Sex chromosome diversity in Armenian toad grasshoppers (Orthoptera, Acridoidea, Pamphagidae)

New insights into the chromosomes of stoneflies: I. Karyotype, C-banding and localization of ribosomal and telomeric DNA markers in Skwalacompacta (McLachlan, 1872) (Polyneoptera, Plecoptera, Perlodidae) from Siberia

This study provides data on chromosome number (2n♂♀=26), sex determination mechanism (XY♂/XX♀), C-banding pattern, distribution of clusters of telomeric TTAGG repeats and 18S ribosomal DNA in the karyotype of the stonefly Skwalacompacta (McLachlan, 1872). For the first time in the history of stoneflies cytogenetics, we provide photos of the chromosomes of the Plecoptera insects. The karyotype of males and females of S.compacta consists of 12 pairs of autosomes. Three pairs of large autosomes and four pairs of medium-sized autosomes are subacrocentric. The remaining pairs of autosomes are small, with unclear morphology. Pericentromeric C-bands were revealed in all autosomes. The sex chromosomes are also subacrocentric. The short arms of X and Y chromosomes are entirely heterochromatic and are rich in ribosomal DNA sequences. In the X chromosome this arm is larger than in the Y chromosome. It is likely that this arm associated with the nucleolar organizer (NOR). Telomeric DNA (TTAGG)n repeats were detected in the terminal regions of all chromosomes.

Karyotypes diversity in some Iranian Pamphagidae grasshoppers (Orthoptera, Acridoidea, Pamphagidae): new insights on the evolution of the neo-XY sex chromosomes

For the first time, cytogenetic features of grasshoppers from Iran have been studied. In this paper we conducted a comparative cytogenetic analysis of six species from the family Pamphagidae. The species studied belong to subfamilies Thrinchinae Stål, 1876 (Eremopeza bicoloripes (Moritz, 1928), E. saussurei (Uvarov, 1918)) and Pamphaginae (Saxetania paramonovi (Dirsh, 1927), Tropidauchen escalerai Bolívar, 1912, Tropidauchen sp., and Paranothrotes citimus Mistshenko, 1951). We report information about the chromosome number and morphology, C-banding patterns, and localization of ribosomal DNA clusters and telomeric (TTAGG)n repeats. Among these species, only S. paramonovi had an ancestral Pamphagidae karyotype (2n=18+X0♂; FN=19♂). The karyotypes of the remaining species differed from the ancestral karyotypes. The karyotypes of E. bicoloripes and E. saussurei, despite having the same chromosome number (2n=18+X0♂) had certain biarmed chromosomes (FN=20♂ and FN=34♂ respectively). The karyotypes of T. escalerai and Tropidauchen sp. consisted of eight pairs of acrocentric autosomes, one submetacentric neo-X chromosome and one acrocentric neo-Y chromosome in males (2n=16+neo-X neo-Y♂). The karyotype of P. citimus consisted of seven pairs of acrocentric autosomes, submetacentric the neo-X1 and neo-Y and acrocentric the neo-X2 chromosomes (2n=14+neo-X1 neo-X2 neo-Y♂). Comparative analysis of the localization and size of C-positive regions, the position of ribosomal clusters and the telomeric DNA motif in the chromosomes of the species studied, revealed early unknown features of their karyotype evolution. The data obtained has allowed us to hypothesize that the origin and early phase of evolution of the neo-Xneo-Y♂ sex chromosome in the subfamily Pamphaginae, are linked to the Iranian highlands.

Rapid chromosomal evolution in the bush-cricket Gonatoxia helleri Hemp, 2016 (Orthoptera, Phaneropterinae)

