Chromosomal dynamics in space and time: evolutionary history of Mycetophylax ants across past climatic changes in the Brazilian Atlantic coast

Chromosome study of the Hymenoptera (Insecta): from cytogenetics to cytogenomics

A brief overview of the current stage of the chromosome study of the insect order Hymenoptera is given. It is demonstrated that, in addition to routine staining and other traditional techniques of chromosome research, karyotypes of an increasing number of hymenopterans are being studied using molecular methods, e.g., staining with base-specific fluorochromes and fluorescence in situ hybridization (FISH), including microdissection and chromosome painting. Due to the advent of whole genome sequencing and other molecular techniques, together with the "big data" approach to the chromosomal data, the current stage of the chromosome research on Hymenoptera represents a transition from Hymenoptera cytogenetics to cytogenomics.

Cytogenetic characterization of solitary wasp Ancistrocerus flavomarginatus (Brèthes, 1906) (Hymenoptera, Vespidae) with insights into the chromosomal evolution in the genus

Cytogenetic studies have enabled the characterization of the chromosomal macrostructure and microstructure and have contributed to the understanding of the evolution of wasp karyotypes. However, studies on Eumeninae solitary wasps are scarce. In this study, we characterized the karyotype of Ancistrocerus flavomarginatus (Brèthes, 1906) and compared it with previous data from other Ancistrocerus (Wesmael, 1836) species to shed light on the chromosomal diversity of the genus. A chromosome number of 2n = 24 in females and n = 12 in males was observed. Comparing the A. flavomarginatus karyotype with that of another Ancistrocerus species showed variations in the morphology of some chromosomal pairs. The presence of two larger chromosome pairs, almost entirely heterochromatic, and the predominance of subtelocentric chromosomes with heterochromatic short arms in A. flavomarginatus support the occurrence of fissions in Ancistrocerus. A single site of ribosomal genes was observed in A. flavomarginatus, in addition to a size polymorphism of these rDNA clusters between the homologues of some analyzed females. This polymorphism may originate from duplications/deletions due to unequal crossing-over or amplification via transposable elements. The (GA)₁₅ microsatellite is located exclusively in euchromatic regions. Our data show that different rearrangements seem to shape chromosomal evolution in Ancistrocerus species.

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

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.

Dynamic development of AT-rich heterochromatin has followed diversification and genome expansion of psammophilous Mycetophylax (Formicidae: Attini: Attina)

Heterochromatin is an important genome constituent comprised by a high density of repetitive DNA sequences that mediate chromosome structure and function. The species Mycetophylax morschi currently harbours three cytotypes: 2n = 26, 2n = 28 and 2n = 30 chromosomes. However, Mycetophylax conformis and Mycetophylax simplex harbour 2n = 30 and 2n = 36 chromosomes, respectively. None of the cytotypes of M. morschi showed any AT-positive blocks, whereas the karyotypes of M. conformis and M. simplex revealed AT-rich blocks around the pericentromeric region and on the short arm of several chromosomes. This AT-rich pattern is coincident with the known heterochromatin distribution of psammophilous Mycetophylax, confirming that heterochromatin is AT-rich, in line with the genome size and AT%. Our results demonstrated that genome size among psammophilous Mycetophylax is correlated with the proportion of base pairs, biased to adenine and thymine. Thus, genome size and the proportion of adenine and thymine in the species studied here suggest that the genome changes in psammophilous Mycetophylax are related to the expansion of repetitive DNA in AT-rich heterochromatin. Considering the phylogenetic relationship of psammophilous Mycetophylax, the dynamic development of AT-rich heterochromatin and karyotype repatterning encompasses the diversification of such ants.

Multiple heterochromatin diversification events in the genome of fungus-farming ants: insights from repetitive sequences

