Cytogenetic and molecular analyses reveal a divergence between Acromyrmex striatus (Roger, 1863) and other congeneric species: taxonomic implications

Isocycloseram: A new active ingredient for leaf-cutting ants control

Leaf-cutting ants are the most important pests in several cropping systems in the Neotropics. Granulated baits containing active ingredients, considered hazardous by the Stockholm Convention, are the usual method to control these ants. Isocycloseram is a new insecticide molecule with high safety margin for mammals, but without registration for the ants in general. Thus, this study investigated the effectiveness of granulated baits with isocycloseram in leaf-cutting ants control under laboratory and field conditions. Initially, the mortality of Atta sexdens workers, fed with dehydrated citrus pulp paste containing different concentrations of isocycloseram was evaluated in the laboratory for 21 days, for toxicological classification. Subsequently, the loading, devolution, and incorporation of baits with different concentrations of isocycloseram and the mortality of A. sexdens colonies were evaluated in the laboratory. After that, the percentages of loading and devolution of baits, foraging activity, and colony mortality treated with 0.05, 0.1, 0.2, and 0.3% of isocycloseram were evaluated for the species A. sexdens, A. laevigata, and Acromyrmex lundii in field conditions. All concentrations of isocycloseram killed more than 15% of ants in 24 h and more than 90% in 21 days in the laboratory, being classified as a fast-acting and highly effective active ingredient. Baits with 0.001 to 0.03% of isocycloseram were highly loaded and exhibited low rate of devolution. The mortality of A. sexdens colony was higher at concentrations between 0.075 and 0.3%, in the laboratory. Baits containing isocycloseram at concentrations of 0.2 and 0.3% were highly loaded, presented low devolution rates, and were highly efficient in controlling A. sexdens, A. laevigata, and A. lundii in the field, at dosages of 6, 10, and 12 g/m² of nest. This is the first report of the use of isocycloseram against leaf-cutting ants, contributing to the development of efficient and toxicologically safer ant baits.

Assessing ploidy levels and karyotype structure of the fire ant Solenopsissaevissima Smith, 1855 (Hymenoptera, Formicidae, Myrmicinae)

The family Formicidae is composed of ants that organize themselves into castes in which every individual has a joint organizational function. Solenopsis Westwood, 1840 is an ant genus with opportunistic and aggressive characteristics, known for being invasive species and stings that cause burning in humans. This genus is particularly difficult to classify and identify since its morphology provides few indications for species differentiation. For this, a tool that has been useful for evolutionary and taxonomic studies is cytogenetics. Here, we cytogenetically studied Solenopsissaevissima Smith, 1855 from Ouro Preto, Minas Gerais, Brazil. We evaluated the occurrence of polyploid cells in individuals and colonies by conventional cytogenetics. A total of 450 metaphases were analyzed and counted. Chromosome counts of individuals and colonies showed varied numbers of ploidies, from n = 16 to 8n = 128. The karyomorphometrical approach allowed determination of the following karyotypes: n = 10 m + 4 sm + 2 st, 2n = 20 m + 8 sm + 4 st, and 4n = 40 m + 16 sm + 8 st. Polyploidy can be found naturally in individuals and colonies and may represent an adaptative trait related to widespread distribution and invasion ability of new habitats.

Karyotype conservation and genomic organization of repetitive sequences in the leaf-cutting ant Atta cephalotes (Linnaeus, 1758) (Formicidae: Myrmicinae)

