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

Structure and evolution of metapolycentromeres

Metapolycentromeres consist of multiple sequential domains of centromeric chromatin associated with a centromere-specific variant of histone H3 (CENP-A), functioning collectively as a single centromere. To date, they have been revealed in nine flowering plant, five insect and six vertebrate species. In this paper, we focus on their structure and possible mechanisms of emergence and evolution. The metapolycentromeres may vary in the number of centromeric domains and in their genetic content and epigenetic modifications. However, these variations do not seem to affect their function. The emergence of metapolycentromeres has been attributed to multiple Robertsonian translocations and segmental duplications. Conditions of genomic instability, such as interspecific hybridization and malignant neoplasms, are suggested as triggers for the de novo emergence of metapolycentromeres. Addressing the "centromere paradox" - the rapid evolution of centromeric DNA and proteins despite their conserved cellular function - we explore the centromere drive hypothesis as a plausible explanation for the dynamic evolution of centromeres in general, and in particular the emergence of metapolycentromeres and holocentromeres. Apparently, metapolycentromeres are more common across different species than it was believed until recently. Indeed, a systematic review of the available cytogenetic publications allowed us to identify 27 candidate species with metapolycentromeres. Тhe list of the already established and newly revealed candidate species thus spans 27 species of flowering plants and eight species of gymnosperm plants, five species of insects, and seven species of vertebrates. This indicates an erratic phylogenetic distribution of the species with metapolycentromeres and may suggest an independent emergence of the metapolycentromeres in the course of evolution. However, the current catalog of species with identified and likely metapolycentromeres remains too short to draw reliable conclusions about their evolution, particularly in the absence of knowledge about related species without metapolycentromeres for comparative analysis. More studies are necessary to shed light on the mechanisms of metapolycentromere formation and evolution.

Chromosomal organization of different repetitive sequences in four wasp species of the genus Trypoxylon Latreille (Hymenoptera: Crabronidae) and insights into the composition of wasp telomeres

This study characterizes the chromosomal organization of DNA repetitive sequences and the karyotypic evolution in four representatives of the solitary wasp genus Trypoxylon using conventional and molecular cytogenetic techniques. Our findings present the first cytogenetic data for Trypoxylon rogenhoferi (2n = 30) and Trypoxylon albonigrum (2n = 32), while the karyotypes of Trypoxylon nitidum (2n = 30) and Trypoxylon lactitarse (2n = 30) were similar to those previously described. Fluorochrome staining and microsatellite distribution data revealed differences in the constitutive heterochromatin composition among species. Trypoxylon nitidum and T. albonigrum exhibited one major rDNA cluster, potentially representing an ancestral pattern for aculeate Hymenoptera, while T. rogenhoferi and T. lactitarse showed two pericentromeric rRNA gene sites, suggesting amplification events in their ancestral clade. The (TCAGG)n motif hybridized in the terminal regions of the chromosomes in all four Trypoxylon species, which may suggest that this sequence represents DNA telomeric repeat. Notably, the presence of this repetitive sequence in the centromeric regions of certain chromosome pairs in two species supports the hypothesis of chromosomal fusions or inversions in the ancestral karyotype of Trypoxylon. The study expands the chromosomal mapping data of repetitive sequences in wasps and offers insights into the dynamic evolutionary landscape of karyotypes in these insects.

Conventional cytogenetics and microsatellite chromosomal distribution in social wasp Mischocyttarus cassununga (Ihering, 1903) (Vespidae, Polistinae, Mischocyttarini)

Cytogenetics has allowed the investigation of chromosomal diversity and repetitive genomic content in wasps. In this study, we characterized the karyotype of the social wasp Mischocyttarus cassununga using conventional cytogenetics and chromosomal mapping of repetitive sequences. This study was undertaken to extend our understanding of the genomic organization of repetitive DNA in social wasps and is the first molecular cytogenetic insight into the genus Mischocyttarus. The karyotype of M. cassununga had a chromosome number of 2n = 64 for females and n = 32 for males. Constitutive heterochromatin exhibited three distribution patterns: centromeric and pericentromeric regions along the smaller arms and extending almost the entire chromosome. The major ribosomal DNA sites were located on chromosome pair in females and one chromosome in males. Positive signals for the microsatellite probes (GA)n and (GAG)n were observed in the euchromatic regions of all chromosomes. The microsatellites, (CGG)n, (TAT)n, (TTAGG)n, and (TCAGG)n were not observed in any region of the chromosomes. Our results contrast with those previously obtained for Polybia fastidiosuscula, which showed that the microsatellites (GAG)n, (CGG)n, (TAT)n, (TTAGG)n, and (TCAGG)n are located predominantly in constitutive heterochromatin. This suggests variations in the diversity and chromosomal organization of repetitive sequences in the genomes of social wasps.

