Patterns of chromosome banding in four nabid species (Heteroptera, Cimicomorpha, Nabidae) with high chromosome number karyotypes

Histone H3 Methylation and Autosomal vs. Sex Chromosome Segregation During Male Meiosis in Heteroptera

Heteropteran insects exhibit a remarkable diversity of meiotic processes, including coexistence of different chromosomes types with different behavior during the first meiotic division, non-chiasmatic segregation, and inverted meiosis. Because of this diversity they represent suitable models to study fundamental questions about the mechanisms of chromosome behavior during cell division. All heteropteran species possess holokinetic chromosomes and in most of them the autosomal chromosomes synapse, recombine, and undergoe pre-reductional meiosis. In contrast, the sex chromosomes are achiasmatic, behave as univalents at metaphase I and present an inverted or post-reductional meiosis. An exception to this typical behavior is found in Pachylis argentinus, where both the autosomes and the X-chromosome divide reductionally at anaphase I and then divide equationally at anaphase II. In the present report, we analyzed the distribution of histones H3K9me2 and H3K9me3 in P. argentinus and in five species that have simple and multiple sex chromosome systems with typical chromosome segregation, Belostoma elegans, B. oxyurum, Holhymenia rubiginosa, Phthia picta, and Oncopeltus unifasciatellus. We found that H3K9me3 is a marker for sex-chromosomes from early prophase I to the end of the first division in all the species. H3K9me2 also marks the sex chromosomes since early prophase but shows different dynamics at metaphase I depending on the sex-chromosome segregation: it is lost in species with equationally dividing sex chromosomes but remains on one end of the X chromosome of P. argentinus, where chromatids migrate together at anaphase I. It is proposed that the loss of H3K9me2 from the sex chromosomes observed at metaphase I may be part of a set of epigenetic signals that lead to the reductional or equational division of autosomes and sex chromosomes observed in most Heteroptera. The present observations suggest that the histone modifications analyzed here evolved in Heteroptera as markers for asynaptic and achiasmatic sex chromosomes during meiosis to allow the distinction from the chiasmatic autosomal chromosomes.

Peaceful revolution in genome size: polyploidy in the Nabidae (Heteroptera); autosomes and nuclear DNA content doubling

Genome size and the position of 18S ribosomal DNA (rDNA) were analysed in two Himacerus, eight Nabis and two Prostemma species from the family Nabidae using flow cytometry and fluorescence in situ hybrization techniques. The karyotypes of Nabis biformis and Nabis maoricus, each with 2n = 16 + XY, and Prostemma aeneicolle, with 2n = 26 + XY, were recorded for the first time. All the species displayed one or two 18S rDNA signals on the X chromosome and up to two signals on the Y chromosome. Several females exhibited two different types of X chromosome breakage, namely within or outside of the 18S rDNA region. Measurements of nuclear DNA content revealed significant differences between all three genera under study. Most notably, the nuclear DNA content of Himacerus species, with 2n = 32/36 + XY (2C = 9–10 pg), was double that of Nabis species, with 2n = 16 + XY (2C = 4–6 pg). Therefore, the previously rejected theory of an autosomal polyploidy event in the evolution of the genus Himacerus is strongly supported by the results of the present study and is now being resurrected.

New cytogenetic data for three species of Pentatomidae (Heteroptera): Dichelops melacanthus (Dallas, 1851), Loxa viridis (Palisot de Beauvois, 1805), and Edessa collaris (Dallas, 1851)

In this paper, we present new cytogenetic data for three species of the family Pentatomidae: Dichelops melacanthus (Dallas, 1851), Loxa viridis (Palisot de Beauvois, 1805), and Edessa collaris (Dallas, 1851). All studied species presented holocentric chromosomes and inverted meiosis for the sex chromosomes. D. melacanthus has 2n = 12 (10A + XY); L. viridis showed 2n = 14 (12A + XY); and E. collaris showed 2n = 14 (12A + XY). C-banding was performed for the first time in these species and revealed terminal and interstitial heterochromatic regions on the autosomes; DAPI/CMA₃ staining showed different fluorescent patterns. In all species, fluorescence in situ hybridization (FISH) with 18S rDNA probe identified signals on one autosomal bivalent, this being the first report of FISH application in the species D. melacanthus and L. viridis. The results obtained add to those already existing in the literature, enabling a better understanding of the meiotic behavior of these insects.