Leaf-cutting ants are among the New World's most conspicuous and studied ant species due to their notable ecological and economic role. Cytogenetic studies carried out in Atta show remarkable karyotype conservation among the species. We performed classical cytogenetics and physical mapping of repetitive sequences in the leaf-cutting ant Atta cephalotes, the type species of the genus. Our goal was to test the karyotype conservation in Atta and to start to understand the genomic organization and diversity regarding repetitive sequences in leaf-cutting ants. Atta cephalotes showed 2n=22 (18m+2sm+2st) chromosomes. The heterochromatin followed a centromeric pattern, and the GC-rich regions and 18S rDNA clusters were co-located interstitially in the 4th metacentric pair. These cytogenetic characteristics were observed in other Atta species that had previously been studied, confirming the karyotype conservation in Atta. Evolutionary implications regarding the conservation of the chromosome number in leaf-cutting ants are discussed. Telomeric motif (TTAGG)n was detected in A. cephalotes as observed in other ants. Five out of the 11 microsatellites showed a scattered distribution exclusively on euchromatic areas of the chromosomes. Repetitive sequences mapped on the chromosomes of A. cephalotes are the first insights into genomic organization and diversity in leaf-cutting ants, useful in further comparative studies.

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.

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.

Cytogenetic Analysis in Trigona spinipes Fabricius (Hymenoptera, Meliponina) Reveals Intraspecific Variation

Trigona spinipes Fabricius is a stingless bee with wide geographical distribution. Although being sometimes considered an agricultural pest, in fact, it has great pollinating potential, and therefore economic interest. Conventional and molecular cytogenetic techniques have been little used to verify genetic diversity in this species, despite its potential to reveal information about the reorganization of the genome having been demonstrated in other species. Conventional cytogenetic techniques, fluorochrome staining, and fluorescent in situ hybridization with 18S rDNA, telomeric, and microsatellite probes (GA)₁₅ were used in this study to characterize and compare T. spinipes from different locations. The karyotypes showed a conserved chromosome number 2n = 34; however, geographic variations were verified in the different features and cytogenetic techniques analyzed, such as karyotype formulas, fluorocrome staining, and FISH. Although the 18S rDNA probe revealed the same number of markings in five rDNA clusters, the chromosomal pairs containing these markers varied between studied locations. The probe for microsatellite (GA)₁₅ also showed polymorphisms within this species. The results reveal that T. spinipes has many intraspecific differences, revealing a higher chromosomal variation than expected.

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.

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.

The tight genome size of ants: diversity and evolution under ancestral state reconstruction and base composition

The mechanisms and processes driving change and variation in the genome size (GS) are not well known, and only a small set of ant species has been studied. Ants are an ecologically successful insect group present in most distinct ecosystems worldwide. Considering their wide distribution and ecological plasticity in different environmental contexts, we aimed to expand GS estimation within Formicidae to examine distribution patterns and variation in GS and base composition and to reconstruct the ancestral state of this character in an attempt to elucidate the generalized pattern of genomic expansions. Genome size estimates were generated for 99 ant species, including new GS estimates for 91 species of ants, and the mean GS of Formicidae was found to be 0.38 pg. The AT/GC ratio was 62.40/37.60. The phylogenetic reconstruction suggested an ancestral GS of 0.38 pg according to the Bayesian inference/Markov chain Monte Carlo method and 0.37 pg according to maximum likelihood and parsimony methods; significant differences in GS were observed between the subfamilies sampled. Our results suggest that the evolution of GS in Formicidae occurred through loss and accumulation of non-coding regions, mainly transposable elements, and occasionally by whole genome duplication. However, further studies are needed to verify whether these changes in DNA content are related to colonization processes, as suggested at the intraspecific level.

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.

Cytogenetic variability in four species of Gnamptogenys Roger, 1863 (Formicidae: Ectatomminae) showing chromosomal polymorphisms, species complex, and cryptic species

Gnamptogenys includes 138 described species that are widely distributed, with high diversity, in the Neotropics. Some Neotropical species have taxonomic issues, as is the case with Gnamptogenys striatula, for which morphological variations have been observed between different populations. For the ant species with taxonomic issues, classical and molecular cytogenetic studies have assisted in the resolution of these issues. Cytogenetic studies of Gnamptogenys are scarce and have only been reported for 14 taxa. These reports have rarely presented chromosomal morphology. Considering the importance of the taxonomic revision of some species, such as G. striatula, the present study cytogenetically characterized four species of Gnamptogenys: G. striatula, G. moelleri, G. regularis, and G. triangularis, discussing their phylogenetic and biogeographic characteristics. The number of chromosomes ranged from 2n = 26 to 2n = 44, with distinct karyotypes at both species and population levels. All four species presented a pair of 18S rDNA gene markers that coincided with GC-rich regions. In the case of G. striatula from the Atlantic rainforest, a chromosomal polymorphism was observed, with chromosomal translocations being the likely origin of this polymorphism. Two populations of G. striatula showed karyotype differences, thus corroborating previous morphological data indicating the existence of a species complex in this taxon. In addition, G. regularis showed a polymorphism involving a chromosome pair bearing ribosomal genes, possibly caused by unequal crossing-over. Although G. moelleri has a well-defined taxonomy, a population from the eastern Amazon rainforest presented a divergent karyotype from the Atlantic rainforest populations, suggesting the existence of a cryptic species in this taxon.

