Valentina G. Kuznetsova

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

Abstract 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.

What genes and chromosomes say about the origin and evolution of insects and other arthropods

At the turn of the 21st century, the use of molecular and molecular cytogenetic methods led to revolutionary advances in systematics of insects and other arthropods. Analysis of nuclear and mitochondrial genes, as well as investigation of structural rearrangements in the mitochondrial chromosome convincingly supported the Pancrustacea hypothesis, according to which insects originated directly from crustaceans, whereas myriapods are not closely related to them. The presence of the specific telomeric motif TTAGG confirmed the monophyletic origin of arthropods (Arthropoda) and the assignment of tongue worms (Pentastomida) to this type. Several different types of telomeric sequences have been found within the class of insects. Investigation of the molecular organization of these sequences may shed light on the relationships between the orders Diptera, Siphonaptera, and Mecoptera and on the origin of such enigmatic groups as the orders Strepsiptera, Zoraptera and suborder Coleorrhyncha.

Holocentric chromosomes in meiosis. I. Restriction of the number of chiasmata in bivalents

The number of chiasmata in bivalents and the behaviour of chiasmata during the meiotic divisions were studied in Psylla foersteri (Psylloidea, Homoptera). Two chiasmata with a frequency of 97% and one or three chiasmata with frequencies of 2% and 0.9%, respectively, were observed in the largest bivalent in male meiosis. Meiosis was normal for the largest bivalents with one or two chiasmata, whereas bivalents with three chiasmata were not capable of completing anaphase I because of their inability to resolve the chiasma located in the middle. Consequently, the bivalent was seen as a laggard joining together two metaphase II daughter plates. Apparently, cells of this kind are eliminated. Inability to resolve the chiasma situated in the middle is attributed to the condensation process, which is unable to change the spatial orientation of successive chiasma loops in holocentric bivalents so that chiasma loops would be arranged perpendicular to each other at metaphase I. Thus they retain their parallel orientation from diplotene to metaphase I. Consequently, sister chromatid cohesion is exposed for release only in the outermost chiasmata but the chiasma in the middle continues to interlock the chromosomes in the bivalent. The elimination of the cells carrying bivalents with more than two chiasmata creates a strong selection against the formation of more than two chiasmata in holocentric bivalents.

Cytogenetic characterization of the Trinidad endemic, Arachnocoris trinitatus Bergroth: the first data for the tribe Arachnocorini [Heteroptera: Cimicomorpha: Nabidae]

As an extension of the ongoing cytogenetic studies of the bug family Nabidae (Heteroptera: Cimicomorpha), the first evidence for the tribe Arachnocorini (the subfamily Nabinae), with reference to the Trinidad endemic, Arachnocoris trinitatus Bergroth, is provided. This is an attempt to gain a better insight into the evolution, systematics and within-family relationships of the family Nabidae. The studies were conducted using a number of cytogenetic techniques. The male karyotype (chromosome number and size; sex chromosome system; NOR location; C-heterochromatin amount, distribution and characterization in terms of the presence of AT-rich and GC-rich DNA), and male meiosis with particular emphasis on the behavior of the sex chromosomes in metaphase II are described. Also investigated are the male and female internal reproductive organs with special reference to the number of follicles in a testis and the number of ovarioles in an ovary. A. trinitatus was found to display a number of characters differentiating it from all hitherto studied nabid species placed in the tribe Nabini of the subfamily Nabinae, and in the tribe Prostemmatini of the subfamily Prostemmatinae. Among these characters are chromosome number 2n = 12 (10 + XY), the lowest within the family, nucleolus organizer regions (NORs) situated on the autosomes rather than on the sex chromosomes as is the case in other nabid species, and testes composed of 3 follicles but not of 7 as in other nabids. All the data obtained suggest many transformations during the evolution ofA. trinitatus.As an extension of the ongoing cytogenetic studies of the bug family Nabidae (Heteroptera: Cimicomorpha), the first evidence for the tribe Arachnocorini (the subfamily Nabinae), with reference to the Trinidad endemic, Arachnocoris trinitatus Bergroth, is provided. This is an attempt to gain a better insight into the evolution, systematics and within-family relationships of the family Nabidae. The studies were conducted using a number of cytogenetic techniques. The male karyotype (chromosome number and size; sex chromosome system; NOR location; C-heterochromatin amount, distribution and characterization in terms of the presence of AT-rich and GC-rich DNA), and male meiosis with particular emphasis on the behavior of the sex chromosomes in metaphase II are described. Also investigated are the male and female internal reproductive organs with special reference to the number of follicles in a testis and the number of ovarioles in an ovary. A. trinitatus was found to display a number of characters differentiating it from all hitherto studied nabid species placed in the tribe Nabini of the subfamily Nabinae, and in the tribe Prostemmatini of the subfamily Prostemmatinae. Among these characters are chromosome number 2n = 12 (10 + XY), the lowest within the family, nucleolus organizer regions (NORs) situated on the autosomes rather than on the sex chromosomes as is the case in other nabid species, and testes composed of 3 follicles but not of 7 as in other nabids. All the data obtained suggest many transformations during the evolution of A. trinitatus.