New data on karyotype, spermatogenesis and ovarian trophocyte ploidy in three aquatic bug species of the families Naucoridae, Notonectidae, and Belostomatidae (Nepomorpha, Heteroptera)

We report the karyotype, some aspects of spermatogenesis, and ovarian trophocytes ploidy in three aquatic bug species: Ilyocoris cimicoides (Linnaeus, 1758), Notonecta glauca Linnaeus, 1758, and Diplonychus rusticus Fabricius, 1871 from previously unexplored regions - South Europe (Bulgaria) and Southeast Asia (Vietnam). Our results add considerable support for the published karyotype data for these species. In I. cimicoides, we observed achiasmate male meiosis - the first report of achiasmy for the family Naucoridae. More comprehensive cytogenetic studies in other species of the Naucoridae are required to elucidate the role of achiasmy as a character in the systematics of the family. Our observations on the association between phases of spermatogenesis and developmental stages in I. cimicoides and N. glauca differ from the previously published data. In these species, we assume that the spermatogenesis phases are not strongly associated with certain developmental stages. For further cytogenetic studies (on the Balkan Peninsula), we recommend July as the most appropriate month for collection of I. cimicoides and N. glauca. In the ovaries of both species, we studied the level of ploidy in metaphase and interphase trophocytes. In I. cimicoides, diploid and tetraploid metaphase trophocytes were found. Heteropycnotic elements, observed in interphase trophocytes of this species, represented the X chromosomes. It allowed us to determine the trophocytes ploidy at interphase (2n was repeated up to 16 times). The situation with N. glauca was different. The metaphase trophocytes were diploid and we were not able to determine the ploidy of interphase trophocytes since such conspicuous heteropycnotic elements were not found. The scarce data available suggest a tendency for a low level of trophocyte ploidy in the basal infraorders (Nepomorpha and Gerromorpha) and for a high level in the more advanced Pentatomomorpha. Data about this character in species from other infraorders are needed to confirm that tendency.

A chromosomal analysis of Nepa cinerea Linnaeus, 1758 and Ranatra linearis (Linnaeus, 1758) (Heteroptera, Nepidae)

An account is given of the karyotypes and male meiosis of the Water Scorpion Nepa cinerea Linnaeus, 1758 and the Water Stick Insect Ranatra linearis (Linnaeus, 1758) (Heteroptera, Nepomorpha, Nepidae). A number of different approaches and techniques were tried: the employment of both male and female gonads and mid-guts as the sources of chromosomes, squash and air-drying methods for chromosome preparations, C-banding and fluorescence in situ hybridization (FISH) for chromosome study. We found that N. cinerea had a karyotype comprising 14 pairs of autosomes and a multiple sex chromosome system, which is X₁X₂X₃X₄Y (♂) / X₁X₁X₂X₂X₃X₃X₄X₄ (♀), whereas R. linearis had a karyotype comprising 19 pairs of autosomes and a multiple sex chromosome system X₁X₂X₃X₄Y (♂) / X₁X₁X₂X₂X₃X₃X₄X₄ (♀). In both N. cinerea and R. linearis, the autosomes formed chiasmate bivalents in spermatogenesis, and the sex chromosome univalents divided during the first meiotic division and segregated during the second one suggesting thus a post-reductional type of behaviour. These results confirm and amplify those of Steopoe (1925, 1927, 1931, 1932) but are inconsistent with those of other researchers. C-banding appeared helpful in pairing up the autosomes for karyotype assembly; however in R. linearis the chromosomes were much more uniform in size and general appearance than in N. cinerea. FISH for 18S ribosomal DNA (major rDNA) revealed hybridization signals on two of the five sex chromosomes in N. cinerea. In R. linearis, rDNA location was less obvious than in N. cinerea; however it is suggested to be similar. We have detected the presence of the canonical "insect" (TTAGG) _n telomeric repeat in chromosomes of these species. This is the first application of C-banding and FISH in the family Nepidae.