Cytogenetic data for sixteen ant species from North-eastern Amazonia with phylogenetic insights into three subfamilies

Ants play essential roles in most terrestrial ecosystems and may be considered pests for agriculture and agroforestry. Recent morphological and molecular data have challenged conventional ant phylogeny and the interpretation of karyotypic variations. Existing Neotropical ant cytogenetic data focus on Atlantic rainforest species, and provide evolutionary and taxonomic insight. However, there are data for only 18 Amazonian species. In this study, we describe the karyotypes of 16 ant species belonging to 12 genera and three subfamilies, collected in the Brazilian state of Amapá, and in French Guiana. The karyotypes of six species are described for the first time, including that of the South American genus Allomerus Mayr, 1878. The karyotype of Crematogaster Lund, 1831 is also described for the first time for the New World. For other species, extant data for geographically distinct populations was compared with our own data, e.g. for the leafcutter ants Acromyrmex balzani (Emery, 1890) and Atta sexdens (Linnaeus, 1758). The information obtained for the karyotype of Dolichoderus imitator Emery, 1894 differs from extant data from the Atlantic forest, thereby highlighting the importance of population cytogenetic approaches. This study also emphasizes the need for good chromosome preparations for studying karyotype structure.

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.

Population-Based Cytogenetic Banding Analysis and Phylogenetic Relationships of the Neotropical Fungus-Farming Ant Trachymyrmex holmgreni Wheeler, 1925

Trachymyrmex is one of the most species-rich genera within fungus-farming ants and presents intraspecific cytogenetic polymorphisms as well as possible cryptic species. This ant genus is currently paraphyletic. Therefore, to unravel systematic and taxonomic misunderstandings, it is necessary to incorporate new information. We aimed to cytogenetically and genetically examine Trachymyrmex holmgreni populations from southern and northern Brazil to identify intraspecific chromosomal variations that support incipient speciation and reveal the species' position in a molecular phylogeny. Our cytogenetic approach did not show population variation in the mapping of both 18S rDNA and the TTAGG(6) motif, presenting instead a pattern characteristic of correlated species. However, the clustered pattern of the microsatellite GA(15) showed significant differences among populations: a well-defined block in each homologue, distinctly irregular signs between homologues, and blocks in 2 pairs of homologues. Our phylogenetic reconstruction yielded unexpected results, grouping representatives of 3 former morphological groups into 1 clade, namely T. urichii, T. papulatus, and T. holmgreni. Previously, it was suggested that northern and southern populations of T. holmgreni may be undergoing incipient speciation, but we can only indicate that the southernmost population differs prominently from the others in its distribution pattern of the microsatellite GA(15). Our study also supports the uniformity of karyotypes and repetitive DNA from both telomeric sequences and ribosomal DNA in Trachymyrmex studied here. In addition, we clarify some phylogenetic uncertainties within the genus and suggest further relevant systematic changes. Finally, additional studies utilizing other probes and additional populations may allow the detection of hidden genetic variation.

