrix and Myrmamblys in genus Camponotus (Formicinae) (Mariano et al 2003) and genus Pheidole (Myrmicinae) (revised in Lorite & Palomeque 2010). However, recent cytogenetic studies on the genus Acromyrmex have revealed differences in the morphology and chromosome banding patterns in 7 species studied in Brazil and Panamá (Barros 2010;Cristiano et al 2013). In the present study, the C-banding technique indicated the existence of a small amount of heterochromatin on the chromosomes of the Atta species, with a distribution pattern in the centromeric region.…”

“…Atta and Acromyrmex are characterized by a conserved chromosome number within each genus of 2n = 22 chromosomes and 2n = 38 chromosomes, respectively, and by similar chromosome morphology within each genera (revised in Barros et al 2011;Cristiano et al 2013). Recent cytogenetic studies on Acromyrmex have reported some differences in the chromosome morphology and banding patterns in 6 species from Minas Gerais, Brazil and Barro Colorado, Panamá (Barros 2010), indicating that further cytogenetic studies in leafcutter ants are warranted.…”

Acromyrmex charruanus: a new inquiline social parasite species of leaf-cutting ants

“…The morphologically and behaviorally specialized leafcutting ants of the genera Atta Fabricius and Acromyrmex Mayr represent an evolutionarily highly specialized clade within the fungus-growing ants (Mayhé-Nunes 1995;Schultz and Meier 1995;Schultz and Brady 2008;Della Lucia 2011;Cristiano et al 2013). Among the leaf-cutting ants, the genus Acromyrmex currently contains 32 species and almost the same number of subspecies (n = 30) (Emery 1905;Gonçalves 1961;Bolton 2014).…”