Interstitial NORs, Fragile Sites, and Chromosome Evolution: A Not So Simple Relationship - The Example of Melolontha melolontha and Genus Protaetia (Coleoptera: Scarabaeidae)

In the present study, the origin of recurrent rearrangements involving chromosome 6 in 3.2% of cells of Melolontha melolontha (Coleoptera, Scarabaeidae) was investigated. Various chromosome staining techniques, including C-banding, Giemsa and silver staining, as well as fluorescence in situ hybridization with a human 28S rDNA probe, were applied to M. melolontha chromosome spreads. In addition, related species of the genera Melolontha and Protaetia were studied. On chromosome 6 of M. melolontha, there is a fragile site-like structure which corresponds to an interstitial nucleolus organizer region (NOR). Despite this instability, the NOR remains unique and interstitial in this species, as well as in the other species studied. It is proposed that the intercalary position of the NOR both facilitates the detection of its fragile site-like instability and correlates with its relative stability during evolution. We explain this apparent paradox by strong counter-selection for imbalances of the chromosome fragment distal to the interstitial NORs, which would recurrently occur in the progeny of translocation carriers. Thus, the frequent telomeric position of the NORs in most animal and plant taxa would have no functional rationale but would be the consequence of selection against the meiotic transmission of chromosome imbalances.

Heterochromatin base pair composition and diversification in holocentric chromosomes of kissing bugs (Hemiptera, Reduviidae)

The subfamily Triatominae (Hemiptera, Reduviidae) includes 150 species of blood-sucking insects, vectors of Chagas disease or American trypanosomiasis. Karyotypic information reveals a striking stability in the number of autosomes. However, this group shows substantial variability in genome size, the amount and distribution of C-heterochromatin, and the chromosome positions of 45S rDNA clusters. Here, we analysed the karyotypes of 41 species from six different genera with C-fluorescence banding in order to evaluate the base-pair richness of heterochromatic regions. Our results show a high heterogeneity in the fluorescent staining of the heterochromatin in both autosomes and sex chromosomes, never reported before within an insect subfamily with holocentric chromosomes. This technique allows a clear discrimination of the heterochromatic regions classified as similar by C-banding, constituting a new chromosome marker with taxonomic and evolutionary significance. The diverse fluorescent patterns are likely due to the amplification of different repeated sequences, reflecting an unusual dynamic rearrangement in the genomes of this subfamily. Further, we discuss the evolution of these repeated sequences in both autosomes and sex chromosomes in species of Triatominae.

Karyotype diversity among predatory Reduviidae (Heteroptera)

Species of infraorder Cimicomorpha of Heteroptera exhibit holokinetic chromosomes with inverted meiosis for sex chromosomes and high variation in chromosome number. The family Reduviidae, which belongs to this infraorder, is also recognized by high variability of heterochromatic bands and chromosome location of 18S rDNA loci. We studied here five species of Reduviidae (Harpactorinae) with predator habit, which are especially interesting because individuals are found solitary and dispersed in nature. These species showed striking variation in chromosome number (including sex chromosome systems), inter-chromosomal asymmetry, different number and chromosome location of 18S rDNA loci, dissimilar location and quantity of autosomal C-heterochromatin, and different types of repetitive DNA by fluorochrome banding, probably associated with occurrence of different chromosome rearrangements. Terminal chromosome location of C-heterochromatin seems to reinforce the model of equilocal dispersion of repetitive DNA families based in the "bouquet configuration".