The evolution of haploid chromosome numbers in Meliponini

It is thought that two evolutionary mechanisms gave rise to chromosomal variation in bees: the first one points to polyploidy as the main cause of chromosomal evolution, while the second, Minimum Interaction Theory (MIT), is more frequently used to explain chromosomal changes in Meliponini and suggests that centric fission is responsible for variations in karyotype. However, differences in chromosome number between Meliponini and its sister taxa and in the karyotype patterns of the Melipona genus cannot be explained by MIT, suggesting that other events were involved in chromosomal evolution. Thus, we assembled cytogenetical and molecular information to reconstruct an ancestral chromosome number for Meliponini and its sister group, Bombini, and propose a hypothesis to explain the evolutionary pathways underpinning chromosomal changes in Meliponini. We hypothesize that the common ancestor shared by the Meliponini and Bombini tribes possessed a chromosome number of n = 18. The karyotype with n = 17 chromosomes was maintained in Meliponini, and variations of haploid numbers possibly originated through additional Robertsonian fissions and fusions. Thus, the low chromosome number would not be an ancestral condition, as predicted by MIT. We then conclude that Robertsonian fission and fusions are unlikely to be the cause of chromosomal rearrangements that originated the current karyotypes in Meliponini.

Molecular phylogeny of the scorpionflies Panorpidae (Insecta: Mecoptera) and chromosomal evolution

Panorpidae is the most species-rich family in Mecoptera with ca. 470 species in the Northern Hemisphere. However, the intergeneric phylogenetic relationships of Panorpidae remain unsatisfactorily resolved to date. Here, we used molecular and cytogenetic approaches to determine the phylogenetic relationships of Panorpidae in the evolutionary scenario of chromosomes, and estimated their divergence times using fossil-calibrated Bayesian analysis. In total, 89 species representing all seven genera of Panorpidae were used to reconstruct the phylogenetic trees using maximum parsimony, maximum likelihood and Bayesian inference based on the nuclear 28S rRNA and mitochondrial cox1 and cox2 genes. The results reveal that Panorpidae is a well-supported monophyletic group that can be categorized into two major clades. Major Clade I comprises Neopanorpa and Leptopanorpa, and Major Clade II consists of all the other genera (Cerapanorpa, Dicerapanorpa, Furcatopanorpa, Panorpa and Sinopanorpa). Neopanorpa and Cerapanorpa are regarded as paraphyletic groups for the first time. BEAST analysis indicates that Panorpidae originated in the Lower Cretaceous approximately 122.5 Ma (96.8-149.3 Ma), and that most diversification occurred from the Selandian (59.8 Ma) to the Middle Pleistocene (0.6 Ma) in the Cenozoic. Cytogenetic data plotted on the cladogram show that the lineage differentiation of Panorpidae is closely related to the chromosomal evolution, especially the reduction of chromosome number. Our study suggests that a taxonomic revision of Panorpidae is urgently needed at the generic level.

Chromosomal variation among populations of a fungus-farming ant: implications for karyotype evolution and potential restriction to gene flow

Background

Intraspecific variation in chromosome structure may cause genetic incompatibilities and thus provides the first step in the formation of species. In ants, chromosome number varies tremendously from 2n = 2 to 2n = 120, and several studies have revealed considerable variation in karyotype within species. However, most previous studies were limited to the description of chromosome number and morphology, and more detailed karyomorphometric analyses may reveal additional, substantial variation. Here, we studied karyotype length, genome size, and phylogeography of five populations of the fungus-farming ant Trachymyrmex holmgreni in order to detect potential barriers to gene flow.

Results

Chromosome number and morphology did not vary among the five populations, but karyotype length and genome size were significantly higher in the southernmost populations than in the northern populations of this ant. Individuals or colonies with different karyotype lengths were not observed. Karyotype length variation appears to result from variation in centromere length.

Conclusion

T. holmgreni shows considerable variation in karyotype length and might provide a second example of centromere drive in ants, similar to what has previously been observed in Solenopsis fire ants. Whether this variation leads to genetic incompatibilities between the different populations remains to be studied.

Electronic supplementary material

The online version of this article (10.1186/s12862-018-1247-5) contains supplementary material, which is available to authorized users.