A substantial portion of the eukaryotic genome includes repetitive DNA, which is important for its stability, regulation, and architecture. Fungus-farming ant genomes show remarkable structural rearrangement rates that were necessary for the establishment of their agriculture-based lifestyle, highlighting the relevance of this peculiar group in understanding the repetitive portion of ant genome. Chromosomal banding studies are in accordance with genomic data because they show that repetitive heterochromatic sequences of basal and derivative Attina species are GC-rich, an uncommon trait in Formicidae. To understand the evolutionary dynamics of heterochromatin in Attina, we compared GC-rich heterochromatin patterns between the Paleoattina and Neoattina clades of this subtribe. To this end, we hybridized the Mrel-C₀t probe (highly and moderately repetitive DNA) obtained from Mycetomoellerius relictus, Neoattina with GC-rich heterochromatin, in karyotypes of Paleoattina and Neoattina species. Additionally, we mapped the repetitive sequences (GA)₁₅ and (TTAGG)₆ in species of the two clades to investigate their organization and evolutionary patterns in the genome of Attina. The Mrel-C₀t probe marked the heterochromatin in M. relictus, in other Mycetomoellerius spp., and in species of Mycetarotes, Cyphomyrmex, and Sericomyrmex (Neoattina). In Mycetomoellerius urichii, only pericentromeric heterochromatin was marked with Mrel-C₀t. No marking was observed in Paleoattina species or in Atta and Acromyrmex (Neoattina). These results indicated that different evolutionary events led to heterochromatin differentiation in Attina. The most likely hypothesis is that GC-rich heterochromatin arose in the common ancestor of the two clades and accumulated various changes throughout evolution. The sequences (GA)₁₅ and (TTAGG)₆ located in euchromatin and telomeres, respectively, showed more homogeneous results among the species.

Phylogenetic Reconstruction of the Ancestral Chromosome Number of the Genera Anochetus Mayr, 1861 and Odontomachus Latreille, 1804 (Hymenoptera: Formicidae: Ponerinae)

Recent phylogenetic and molecular data are changing our knowledge about the relations between species and evolutionary processes resulting in the chromosome variation observed in ants (Hymenoptera: Formicidae). Ants exhibit remarkable variations in morphology, behavior, karyotypes, and chromosome structure. By assembling genetic and chromosome information about the trap-jaw ants from the subfamily Ponerinae, we reconstructed the phylogenetic relationships that inferred the monophyletic condition between the Anochetus and Odontomachus genera and estimated their ancestral haploid chromosome number. According to our inferences, these clades have an ancestral haploid chromosome number n = 15. The most recent common ancestor of Anochetus and Odontomachus has arisen between the Early Paleocene and the Early Eocene periods (time of the most recent common ancestor). In the Anochetus genus, we observed maintenance of the ancestral chromosome number estimated here in most species. This also suggests that pericentric inversions were the primary chromosomal rearrangement modulating the karyotype evolution of this genus. However, a reduction from n = 15–14 is observed in Anochetus emarginatus and Anochetus cf. madaraszi, which likely occurred by centromeric fusion. In contrast, the increase from the ancestral karyotype number in Anochetus horridus suggested centromeric fissions. Odontomachus showed maintenance of the ancestral chromosome number in the “rixosus group” and several gains in all species from the “haematodus group.” Our findings suggest that centromeric fissions and pericentric rearrangements lead to chromosomal changes in trap-jaw ants. Considering the ancestral state estimated here, changes in chromosome morphology are likely due to pericentric inversions, and chromosome number increases are likely due to centric fissions. The higher number of acrocentric or telocentric chromosomes in the karyotypes with n < 15 haploid chromosomes supports such an idea.

Chromosome Evolution in the Genus Partamona (Apidae: Meliponini), with Comments on B Chromosome Origin

The genus Partamona includes 33 species of stingless bees, of which 11 were studied cytogenetically. The main goal of this study was to propose a hypothesis about chromosomal evolution in Partamona by combining molecular and cytogenetic data. Cytogenetic analyses were performed on 3 Partamona species. In addition, the molecular phylogeny included mitochondrial sequences of 11 species. Although the diploid number was constant within the genus, 2n = 34, B chromosomes were reported in 7 species. Cytogenetic data showed karyotypic variations related to chromosome morphology and the amount and distribution of heterochromatin and repetitive DNA. The molecular phylogenetic reconstruction corroborated the monophyly of the genus and separated the 2 clades (A and B). This separation was also observed in the cytogenetic data, in which species within each clade shared most of the cytogenetic characteristics. Furthermore, our data suggested that the B chromosome in the genus Partamona likely originated from a common ancestor of the species that have it in clade B and, through interspecific hybridization, it appeared only in Partamona rustica from clade A. Based on the above, Partamona is an interesting genus for further investigations using molecular mapping of B chromosomes as well as for broadening phylogenetic data.