First report on C-banding, fluorochrome staining and NOR location in holocentric chromosomes of Elasmolomus (Aphanus) sordidus Fabricius, 1787 (Heteroptera, Rhyparochromidae)

In spite of various cytogenetic works on suborder Heteroptera, the chromosome organization, function and its evolution in this group is far from being fully understood. Cytologically, the family Rhyparochromidae constitutes a heterogeneous group differing in chromosome numbers. This family possesses XY sex mechanism in the majority of the species with few exceptions. In the present work, multiple banding techniques viz., C-banding, base-specific fluorochromes (DAPI/CMA₃) and silver nitrate staining have been used to cytologically characterize the chromosomes of the seed plant pest Elasmolomus (Aphanus) sordidus Fabricius, 1787 having 2n=12=8A+2m+XY. One pair of the autosomes was large while three others were of almost equal size. At diplotene, C-banding technique revealed, that three autosomal bivalents show terminal constitutive heterochromatic bands while one medium sized bivalent was euchromatic. Microchromosomes (m-chromosomes) were positively heteropycnotic. After DAPI and CMA₃ staining, all the autosomal bivalents showed equal fluorescence, except CMA₃ positive signals, observed at both telomeric heterochromatic regions of one medium sized autosomal bivalent. Silver nitrate staining further revealed that this chromosome pair carries Nucleolar Organizer Regions (NORs) at the location of CMA₃ positive signals. The X chromosome showed a thick C-band, positive to both DAPI /CMA₃ while Y, otherwise C-negative, was weakly positive to DAPI and negative to CMA₃, m-chromosomes were DAPI bright and CMA₃ dull.

The significance of cytogenetics for the study of karyotype evolution and taxonomy of water bugs (Heteroptera, Belostomatidae) native to Argentina

Male meiosis behaviour and heterochromatin characterization of three big water bug species were studied. Belostoma dentatum (Mayr, 1863), Belostoma elongatum Montandon, 1908 and Belostoma gestroi Montandon, 1903 possess 2n = 26 + X1X2Y (male). In these species, male meiosis is similar to that previously observed in Belostoma Latreille, 1807. In general, autosomal bivalents show a single chiasma terminally located and divide reductionally at anaphase I. On the other hand, sex chromosomes are achiasmatic, behave as univalents and segregate their chromatids equationally at anaphase I. The analysis of heterochromatin distribution and composition revealed a C-positive block at the terminal region of all autosomes in Belostoma dentatum, a C-positive block at the terminal region and C-positive interstitial dots on all autosomes in Belostoma elongatum, and a little C-positive band at the terminal region of autosomes in Belostoma gestroi. A C-positive band on one bivalent was DAPI negative/CMA3 positive in the three species. The CMA3-bright band, enriched in GC base pairs, was coincident with a NOR detected by FISH. The results obtained support the hypothesis that all species of Belostoma with multiple sex chromosome systems preserve NORs in autosomal bivalents. The karyotype analyses allow the cytogenetic characterization and identification of these species belonging to a difficult taxonomic group. Besides, the cytogenetic characterization will be useful in discussions about evolutionary trends of the genome organization and karyotype evolution in this genus.

Cytogenetics of the true bug infraorder Cimicomorpha (Hemiptera, Heteroptera): a review

The Cimicomorpha is one of the largest and highly diversified infraorders of the Heteroptera. This group is also highly diversified cytogenetically and demonstrates a number of unusual cytogenetic characters such as holokinetic chromosomes; m-chromosomes; multiple sex chromosome systems; post-reduction of sex chromosomes in meiosis; variation in the presence/absence of chiasmata in spermatogenesis; different types of achiasmate meiosis. We present here a review of essential cytogenetic characters of the Cimicomorpha and outline the chief objectives and goals of future investigations in the field.

Male meiosis, heterochromatin characterization and chromosomal location of rDNA in Microtomus lunifer (Berg, 1900) (Hemiptera: Reduviidae: Hammacerinae)