Genetic Characterization of Some Neoponera (Hymenoptera: Formicidae) Populations Within the foetida Species Complex

The foetida species complex comprises 13 Neotropical species in the ant genus Neoponera. Neoponera villosa Fabricius (1804) , Neoponera inversa Smith (1858), Neoponera bactronica Fernandes, Oliveira & Delabie (2013), and Neoponera curvinodis (Forel, 1899) have had an ambiguous taxonomic status for more than two decades. In southern Bahia, Brazil, these four species are frequently found in sympatry. Here we used Bayesian Inference and maximum likelihood analyses of COI and 16S mtDNA sequence data and conventional cytogenetic data together with observations on morphology to characterize sympatric populations of N. villosa, N. inversa, N. bactronica, and N. curvinodis. Our results showed marked differences in the karyotype of these ants. Both N. curvinodis and N. inversa have chromosome number of 2n = 30. Their chromosome composition, however, is distinct, which indicates that N. curvinodis is more closely related to N. bactronica. These four species clustered into three distinct groups. The close relationship between N. bactronica and N. curvinodis deserves further investigation since it has not been fully resolved here. Our results confirm that N. inversa, N. villosa, N. bactronica + N. curvinodis indeed represent four distinct taxa within the foetida species complex.

Phylogenetic patterns of ant-fungus associations indicate that farming strategies, not only a superior fungal cultivar, explain the ecological success of leafcutter ants

To elucidate fungicultural specializations contributing to ecological dominance of leafcutter ants, we estimate the phylogeny of fungi cultivated by fungus-growing (attine) ants, including fungal cultivars from (i) the entire leafcutter range from southern South America to southern North America, (ii) all higher-attine ant lineages (leafcutting genera Atta, Acromyrmex; nonleafcutting genera Trachymyrmex, Sericomyrmex) and (iii) all lower-attine lineages. Higher-attine fungi form two clades, Clade-A fungi (Leucocoprinus gongylophorus, formerly Attamyces) previously thought to be cultivated only by leafcutter ants, and a sister clade, Clade-B fungi, previously thought to be cultivated only by Trachymyrmex and Sericomyrmex ants. Contradicting this traditional view, we find that (i) leafcutter ants are not specialized to cultivate only Clade-A fungi because some leafcutter species ranging across South America cultivate Clade-B fungi; (ii) Trachymyrmex ants are not specialized to cultivate only Clade-B fungi because some Trachymyrmex species cultivate Clade-A fungi and other Trachymyrmex species cultivate fungi known so far only from lower-attine ants; (iii) in some locations, single higher-attine ant species or closely related cryptic species cultivate both Clade-A and Clade-B fungi; and (iv) ant-fungus co-evolution among higher-attine mutualisms is therefore less specialized than previously thought. Sympatric leafcutter ants can be ecologically dominant when cultivating either Clade-A or Clade-B fungi, sustaining with either cultivar-type huge nests that command large foraging territories; conversely, sympatric Trachymyrmex ants cultivating either Clade-A or Clade-B fungi can be locally abundant without achieving the ecological dominance of leafcutter ants. Ecological dominance of leafcutter ants therefore does not depend primarily on specialized fungiculture of L. gongylophorus (Clade-A), but must derive from ant-fungus synergisms and unique ant adaptations.

Molecular phylogenetic reconstruction and localization of the (TTAGG)n telomeric repeats in the chromosomes of Acromyrmex striatus (Roger, 1863) suggests a lower ancestral karyotype for leafcutter ants (Hymenoptera)