Karyotype Diversity, Mode, and Tempo of the Chromosomal Evolution of Attina (Formicidae: Myrmicinae: Attini): Is There an Upper Limit to Chromosome Number?

Simple Summary

Ants are an important insect group that includes a considerable number of species. Along with this diversity in species, they also exhibit a wide variation in chromosome numbers, from 1 up to 60 chromosomes. DNA molecules can be counted in a specific stage of the cell life cycle and quantified. These DNA molecules are very tightly packed together with several proteins and are called chromosomes. Each species shows a specific number of chromosomes with different shapes and sizes, as well as different quantities of DNA. We can use such information (the number of chromosomes, shape of the chromosomes, and quantity of DNA) as morphological attributes to study evolution at the species level. In this study, we describe new karyotypes of several ant species. In addition, from previous studies, we have compiled all the available information regarding the chromosome number and DNA quantity in fungus-farming ant cells. Different processes, called rearrangements, can change chromosomes over time, producing new character states. Such states can be tracked, along with the species and groups of similar species, using their relationships to identify patterns. We use DNA sequences to reconstruct the relationships of fungus-farming ant species (molecular phylogeny). By comparing such phylogeny with the chromosome number and DNA quantity, we discuss the evolution of chromosomes and DNA quantity (or genome size), and the potential limits to these features across fungus-farming ants.

Abstract

Ants are an important insect group that exhibits considerable diversity in chromosome numbers. Some species show only one chromosome, as in the males of the Australian bulldog ant Myrmecia croslandi, while some have as many as 60 chromosomes, as in the males of the giant Neotropical ant Dinoponera lucida. Fungus-growing ants are a diverse group in the Neotropical ant fauna, engaged in a symbiotic relationship with a basidiomycete fungus, and are widely distributed from Nearctic to Neotropical regions. Despite their importance, new chromosome counts are scarcely reported, and the marked variation in chromosome number across species has been poorly studied under phylogenetic and genome evolutionary contexts. Here, we present the results of the cytogenetic examination of fungus-farming ants and compile the cytogenetic characteristics and genome size of the species studied to date to draw insights regarding the evolutionary paths of karyotype changes and diversity. These data are coupled with a fossil-calibrated phylogenetic tree to discuss the mode and tempo of chromosomal shifting, considering whether there is an upper limit for chromosome number and genome size in ants, using fungus-farming ants as a model study. We recognize that karyotypes are generally quite variable across fungus-farming ant phylogeny, mostly between genera, and are more numerically conservative within genera. A low chromosome number, between 10 and 12 chromosomes, seems to present a notable long-term evolutionary stasis (intermediate evolutionary stasis) in fungus-farming ants. All the genome size values were inside a limited spectrum below 1 pg. Eventual departures in genome size occurred with regard to the mean of 0.38 pg, indicating that there is a genome, and likely a chromosome, number upper limit.

Reconstruction of the Doradinae (Siluriformes-Doradidae) ancestral diploid number and NOR pattern reveals new insights about the karyotypic diversification of the Neotropical thorny catfishes

Doradinae (Siluriformes: Doradidae) is the most species-rich subfamily among thorny catfishes, encompassing over 77 valid species, found mainly in Amazon and Platina hydrographic basins. Here, we analyzed seven Doradinae species using combined methods (e.g., cytogenetic tools and Mesquite ancestral reconstruction software) in order to scrutinize the processes that mediated the karyotype diversification in this subfamily. Our ancestral reconstruction recovered that 2n=58 chromosomes and simple nucleolar organizer regions (NOR) are ancestral features only for Wertheimerinae and the most clades of Doradinae. Some exceptions were found in Trachydoras paraguayensis (2n=56), Trachydoras steindachneri (2n=60), Ossancora punctata (2n=66) and Platydoras hancockii whose karyotypes showed a multiple NOR system. The large thorny catfishes, such as Pterodoras granulosus, Oxydoras niger and Centrodoras brachiatus share several karyotype features, with subtle variations only regarding their heterochromatin distribution. On the other hand, a remarkable karyotypic variability has been reported in the fimbriate barbells thorny catfishes. These two contrasting karyoevolution trajectories emerged from a complex interaction between chromosome rearrangements (e.g., inversions and Robertsonian translocations) and mechanisms of heterochromatin dispersion. Moreover, we believe that biological features, such as microhabitats preferences, populational size, low vagility and migratory behavior played a key role during the origin and maintenance of chromosome diversity in Doradinae subfamily.