In the present work, we analysed the male meiosis, the content and distribution of heterochromatin and the number and location of nucleolus organizing regions in Microtomus lunifer (Berg, 1900) by means of standard technique, C- and fluorescent bandings, and fluorescent in situ hybridization with an 18S rDNA probe. This species is the second one cytogenetically analysed within the Hammacerinae. Its male diploid chromosome number is 31 (2n=28+X1X2Y), including a minute pair of m-chromosomes. The diploid autosomal number and the presence of m-chromosomes are similar to those reported in Microtomus conspicillaris (Drury, 1782) (2n=28+XY). However, Microtomus lunifer has a multiple sex chromosome system X1X2Y (male) that could have originated by fragmentation of the ancestral X chromosome. Taking into account that Microtomus conspicillaris and Microtomus lunifer are the only two species within Reduviidae that possess m-chromosomes, the presence of this pair could be a synapomorphy for the species of this genus. C- and fluorescent bandings showed that the amount of heterochromatin in Microtomus lunifer was small, and only a small CMA3 bright band was observed in the largest autosomal pair at one terminal region. FISH with the 18S rDNA probe demonstrated that ribosomal genes were terminally placed on the largest autosomal pair. Our present results led us to propose that the location of rDNA genes could be associated with variants of the sex chromosome systems in relation with a kind of the sex chromosome systems within this family. Furthermore, the terminal location of NOR in the largest autosomal pair allowed us to use it as a chromosome marker and, thus, to infer that the kinetic activity of both ends is not a random process, and there is an inversion of this activity.

Modeling protein network evolution under genome duplication and domain shuffling

Background

Successive whole genome duplications have recently been firmly established in all major eukaryote kingdoms. Such exponential evolutionary processes must have largely contributed to shape the topology of protein-protein interaction (PPI) networks by outweighing, in particular, all time-linear network growths modeled so far.

Results

We propose and solve a mathematical model of PPI network evolution under successive genome duplications. This demonstrates, from first principles, that evolutionary conservation and scale-free topology are intrinsically linked properties of PPI networks and emerge from i) prevailing exponential network dynamics under duplication and ii) asymmetric divergence of gene duplicates. While required, we argue that this asymmetric divergence arises, in fact, spontaneously at the level of protein-binding sites. This supports a refined model of PPI network evolution in terms of protein domains under exponential and asymmetric duplication/divergence dynamics, with multidomain proteins underlying the combinatorial formation of protein complexes. Genome duplication then provides a powerful source of PPI network innovation by promoting local rearrangements of multidomain proteins on a genome wide scale. Yet, we show that the overall conservation and topology of PPI networks are robust to extensive domain shuffling of multidomain proteins as well as to finer details of protein interaction and evolution. Finally, large scale features of direct and indirect PPI networks of S. cerevisiae are well reproduced numerically with only two adjusted parameters of clear biological significance (i.e. network effective growth rate and average number of protein-binding domains per protein).

Conclusion

This study demonstrates the statistical consequences of genome duplication and domain shuffling on the conservation and topology of PPI networks over a broad evolutionary scale across eukaryote kingdoms. In particular, scale-free topologies of PPI networks, which are found to be robust to extensive shuffling of protein domains, appear to be a simple consequence of the conservation of protein-binding domains under asymmetric duplication/divergence dynamics in the course of evolution.

Heterochromatin characterization in five species of Heteroptera

The amount, composition and location of heterochromatin in Athaumastus haematicus (Stål, 1859), Leptoglossus impictus (Stål, 1859), Phthia picta (Drury, 1770) (Coreidae), Largus rufipennis Laporte, 1832 (Largidae) and Jadera sanguinolenta (Fabricius, 1775) (Rhopalidae) are analyzed by C-banding and DAPI/ CMA fluorescent banding. As the rule for Heteroptera the possession of holokinetic chromosomes and a pre-reductional type of meiosis cytogenetically characterize these five species. Besides, all of them (except L. rufipennis) present a pair of m chromosomes. C-banding technique reveals the absence of constitutive heterochromatin in A. haematicus, scarce C-positive blocks in L. impictus and J. sanguinolenta, and C-positive heterochromatin terminally located in P. picta and L. rufipennis. All C-bands are DAPI bright, except for a DAPI dull/CMA bright band at one telomeric end of the X chromosome in L. rufipennis, which probably corresponds to a nucleolar organizing region. The results of the banding techniques are analyzed in relation to the chiasma frequency and distribution in the five species, and it is concluded that there should exist some constraints to the acquisition and/ or accumulation of heterochromatin in their karyotypes.