Chromosome counts and karyotype characterization have proved to be important features of a genome. Chromosome changes during the diversification of ants might play an important role, given the diversity and success of Formicidae. Comparative karyotype analyses on ants have enriched and helped ant systematics. Among leafcutter ants, two major chromosome counts have been described, one frequent in Atta Fabricius, 1804 (2n = 22 in all Atta spp. whose karyotype is known) and the other frequent in Acromyrmex Mayr, 1865 (2n = 38 in the majority of species whose karyotype is known). The main exception is Acromyrmex striatus (Roger, 1863), which harbors a diploid chromosome set of 22. Here we describe the use of fluorescence in situ hybridization (FISH) with telomeric probes with (TTAGG)₆ repeats to describe the telomere composition of A. striatus and to recover potential interstitial non-telomeric signals that may reflect fusion events during the evolution of leafcutter lineage from 38 to 22 chromosomes. Further, we reconstruct the ancestral chromosome numbers of the leafcutter clade based on a recently proposed molecular phylogenetic hypothesis and phylogenomic tree. Distinct signals have been observed in both extremities on the telomere chromosomes of A. striatus. Non-telomeric signals have not been retrieved in our analysis. It could be supposed that the low-numbered karyotype indeed represents the ancestral chromosome number of leafcutters. The phylogenetic reconstruction also recovered a low chromosome number from the diverse approaches implemented, suggesting that n = 11 is the most likely ancestral karyotype of the leafcutter ants and is a plesiomorphic feature shared between A. striatus and Atta spp.

Biogeography of mutualistic fungi cultivated by leafcutter ants

Leafcutter ants propagate co-evolving fungi for food. The nearly 50 species of leafcutter ants (Atta, Acromyrmex) range from Argentina to the United States, with the greatest species diversity in southern South America. We elucidate the biogeography of fungi cultivated by leafcutter ants using DNA sequence and microsatellite-marker analyses of 474 cultivars collected across the leafcutter range. Fungal cultivars belong to two clades (Clade-A and Clade-B). The dominant and widespread Clade-A cultivars form three genotype clusters, with their relative prevalence corresponding to southern South America, northern South America, Central and North America. Admixture between Clade-A populations supports genetic exchange within a single species, Leucocoprinus gongylophorus. Some leafcutter species that cut grass as fungicultural substrate are specialized to cultivate Clade-B fungi, whereas leafcutters preferring dicot plants appear specialized on Clade-A fungi. Cultivar sharing between sympatric leafcutter species occurs frequently such that cultivars of Atta are not distinct from those of Acromyrmex. Leafcutters specialized on Clade-B fungi occur only in South America. Diversity of Clade-A fungi is greatest in South America, but minimal in Central and North America. Maximum cultivar diversity in South America is predicted by the Kusnezov-Fowler hypothesis that leafcutter ants originated in subtropical South America and only dicot-specialized leafcutter ants migrated out of South America, but the cultivar diversity becomes also compatible with a recently proposed hypothesis of a Central American origin by postulating that leafcutter ants acquired novel cultivars many times from other nonleafcutter fungus-growing ants during their migrations from Central America across South America. We evaluate these biogeographic hypotheses in the light of estimated dates for the origins of leafcutter ants and their cultivars.

Reassessing the Chromosome Number and Morphology of the Turtle Ant Cephalotes pusillus (Klug, 1824) Using Karyomorphometrical Analysis and Observations of New Nesting Behavior

Here we use karyomorphometrical analysis to characterize and evaluate the karyotype of the turtle ant Cephalotes pusillus (Klug, 1824). This is the first representative of this diverse ant genus to be cytogenetically studied. They bear a diploid chromosome set of 44 chromosomes, which, according to the centromeric index, are metacentric, submetacentric, and subtelocentric. This small ant is quite widely distributed in the Neotropics and seems to be well adapted to living in disturbed areas. Here we report the species nesting on dead trunks used to build fences at countryside houses and farms. On these nests, we observed some never reported behavior of C. pusillus: the ants appear to be able to dig by actively removing small fragments of dead wood fiber, hence expanding their nest cavities. It was not thought that Cephalotes species had this ability, given that they nest in preexisting cavities. Our observations are initial remarks that the small plier-like mandibles of C. pusillus may not be a constraint for this species, adding to our knowledge on ant nesting biology.