Distribution of GC-rich heterochromatin and ribosomal genes in three fungus-farming ants (Myrmicinae, Attini, Attina): insights on chromosomal evolution

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 Cyphomyrmextransversus Emery, 1894, Sericomyrmexmaravalhas Ješovnik et Schultz, 2017, and Mycetomoelleriusrelictus (Borgmeier, 1934), to provide insights into the chromosomal evolution in these genera and to investigate the presence the GC-rich heterochromatin in these species. Cyphomyrmextransversus (2n = 18, 10m + 2sm + 6a) and S.maravalhas (2n = 48, 28m + 20sm) showed karyotypes distinct from other species from their genera. Mycetomoelleriusrelictus (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.

Highly divergent karyotypes and barcoding of the East African genus Gonatoxia Karsch (Orthoptera: Phaneropterinae)

East Africa is a hotspot of biodiversity of many orthopteran taxa, including bushcrickets. Gonatoxia Karsch, 1889 species are fully alate Phaneropterinae, which are perfectly adapted to the foliage of forests. We examined five species using combined cytogenetic and molecular data to determine the inter- and intraspecific genetic diversity. The variation in the diploid number of chromosomes in males ranged from 2n = 28 + X0 and 26 + X0 to 2n = 6 + X0. Fluorescence in situ hybridization showed from one to many 18S rDNA loci as well as interstitial sequences, especially in G. helleri. 18S rDNA loci coincided with active NOR and C-banding patterns. The isolation of populations of the species explains differences in the number of chromosomes (G. maculata), chromosomal polymorphism and chromosomal heterozygosity (G. helleri). Our molecular phylogeny based on the COI locus supported the monophyly of the genus Gonatoxia and separateness of the five examined species in accordance with their morphological features and chromosome numbers as well as the species' distribution.

Karyotype structure and cytogenetic markers of Amoimyrmex bruchi and Amoimyrmex silvestrii: contribution to understanding leaf-cutting ant relationships

Leaf-cutting ants are considered the most important herbivores in terrestrial environments throughout the Neotropics. Amoimyrmex Cristiano, Cardoso, & Sandoval, 2020 is the sister clade of the remaining leaf-cutting ants from the genera Atta and Acromyrmex. Amoimyrmex striatus was the only species cytogenetically studied within the genus and shares the same chromosomal number as Atta, bearing 22 chromosomes, whereas Acromyrmex bears 38 chromosomes, with the exception of the social parasite Acromyrmex ameliae (2n = 36). Our objective here was to cytogenetically analyze the species of Amoimyrmex bruchi and Amoimyrmex silvestrii, as well as to describe the karyotype of these sister species, using an integrative approach using classical and molecular cytogenetics. We aimed to characterize the cytogenetic markers that contribute to the systematics and taxonomy of the genus. Our results showed that the karyotypes of these two species are very similar, with an identical chromosome number (2n = 22), chromosome morphology (2K = 20m + 2sm), and location of 18S rDNA and telomeric repeat TTAGG on the chromosomes. However, the microsatellite probe GA(15) showed variation across the species and populations studied. We suggest that both species diverged relatively recently and are unmistakably sisters because of the many shared characteristics, including the highly conserved karyotypes.

Cytogenetic Evidence for Sex Chromosomes and Karyotype Evolution in Anguimorphan Lizards

Anguimorphan lizards are a morphologically variable group of squamate reptiles with a wide geographical distribution. In spite of their importance, they have been cytogenetically understudied. Here, we present the results of the cytogenetic examination of 23 species from five anguimorphan families (Anguidae, Helodermatidae, Shinisauridae, Varanidae and Xenosauridae). We applied both conventional (Giemsa staining and C-banding) and molecular cytogenetic methods (fluorescence in situ hybridization with probes for the telomeric motifs and rDNA loci, comparative genome hybridization), intending to describe the karyotypes of previously unstudied species, to uncover the sex determination mode, and to reveal the distribution of variability in cytogenetic characteristics among anguimorphan lizards. We documented that karyotypes are generally quite variable across anguimorphan lineages, with anguids being the most varying. However, the derived chromosome number of 2n = 40 exhibits a notable long-term evolutionary stasis in monitors. Differentiated ZZ/ZW sex chromosomes were documented in monitors and helodermatids, as well as in the anguids Abronia lythrochila, and preliminary also in Celestus warreni and Gerrhonotus liocephalus. Several other anguimorphan species have likely poorly differentiated sex chromosomes, which cannot be detected by the applied cytogenetic methods, although the presence of environmental sex determination cannot be excluded. In addition, we uncovered a rare case of spontaneous triploidy in a fully grown Varanus primordius.

