Cytogenetic Analysis of the Fungus-Farming Ant Cyphomyrmex rimosus (Spinola, 1851) (Formicidae: Myrmicinae: Attini) Highlights Karyotypic Variation

Abstract

The fungus-farming ant genus Cyphomyrmex (subtribe Attina, clade Neoattina) comprises 23 described species that are widely distributed throughout the Neotropics. Species within Cyphomyrmex have taxonomic issues such as Cyphomyrmex rimosus (Spinola, 1851) which is likely a species complex. Cytogenetics is a useful tool for evolutionary studies and understanding species with dubious taxonomy. In this study, we characterized the karyotype of C. rimosus from Viçosa, Minas Gerais state, southeastern Brazil using classical and molecular cytogenetic techniques to enrich the chromosomal information about Cyphomyrmex. The karyotype of C. rimosus from the rainforest of southeastern Brazil (2n = 22, 18m + 4sm) notably contrasts with that previously described for this species in Panama (2n = 32). This intraspecific chromosomal variation suggests the existence of a species complex within this taxon according to the previous hypothesis derived from morphological analysis. We detected GC-rich heterochromatic regions in C. rimosus and using repetitive DNA probes showed that this heterochromatin shares repetitive sequences with other Neoattina species already studied, enhancing the importance of this specific genome region in the understanding of Attina evolution. Mapping of microsatellite (GA)15 on C. rimosus was restricted to the euchromatic regions of all chromosomes. The single intrachromosomal rDNA sites observed in C. rimosus follow the general genomic organization trend of ribosomal genes in Formicidae. Our study extends the data of chromosome mapping on Cyphomyrmex and reinforces the importance of cytogenetic studies in different localities to better understand taxonomic issues in widely distributed taxa such as C. rimosus.

Physical chromosomal mapping of major ribosomal genes in 15 ant species with a review of hypotheses regarding evolution of the number and position of NORs in ants

“…The data obtained in this study reinforce this hypothesis, since C. laevigatus Weber, 1938 (2n=14) showed rDNA sites on the short arm of the 4 th metacentric pair, differing from the other two Cyphomyrmex species previously studied, C. transversus Emery, 1894 (2n=18), andC. rimosus (Spinola, 1851) (2n=24), where rDNA clusters are located on the short arm of the 2 nd pair and the long arm of the 3 rd pair, respectively (Teixeira et al 2021b(Teixeira et al , 2023.…”

“…The occurrence of chromosomal rearrangements involving the rDNA region during karyotypic evolution in the fungus-growing ant genus Cyphomyrmex (comprising 23 valid species, Bolton 2024 ) has also been suggested ( Teixeira et al 2023 ). The data obtained in this study reinforce this hypothesis, since C. laevigatus Weber, 1938 (2n=14) showed rDNA sites on the short arm of the 4 th metacentric pair, differing from the other two Cyphomyrmex species previously studied, C. transversus Emery, 1894 (2n=18), and C. rimosus (Spinola, 1851) (2n=24), where rDNA clusters are located on the short arm of the 2 nd pair and the long arm of the 3 rd pair, respectively ( Teixeira et al 2021b , 2023 ).…”

“…In Formicidae , molecular cytogenetic studies involving fluorescence in situ hybridization ( FISH ) for physical mapping of major ribosomal genes, 45S ribosomal DNA ( rDNA ), here referred to as rDNA clusters, were first conducted in Australian ants of the genus Myrmecia Fabricius, 1804 ( Imai et al 1992 ; Hirai et al 1994 , 1996 ). In recent years, FISH has been widely employed in several ant species, particularly in the Neotropical region ( Santos et al 2016 ; Aguiar et al 2017 ; Micolino et al 2019 , 2022 ; Barros et al 2021a , b , 2022a , b ; Murakami et al 2021 ; Silveira 2022 ; Teixeira et al 2022 , 2023 ; Jacintho et al 2023 ). These molecular cytogenetic studies have provided valuable insights into various biological aspects of these insects, including evolution, taxonomy, and reproduction.…”