Effects of hTERT transfection on the telomere and telomerase of Periplaneta americana cells in vitro

Telomere and telomerase are crucial factors in cell division and chromosome stability. Telomerase activity in most cells depends on the transcription control by the telomerase reverse transcriptase (TERT). The introduction of an exogenous human TERT (hTERT) in cultured cells could enhance telomerase activity and elongate the lifespan of various cells. Telomere elongation mechanisms vary between insects and are complex and unusual. Whether the use of exogenous hTERT can immortalize primary insect cells remains to be investigated. In this study, we used a recombinant virus expressing hTERT to infect primary cultured cells of Periplaneta americana and evaluated its effects on insect cell immortalization. We found that hTERT was successfully expressed and promoted the growth of P. americana cells, shortening their doubling time. This was due to the ability of hTERT to increase the activity of telomerase in P. americana cells, thus prolonging the telomeres. Our study lays the foundation for understanding the mechanisms of telomere elongation in P. americana, and suggests that the introduction of hTERT into insect cells could be an efficient way to establish certain insect cell lines.

Differential Spreading of Microsatellites in Holocentric Chromosomes of Chagas Disease Vectors: Genomic and Evolutionary Implications

This study focused on analyzing the distribution of microsatellites in holocentric chromosomes of the Triatominae subfamily, insect vectors of Chagas disease. We employed a non-denaturing FISH technique to determine the chromosomal distribution of sixteen microsatellites across twenty-five triatomine species, involving five genera from the two principal tribes: Triatomini and Rhodniini. Three main hybridization patterns were identified: strong signals in specific chromosomal regions, dispersed signals dependent on microsatellite abundance and the absence of signals in certain chromosomal regions or entire chromosomes. Significant variations in hybridization patterns were observed between Rhodniini and Triatomini species. Rhodniini species displayed weak and scattered hybridization signals, indicating a low abundance of microsatellites in their genomes. In contrast, Triatomini species exhibited diverse and abundant hybridization patterns, suggesting that microsatellites are a significant repetitive component in their genomes. One particularly interesting finding was the high abundance of GATA repeats, and to a lesser extent AG repeats, in the Y chromosome of all analyzed Triatomini species. In contrast, the Y chromosome of Rhodniini species did not show enrichment in GATA and AG repeats. This suggests that the richness of GATA repeats on the Y chromosome likely represents an ancestral trait specific to the Triatomini tribe. Furthermore, this information can be used to elucidate the evolutionary relationships between Triatomini and other groups of reduviids, contributing to the understanding of the subfamily's origin. Overall, this study provides a comprehensive understanding of the composition and distribution of microsatellites within Triatominae genomes, shedding light on their significance in the evolutionary processes of these species.

Supernumerary B Chromosomes of Tetragonisca fiebrigi Share Repeat Content with Standard Chromosome Set of both T. fiebrigi and Tetragonisca angustula (Apidae: Meliponini)

The stingless bees Tetragonisca angustula and Tetragonisca fiebrigi are widely distributed in Brazil, and both are commonly known as "jataí." Our goal was to investigate the possible origin of the B chromosomes in T. fiebrigi, a cytotaxonomic trait that differentiates T. fiebrigi from T. angustula. We analyzed diploid chromosome number (2n), B chromosome incidence, patterns of constitutive heterochromatin, and in situ localization of different repetitive DNA probes in T. angustula and T. fiebrigi. Both species displayed 2n = 34, with similar karyotype structures. One to three B chromosomes were observed in T. fiebrigi only. Constitutive heterochromatin was distributed on one arm of all chromosomes in both species, and T. fiebrigi B chromosomes were mainly heterochromatic with one euchromatic extremity. The (GA)15 and (CAA)10 microsatellite probes marked the euchromatic arms of all chromosomes in both species without marking the B chromosomes. The 18S ribosomal DNA (rDNA) probe marked 10 chromosomes in T. angustula and 6 A chromosomes in T. fiebrigi with an additional marking on 1B in individuals with 3B. The Tan-Bsp68I repetitive DNA probe marked the heterochromatic portion of all T. fiebrigi A and B chromosomes. This probe also marked the heterochromatic portion of all T. angustula chromosomes; therefore, both alternative hypotheses to the B chromosome origin are possible: (i) from the A chromosome complement of T. fiebrigi (intraspecific origin); or (ii) a by-product of genome reshuffling following the hybridization between T. fiebrigi and T. angustula (interspecific origin).

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.

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.