Evolutionary insights into the genomic organization of major ribosomal DNA in ant chromosomes

The major rDNA genes are composed of tandem repeats and are part of the nucleolus organizing regions (NORs). They are highly conserved and therefore useful in understanding the evolutionary patterns of chromosomal locations. The evolutionary dynamics of the karyotype may affect the organization of rDNA genes within chromosomes. In this study, we physically mapped 18S rDNA genes in 13 Neotropical ant species from four subfamilies using fluorescence in situ hybridization. Furthermore, a survey of published rDNA cytogenetic data for 50 additional species was performed, which allowed us to detect the evolutionary patterns of these genes in ant chromosomes. Species from the Neotropical, Palearctic, and Australian regions, comprising a total of 63 species from 19 genera within six subfamilies, were analysed. Most of the species (48 out of 63) had rDNA genes restricted to a single chromosome pair in their intrachromosomal regions. The position of rDNA genes within the chromosomes appears to hinder their dispersal throughout the genome, as translocations and ectopic recombination are uncommon in intrachromosomal regions because they can generate meiotic abnormalities. Therefore, rDNA genes restricted to a single chromosome pair seem to be a plesiomorphic feature in ants, while multiple rDNA sites, observed in distinct subfamilies, may have independent origins in different genera.

Comparative Cytogenetic Analysis of Three Eumeninae Species (Hymenoptera, Vespidae)

Eumeninae represents the largest subfamily within Vespidae, with 3,600 species described. Of these, only 18 have been cytogenetically analysed. In the present study, we used both classical and molecular techniques to characterise and compare the karyotypes of 3 Eumeninae species, namely, Ancistrocerus sp., Pachodynerus grandis, and Pachodynerus nasidens. Ancistrocerus sp. presented a haploid chromosome number of n = 12, with the first 2 chromosomes of the karyotype being almost entirely heterochromatic and much larger than the remaining chromosomes. The 2 Pachodynerus species presented the same chromosome number (n = 11 and 2n = 22) but displayed different karyotypic formulae. Additionally, chromosomal polymorphisms were observed in the analysed P. nasidens female. In the 3 species, heterochromatin was located in one of the chromosome arms. Fluorochrome staining revealed a balanced composition of AT and GC bases within the chromatin for each of the 3 species, except for few regions that were visibly GC-rich. All species had a single 18S rDNA site that co-localised with GC-rich regions; however, this localisation varied from species to species and not all GC-rich regions corresponded to ribosomal genes. Based on the cytogenetic data obtained here, we discuss the possible numerical/structural rearrangements that may be involved in the karyotypic evolution of the 3 studied species. In addition to the first description of the molecular cytogenetic characteristics of the Eumeninae subfamily and the genus Pachodynerus, this study also provides a relevant contribution towards the discussion of chromosomal evolution in Eumeninae wasps.

Diversification of the Balloon bushcrickets (Orthoptera, Hexacentrinae, Aerotegmina) in the East African mountains

East African mountains constitute a network of isolated habitat islands among dry savannah and are thus ideal for studying species diversification processes. This study elucidated the phylogenetic and phylogeographic relationships of all bushcricket species comprising the genus Aerotegmina. Our analysis indicated that large-scale climatic and topographic processes in Africa are likely to have driven speciation in this group, and revealed the cytogenetic traits of the species. Molecular phylogeny supported the monophyly of Aerotegmina and showed that the genus probably originated in the old Eastern Arc Mountains of Tanzania and Kenya. Two lineages were distinguished: small- and large-sized species with geographically distinct habitats. The underlying processes are thought to be eight dispersals, ten vicariance events, and one extinction event linked to repeated fragmentation of the African rainforest. Those processes, in conjunction with habitat change, probably also led to the spatial separation of the species into a northern clade with a diploid number of chromosomes 2n = 32 + X0 or 2n = 30 + neo-XY and a southern clade with a reduced number of chromosomes (2n = 28 + X0 or 24 + neo-X₁X₂Y). Karyotype analysis suggests that Aerotegmina is currently in the process of speciation.