Cytogenetic data on the agro-predatory ant Megalomyrmex incisus Smith, 1947 and its host, Mycetophylax conformis (Mayr, 1884) (Hymenoptera, Formicidae)

We provide the first karyotype description of the agro-predatory ant species Megalomyrmex incisus Smith, 1947 (Myrmicinae, Formicidae), and chromosome counts of its host Mycetophylax conformis (Mayr, 1884) (Myrmicinae, Formicidae) from geographically distinct populations. Colonies of both species were sampled from coastal areas of Ilhéus, Bahia, Brazil, and transferred to the laboratory. Metaphase spreads were prepared from the cerebral ganglia of defecated larvae. The slides were examined and pictures of the best metaphases were taken. The chromosome number for Megalomyrmex incisus was 2n=50 and n=25. The karyotype of this species consists of 20 metacentric and 5 submetacentric pairs. Thus, the karyotype formula of the diploid set was 2K=40M + 10SM and a fundamental number FN=100. The host species Mycetophylax conformis has 2n=30 and the karyotype consisting of 11 metacentric and 4 submetacentric pairs. The karyotype formula was 2K=22M + 8SM, and a fundamental number FN=60. Megalomyrmex incisus showed a slightly higher chromosome number, placed at the marginal range of the known distribution of haploid karyotypes of the Myrmicinae. The chromosome number and chromosomal morphology of Mycetophylax conformis corresponded to those of previously studied populations, suggesting its karyotype stability.

Geographical Distribution Patterns and Niche Modeling of the Iconic Leafcutter Ant Acromyrmex striatus (Hymenoptera: Formicidae)

Ants are considered one of the most successful groups in the planet's evolutionary history. Among them highlights the fungus-farming ants of the genera Atta and Acromyrmex that occur throughout most of the Americas. Within the Acromyrmex genus, the species A. striatus distinguishes from other Acromyrmex species as its morphology and karyotype differ from its congeners. This species is found in open environments of dry climate in the southern States of Brazil, Argentina, Paraguay and Uruguay; however, little is known about the current distribution of the species. This article aimed to investigate the current distribution of the species by compiling its known distribution and discussing its distributional range. To achieve this, published and unpublished data obtained through a literature search and active collections in various locations were compiled. Published and unpublished data revealed that 386 colonies were recorded, distributed across four countries where its occurrence is known. Environmental factors, such as temperature, humidity, soil type and vegetation, as well as historical geological and climate events that have modified Earth's surface may have influenced species distribution patterns. In the Neotropics, the environmental factors that most impacted the distribution of species were the glaciation periods that occurred in the Quaternary, leading to a great migratory process. These factors may have contributed to the current geographical distribution of A. striatus.

The Evolution of Haploid Chromosome Numbers in the Sunflower Family

Chromosome number changes during the evolution of angiosperms are likely to have played a major role in speciation. Their study is of utmost importance, especially now, as a probabilistic model is available to study chromosome evolution within a phylogenetic framework. In the present study, likelihood models of chromosome number evolution were fitted to the largest family of flowering plants, the Asteraceae. Specifically, a phylogenetic supertree of this family was used to reconstruct the ancestral chromosome number and infer genomic events. Our approach inferred that the ancestral chromosome number of the family is n = 9. Also, according to the model that best explained our data, the evolution of haploid chromosome numbers in Asteraceae was a very dynamic process, with genome duplications and descending dysploidy being the most frequent genomic events in the evolution of this family. This model inferred more than one hundred whole genome duplication events; however, it did not find evidence for a paleopolyploidization at the base of this family, which has previously been hypothesized on the basis of sequence data from a limited number of species. The obtained results and potential causes of these discrepancies are discussed.

