The mechanisms behind the regular segregation of the m-chromosomes in Coreus marginatus L. (Coreidae, Hemiptera)

Karyotype diversity in the genus Nysius Dallas, 1852 (Hemiptera, Heteroptera, Lygaeidae) is much greater than you might think

We studied the karyotype and chromosomal distribution of 18S rDNA clustered in nucleolar organizer regions (NORs) in Nysiusgraminicola (Kolenati, 1845), belonging to the subfamily Orsillinae (Lygaeidae). It is shown that this species has a karyotype with 2n = 22(18+mm+XY), previously known in only one of 24 studied species of the genus Nysius Dallas, 1852, characterized by a similar karyotype, 2n = 14(12+mm+XY). In N.graminicola, 18S loci are located on sex chromosomes, which is a previously unknown trait for this genus. Our results in a compilation with previous data revealed dynamic evolution of rDNA distribution in Nysius. It is concluded that molecular chromosomal markers detected by FISH contribute to a better understanding of the structure and evolution of the taxonomically complex genus Nysius.

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.

Comparative analysis of chromosome numbers and sex chromosome systems in Paraneoptera (Insecta)

This article is part (the 4th article) of the themed issue (a monograph) "Aberrant cytogenetic and reproductive patterns in the evolution of Paraneoptera". The purpose of this article is to consider chromosome structure and evolution, chromosome numbers and sex chromosome systems, which all together constitute the chromosomal basis of reproduction and are essential for reproductive success. We are based on our own observations and literature data available for all major lineages of Paraneoptera including Zoraptera (angel insects), Copeognatha (=Psocoptera; bark lice), Parasita (=Phthiraptera s. str; true lice), Thysanoptera (thrips), Homoptera (scale insects, aphids, jumping plant-lice, whiteflies, and true hoppers), Heteroptera (true bugs), and Coleorrhyncha (moss bugs). Terminology, nomenclature, classification, and the study methods are given in the first paper of the issue (Gavrilov-Zimin et al. 2021).

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.

Achiasmate male meiosis in two Cymatia species (Hemiptera, Heteroptera, Corixidae)

The karyotype and male meiosis, with a particular focus on the presence or absence of chiasmata between the homologs, were studied in the water boatman species Cymatiarogenhoferi (Fieber) and Cymatiacoleoptrata (Fabricius) (Corixidae, Cymatiainae). It is shown that the species have 2n = 33 (28A+2m+X₁X₂Y) and 2n = 24 (20A+2m+XY) respectively, post-reduction of sex chromosomes, and achiasmate meiosis of an alignment type in males. Cytogenetic and some morphological diagnostic characters separating Cymatia Flor from the rest of Corixidae are discussed.

Meiosis in rare males in parthenogenetic Cacopsylla myrtilli (Wagner, 1947) (Hemiptera, Psyllidae) populations from northern Europe

For studying meiosis in males, large samples of Cacopsylla myrtilli (Wagner, 1947) (Hemiptera, Psyllidae) were collected in Norway, Sweden, Finland and northwest Russia. In addition to all-female populations, males were present in 10 out of 47 populations; still, all populations were highly female-biased, the proportion of males varying from 0.1% to 9.1%. These males are thus rare or so-called spanandric males. Males in northern Norway, Finland and northwest Russia showed normal chiasmate meiosis, while complete absence of chiasmata due to asynapsis was found in males collected in Norway and northern Sweden. In asynaptic meiosis, all univalent chromosomes divided during the first meiotic division resulting in incomplete second meiotic division and formation of diploid sperms. Hence, males in these populations are nonfunctional and do not contribute to the genetic constitution of the population, but appear in every generation as reversals from apomictic parthenogenesis and the mode of parthenogenesis is of obligatory type.

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.

Synapsis with and without recombination in the male meiosis of the leaf-footed bug Holhymenia rubiginosa (Coreidae, Heteroptera)

In organisms with chiasmatic meiosis two different relationships have been described between crossing over and synapsis: in one group of organisms synapsis depends on the initiation of meiotic recombination while in the other group it is independent of this initiation. These patterns have been observed mainly in organisms where all meiotic bivalents in the set have similar behaviors. In some heteropteran insects a pair of chromosomes named m chromosomes is known to behave differently from autosomes regarding synapsis and recombination. Here we used immunodetection of a synaptonemal complex component and acid-fixed squashes to investigate the conduct of the small m chromosome pair during the male meiosis in the coreid bug Holhymenia rubiginosa. We found that the m chromosomes form a synaptonemal complex during pachytene, but they are not attached by a chiasma in diakinesis. On the other hand, the autosomal bivalents synapse and recombine regularly. The co-existence of these variant chromosome behaviors during meiosis I add further evidence to the absence of unique patterns regarding the interdependence of synapsis and recombination.

A chromosomal investigation of the west European species of Corixa Geoffroy (Heteroptera: Corixidae)

C-banded karyotypes are provided for the five west European species of Corixa Geoffroy. All have 2n = 22 autosomes + sex chromosomes which are XY (male symbol), XX (female symbol). Some specimens of C. punctata have a small B-chromosome, while some C. panzeri have a fairly large one. The karyotypes of C. punctata, C. dentipes, C. affinis and C. panzeri are all distinctive, but the karyotypes of C. punctata and C. iberica appear indistinguishable. The Scottish material of C. punctata and C. iberica used in this study shows morphological intergradation between the two, suggesting that they hybridise where their ranges overlap. Meiosis shows very fine even pachytene bands in C. punctata, and that the autosomes form true bivalents during diplotene in C. affinis and C. panzeri, while the sex chromosomes appear closely associated. At first metaphase the sex chromosomes may appear either closely associated or may have separated. There is no evidence of the presence of m-chromosomes.