Intraspecific variation in the karyotype length and genome size of fungus-farming ants (genus Mycetophylax), with remarks on procedures for the estimation of genome size in the Formicidae by flow cytometry

Ants (Formicidae) present considerable diversity in chromosome numbers, which vary from n = 1 to n = 60, although this variation is not proportional to that in genome size, for which estimates range from 0.18 pg to 0.77 pg. Intraspecific variation in the chromosome number and karyotype structure has been reported among species, although the variation among populations of the same species has received much less attention, and there are few data on genome size. Here, we studied the karyotype length and genome size of different populations of the fungus-farming ants Mycetophylax conformis (Mayr, 1884) and Mycetophylax morschi (Emery, 1888). We also provide remarks on procedure for the estimation of ant genome size by Flow Cytometry (FCM) analysis. Chromosome number and morphology did not vary among the populations of M. conformis or the cytotypes of M. morschi, but karyotype length and genome size were significantly distinct among the populations of these ants. Our results on the variation in karyotype length and genome size among M. morschi and M. conformis populations reveal considerable diversity that would be largely overlooked by more traditional descriptions of karyotypes, which were also supported by the estimates of genome size obtained using flow cytometry. Changes in the amount of DNA reflect variation in the fine structure of the chromosomes, which may represent the first steps of karyotype evolution and may occur previously to any changes in the chromosome number.

Karyotype and putative chromosomal inversion suggested by integration of cytogenetic and molecular data of the fungus-farming ant Mycetomoellerius iheringi Emery, 1888

Comparative cytogenetic analyses are being increasingly used to collect information on species evolution, for example, diversification of closely related lineages and identification of morphologically indistinguishable species or lineages. Here, we have described the karyotype of the fungus-farming ant Mycetomoellerius iheringi Emery, 1888 and investigated its evolutionary relationships on the basis of molecular and cytogenetic data. The M. iheringi karyotype consists of 2n = 20 chromosomes (2K = 18M + 2SM). We also demonstrated that this species has the classical insect TTAGG telomere organization. Phylogenetic reconstruction showed that M. iheringi is phylogenetically closer to M. cirratus Mayhé-Nunes & Brandão, 2005 and M. kempfi Fowler, 1982. We compared M. iheringi with other congeneric species such as M. holmgreni Wheeler, 1925 and inferred that M. iheringi probably underwent a major pericentric inversion in one of its largest chromosomes, making it submetacentric. We discussed our results in the light of the phylogenetic relationships and chromosomal evolution.

Comparative FISH-mapping of TTAGG telomeric sequences to the chromosomes of leafcutter ants (Formicidae, Myrmicinae): is the insect canonical sequence conserved?

Telomeric sequences are conserved across species. The most common sequence reported among insects is (TTAGG)_n, but its universal occurrence is not a consensus because other canonical motifs have been reported. In the present study, we used fluorescence in situ hybridization (FISH) using telomeric probes with (TTAGG)₆ repeats to describe the telomere composition of leafcutter ants. We performed the molecular cytogenetic characterization of six Acromyrmex Mayr, 1865 and one Atta Fabricius, 1804 species (Acromyrmex ambiguus (Emery, 1888), Ac. crassispinus (Forel, 1909), Ac. lundii (Guérin-Mèneville, 1838), Ac. nigrosetosus (Forel, 1908), Ac. rugosus (Smith, 1858), Ac. subterraneus subterraneus (Forel, 1893), and Atta sexdens (Linnaeus, 1758)) and described it using a karyomorphometric approach on their chromosomes. The diploid chromosome number 2n = 38 was found in all Acromyrmex species, and the karyotypic formulas were as follows: Ac. ambiguus 2K = 14M + 12SM + 8ST + 4A, Ac. crassispinus 2K = 12M + 20SM + 4ST + 2A, Ac. lundii 2K = 10M + 14SM + 10ST + 4A, Ac. nigrosetosus 2K = 12M + 14SM + 10ST + 2A, and Ac. subterraneus subterraneus 2K = 14M + 18SM + 4ST + 2A. The exact karyotypic formula was not established for Ac. rugosus. FISH analyses revealed the telomeric regions in all the chromosomes of the species studied in the present work were marked by the (TTAGG)₆ sequence. These results reinforce the premise that Formicidae presents high homology between their genera for the presence of the canonical sequence (TTAGG)_n.