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

Cytogenetic data for the genus Acromyrmex Mayr, 1865 are available, to date, for a few species from Brazil and Uruguay, which have uniform chromosome numbers (2n = 38). The recent cytogenetic data of Acromyrmex striatus (Roger, 1863), including its banding patterns, showed a distinct karyotype (2n = 22), similar to earlier studied Atta Fabricius, 1804 species. Karyological data are still scarce for the leafcutter ants and many gaps are still present for a proper understanding of this group. Therefore, this study aimed at increasing cytogenetic knowledge of the genus through the characterization of other six species: Acromyrmex balzani (Emery, 1890), Acromyrmex coronatus Fabricius, 1804, Acromyrmex disciger (Mayr, 1887), Acromyrmex echinatior (Forel, 1899), Acromyrmex niger (Smith, 1858) and Acromyrmex rugosus (Smith, 1858), all of which were collected in Minas Gerais - Brazil, except for Acromyrmex echinatior which was collected in Barro Colorado - Panama. The number and morphology of the chromosomes were studied and the following banding techniques were applied: C-banding, fluorochromes CMA3 and DAPI, as well as the detection of 45S rDNA using FISH technique. All the six species had the same chromosome number observed for already studied species, i.e. 2n = 38. Acromyrmex balzani had a different karyotype compared with other species mainly due to the first metacentric pair. The heterochromatin distribution also showed interspecific variation. Nevertheless, all the studied species had a pair of bands in the short arm of the first subtelocentric pair. The fluorochrome CMA3 visualized bands in the short arm of the first subtelocentric pair for all the six species, while Acromyrmex rugosus and Acromyrmex niger also demonstrated in the other chromosomes. The AT-rich regions with differential staining using DAPI were not observed. 45S ribosomal genes were identified by FISH in the short arm of the first subtelocentric pair in Acromyrmex coronatus, Acromyrmex disciger and Acromyrmex niger. The uniform chromosome number in the genus Acromyrmex (2n = 38) suggests that Acromyrmex striatus (2n = 22) should be transferred to a new genus. Other aspects of the chromosome evolution in ants are also discussed.

Integrating Paleodistribution Models and Phylogeography in the Grass-Cutting Ant Acromyrmex striatus (Hymenoptera: Formicidae) in Southern Lowlands of South America

Past climate changes often have influenced the present distribution and intraspecific genetic diversity of organisms. The objective of this study was to investigate the phylogeography and historical demography of populations of Acromyrmex striatus (Roger, 1863), a leaf-cutting ant species restricted to the open plains of South America. Additionally, we modeled the distribution of this species to predict its contemporary and historic habitat. From the partial sequences of the mitochondrial gene cytochrome oxidase I of 128 A. striatus workers from 38 locations we estimated genetic diversity and inferred historical demography, divergence time, and population structure. The potential distribution areas of A. striatus for current and quaternary weather conditions were modeled using the maximum entropy algorithm. We identified a total of 58 haplotypes, divided into five main haplogroups. The analysis of molecular variance (AMOVA) revealed that the largest proportion of genetic variation is found among the groups of populations. Paleodistribution models suggest that the potential habitat of A. striatus may have decreased during the Last Interglacial Period (LIG) and expanded during the Last Maximum Glacial (LGM). Overall, the past potential distribution recovered by the model comprises the current potential distribution of the species. The general structuring pattern observed was consistent with isolation by distance, suggesting a balance between gene flow and drift. Analysis of historical demography showed that populations of A. striatus had remained constant throughout its evolutionary history. Although fluctuations in the area of their potential historic habitat occurred during quaternary climate changes, populations of A. striatus are strongly structured geographically. However, explicit barriers to gene flow have not been identified. These findings closely match those in Mycetophylax simplex, another ant species that in some areas occurs in sympatry with A. striatus. Ecophysiological traits of this species and isolation by distance may together have shaped the phylogeographic pattern.

Colony size predicts division of labour in attine ants

Division of labour is central to the ecological success of eusocial insects, yet the evolutionary factors driving increases in complexity in division of labour are little known. The size-complexity hypothesis proposes that, as larger colonies evolve, both non-reproductive and reproductive division of labour become more complex as workers and queens act to maximize inclusive fitness. Using a statistically robust phylogenetic comparative analysis of social and environmental traits of species within the ant tribe Attini, we show that colony size is positively related to both non-reproductive (worker size variation) and reproductive (queen-worker dimorphism) division of labour. The results also suggested that colony size acts on non-reproductive and reproductive division of labour in different ways. Environmental factors, including measures of variation in temperature and precipitation, had no significant effects on any division of labour measure or colony size. Overall, these results support the size-complexity hypothesis for the evolution of social complexity and division of labour in eusocial insects. Determining the evolutionary drivers of colony size may help contribute to our understanding of the evolution of social complexity.