Gonatoxia helleri Hemp, 2016 is one of the most widespread bush-crickets of the genus Gonatoxia Karsch, 1889 in East Africa. This species with seven large chromosomes (2n♂ = 7) differs from other representatives of the genus Gonatoxia drastically by its reduced chromosome number, the asymmetrical karyotype including karyomorphs rarely found in tettigoniids, as well as in irregularities in the course of meiosis. To better understand the origin of such an exceptional karyotype, chromosomes of 29 specimens from four populations/localities were studied using classical techniques, such as C-banding, silver impregnation, fluorochrome double staining and fluorescence in situ hybridization (FISH) technique with 18S rDNA and (TTAGG) _n telomeric probes. FISH showed many 18S rDNA loci as well as interstitial telomeric sequences, where chromosome morphology varied in these components in terms of quantity and distribution. The 18S rDNA loci coincided with active NORs and C-banding patterns. We suggest that a combination of Robertsonian rearrangements and/or multiple common tandem fusions involving the same chromosomes contributed to the formation of this karyotype/karyomorphs. The results are the first step towards a better understanding of chromosomal reorganization and evolution within the genus Gonatoxia. Low chromosome number, together with the incidence of chromosomal polymorphism that is higher in G. helleri than previously reported in bush-crickets, implies that this species can be a valuable new model for cytogenetic and speciation studies. Our findings suggest that chromosomal translocations lead to diversification and speciation in this species and could be the driving force of adaptive radiation.

Comparative analysis of C-heterochromatin, ribosomal and telomeric DNA markers in chromosomes of Pamphagidae grasshoppers from Morocco

The karyotypes and the localization of C-bands, clusters of ribosomal DNA and telomeric repeats of 10 species of the family Pamphagidae from Morocco are described for the first time. The species studied belong to the subfamilies Pamphaginae and Thrinchinae. All species have karyotypes consisting of 19 and 20 acrocentric chromosomes and X0/XX sex chromosome system in males and females, respectively (2n♂=19, NF=19; 2n♀=20, NF=20). Despite the karyotype conservatism, we revealed differences in the location and size of C-heterochromatin blocks and ribosomal DNA clusters. A comparative analysis of these differences shows that karyotype divergences in this group is connected not to structural chromosome rearrangements, but to the evolution of repetitive DNA.

B Chromosomes in Grasshoppers: Different Origins and Pathways to the Modern Bs

B chromosomes (B_s) were described in most taxa of eukaryotes and in around 11.9% of studied Orthopteran species. In some grasshopper species, their evolution has led to many B chromosome morphotypes. We studied the B_s in nine species (Nocaracris tardus, Nocaracris cyanipes, Aeropus sibiricus, Chorthippus jacobsoni, Chorthippus apricarius, Bryodema gebleri, Asiotmethis heptapotamicus songoricus, Podisma sapporensis, and Eyprepocnemis plorans), analyzing their possible origin and further development. The studied B_s consisted of C-positive or C-positive and C-negative regions. Analyzing new data and considering current hypotheses, we suggest that B_s in grasshoppers could arise through different mechanisms and from different chromosomes of the main set. We gave our special attention to the B_s with C-negative regions and suggest a new hypothesis of B chromosome formation from large or medium autosomes. This hypothesis includes dissemination of repetitive sequences and development of intercalary heterochromatic blocks in euchromatic chromosome arm followed by deletion of euchromatic regions located between them. The hypothesis is based on the findings of the Eyprepocnemis plorans specimens with autosome containing numerous intercalary repeat clusters, analysis of C-positive B_s in Eyprepocnemis plorans and Podisma sapporensis containing intercalary and terminal C-negative regions, and development of heterochromatic neo-Y chromosome in some Pamphagidae grasshoppers.

Origin and Evolution of the Neo-Sex Chromosomes in Pamphagidae Grasshoppers through Chromosome Fusion and Following Heteromorphization

In most phylogenetic lineages, the evolution of sex chromosomes is accompanied by their heteromorphization and degradation of one of them. The neo-sex chromosomes are useful model for studying early stages of these processes. Recently two lineages of the neo-sex chromosomes on different stages of heteromorphization was discovered in Pamphagidae family. The neo-sex chromosome heteromorphization was analyzed by generation of DNA probes derived from the neo-Xs and neo-Ys followed with chromosome painting in nineteen species of Pamphagidae family. The homologous regions of the neo-sex chromosomes were determined in closely related species with the painting procedure and image analysis with application of the Visualization of the Specific Signal in Silico software package. Results of these analyses and distribution of C-positive regions in the neo-sex chromosomes revealed details of the heteromorphization of the neo-sex chromosomes in species from both phylogenetic lineages of Pamphagidae grasshoppers. The hypothetical mechanism of the neo-Y degradation was suggested. It includes expansion of different repeats from the proximal neo-Y chromosome region by inversions, spreading them towards distal region. Amplification of these repeats leads to formation of C-positive regions and elimination of the C-negative regions located between them.

Comparative analysis of chromosomal localization of ribosomal and telomeric DNA markers in three species of Pyrgomorphidae grasshoppers

The karyotypes of three species of Pyrgomorphidae grasshoppers were studied: Zonocerus elegans (Thunberg, 1815), Pyrgomorpha guentheri (Burr, 1899) and Atractomorpha lata (Mochulsky, 1866). Data on karyotypes of P. guentheri and Z. elegans are reported here for the first time. All species have karyotypes consisting of 19 acrocentric chromosomes in males and 20 acrocentric chromosomes in females (2n♂=19, NF=19; 2n♀=20, NF=20) and X0/XX sex determination system. A comparative analysis of the localization of C-heterochromatin, clusters of ribosomal DNA, and telomere repeats revealed inter-species diversity in these cytogenetic markers. These differences indicate that the karyotype divergence in the species studied is not associated with structural chromosome rearrangements, but with the evolution of repeated DNA sequences.

High-throughput analysis of the satellitome revealed enormous diversity of satellite DNAs in the neo-Y chromosome of the cricket Eneoptera surinamensis

Satellite DNAs (satDNAs) constitute large portion of eukaryote genomes, comprising non-protein-coding sequences tandemly repeated. They are mostly found in heterochromatic regions of chromosomes such as around centromere or near telomeres, in intercalary heterochromatin, and often in non-recombining segments of sex chromosomes. We examined the satellitome in the cricket Eneoptera surinamensis (2n = 9, neo-X1X2Y, males) to characterize the molecular evolution of its neo-sex chromosomes. To achieve this, we analyzed illumina reads using graph-based clustering and complementary analyses. We found an unusually high number of 45 families of satDNAs, ranging from 4 bp to 517 bp, accounting for about 14% of the genome and showing different modular structures and high diversity of arrays. FISH mapping revealed that satDNAs are located mostly in C-positive pericentromeric regions of the chromosomes. SatDNAs enrichment was also observed in the neo-sex chromosomes in comparison to autosomes. Especially astonishing accumulation of satDNAs loci was found in the highly differentiated neo-Y, including 39 satDNAs over-represented in this chromosome, which is the greatest satDNAs diversity yet reported for sex chromosomes. Our results suggest possible involvement of satDNAs in genome increasing and in molecular differentiation of the neo-sex chromosomes in this species, contributing to the understanding of sex chromosome composition and evolution in Orthoptera.

Molecular cytogenetic analysis reveals the existence of two independent neo-XY sex chromosome systems in Anatolian Pamphagidae grasshoppers

BACKGROUND

Neo-XY sex chromosome determination is a rare event in short horned grasshoppers, but it appears with unusual frequency in the Pamphagidae family. The neo-Y chromosomes found in several species appear to have undergone heterochromatinization and degradation, but this subject needs to be analyzed in other Pamphagidae species. We perform here karyotyping and molecular cytogenetic analyses in 12 Pamphagidae species from the center of biodiversity of this group in the previously-unstudied Anatolian plateau.

RESULTS

The basal karyotype for the Pamphagidae family, consisting of 18 acrocentric autosomes and an acrocentric X chromosome (2n♂ = 19, X0; 2n♀ = 20, XX), was found only in G. adaliae. The karyotype of all other studied species consisted of 16 acrocentric autosomes and a neo-XY sex chromosome system (2n♂♀ = 18, neo-XX♀/neo-XY♂). Two different types of neo-Y chromosomes were found. One of them was typical for three species of the Glyphotmethis genus, and showed a neo-Y chromosome being similar in size to the XR arm of the neo-X, with the addition of two small subproximal interstitial C-blocks. The second type of the neo-Y chromosome was smaller and more heterochromatinized than the XR arm, and was typical for all Nocarodeini species studied. The chromosome distribution of C-positive regions and clusters of ribosomal DNA (rDNA) and telomeric repeats yielded additional information on evolution of these neo-XY systems.

CONCLUSION

Most Pamphagidae species in the Anatolian region were found to have neo-XY sex chromosome systems, belonging to two different evolutionary lineages, marked by independent X-autosome fusion events occurred within the Trinchinae and Pamphaginae subfamilies. The high density of species carrying neo-XY systems in the Anatolian region, and the different evolutionary stage for the two lineages found, one being older than the other, indicates that this region has a long history of neo-XY sex chromosome formation.