Homoploid hybrid speciation and genome evolution via chromosome sorting

Genomes of numerous diploid plant and animal species possess traces of interspecific crosses, and many researches consider them as support for homoploid hybrid speciation (HHS), a process by which a new reproductively isolated species arises through hybridization and combination of parts of the parental genomes, but without an increase in ploidy. However, convincing evidence for a creative role of hybridization in the origin of reproductive isolation between hybrid and parental forms is extremely limited. Here, through studying Agrodiaetus butterflies, we provide proof of a previously unknown mode of HHS based on the formation of post-zygotic reproductive isolation via hybridization of chromosomally divergent parental species and subsequent fixation of a novel combination of chromosome fusions/fissions in hybrid descendants. We show that meiotic segregation, operating in the hybrid lineage, resulted in the formation of a new diploid genome, drastically rearranged in terms of chromosome number. We also demonstrate that during the heterozygous stage of the hybrid species formation, recombination was limited between rearranged chromosomes of different parental origin, representing evidence that the reproductive isolation was a direct consequence of hybridization.

The origin of the achiasmatic XY sex chromosome system in Cacopsylla peregrina (Frst.) (Psylloidea, Homoptera)

The status of an extra univalent, if it is a B chromosome or an achiasmatic Y chromosome, associating with the X chromosome in male meiosis of Cacopsylla peregrina (Frst.) (Homoptera, Psylloidea) was analysed. One extra univalent was present in all males collected from three geographically well separated populations, it was mitotically stable, and showed precise segregation from the X chromosome. These findings led us to propose that the univalent represents in fact a Y chromosome. The behaviour of the X and Y chromosomes during meiotic prophase suggested that their regular segregation was based on an achiasmatic segregation mechanism characterised by a ‘touch and go’ pairing of segregating chromosomes at metaphase I. To explain the formation of the achiasmatic Y within an insect group with X0 sex chromosome system, it was suggested that the Y chromosome has evolved from a mitotically stable B chromosome that was first integrated into an achiasmatic segregation system with the X chromosome, and has later become fixed in the karyotype as a Y chromosome.

Distribution of 18S rDNA sites and absence of the canonical TTAGG insect telomeric repeat in parasitoid Hymenoptera

Karyotypes of six species belonging to three main clades of parasitoid Hymenoptera, the superfamilies Ichneumonoidea (Ichneumonidae: Ichneumon amphibolus), Cynipoidea (Cynipidae: Diplolepis rosae) and Chalcidoidea (Eurytomidae: Eurytoma robusta, Eu. serratulae and Eu. compressa, and Torymidae: Torymus bedeguaris) were studied using FISH with 18S rDNA and telomeric (TTAGG)n probes. Haploid karyotypes of D. rosae, Eu. robusta and Eu. serratulae carried the only 18S rDNA hybridization signal, whereas those of I. amphibolus and Eu. compressa carried three and two rDNA clusters respectively. In addition, three rDNA sites were visualized in the aneuploid female of T. bedeguaris. The number of rDNA clusters in parasitoid Hymenoptera generally correlates to the chromosome number. Apart from the overwhelming majority of the studied species of aculeate Hymenoptera, no hybridization signals were obtained from FISH with the telomeric (TTAGG)n probe in the examined parasitoid species. These data suggest absence of the canonical (TTAGG)n insect telomeric motif in the Ichneumonoidea, Cynipoidea and Chalcidoidea, and perhaps in parasitoid Hymenoptera in general.

Molecular phylogeny of the Mediterranean species of Philaenus (Hemiptera: Auchenorrhyncha: Aphrophoridae) using mitochondrial and nuclear DNA sequences.

The phylogenies of all eight European species of Philaenus were estimated from cytochrome oxidase subunit I, cytochrome B and internal transcribed spacer 2 (ITS2) fragments of DNA using phylogenetic reconstruction methods: maximum parsimony (MP), maximum likelihood (ML) and Bayesian inference (BI) analyses. Based on the topologies of all obtained phylogenetic trees, the monophyly of Philaenus is well supported, being congruent with morphological, ecological and chromosomal data. Three phylogenetic lineages were distinguished in the mitochondrial and combined (mtDNA with ITS2) trees. The first lineage is represented by only one species, Philaenus maghresignus, which inhabits Maghreb and southern Spain. Clade A includes three species: P. tarifa (Southern Iberia), P. italosignus (Sicily and Southern Italy) and P. signatus (the Balkans and Middle East). In clade B two subclades were recognized: B1 represented by P. loukasi (Southern Balkans) and P. arslani (Middle East), and B2 comprising P. spumarus (the most widespread Palaearctic species) and P. tesselatus (from Southern Iberia and Maghreb). These clades were also retrieved in trees reconstructed from nuclear sequences. However, four species (P. maghresignus, P. tarifa, P. italosignus and P. signatus) showed unresolved polytomy at the base of the nuclear tree. Clade A together with P. maghresignus clustered with the ‘signatus’ group defined from morphology, and clade B with the ‘spumarius’ group; these might be considered separate subgenera. Genetic distances in mitochondrial DNA between ingroup species ranged from 14.0% between P. signatus and P. spumarius to 2.4% between P. tesselatus and P. spumarius. By contrast, genetic divergence of ITS2 between ingroup species was very low, at most 2.1%. The divergence of Philaenus species is estimated to have occcurred between 7.9 and 0.6 Ma. Possibly three main speciation events occurred: the first at the Miocene/Pliocene boundary (c. 5.5 Ma) for deeper splits; the second between 4.2 and 2.5 Ma in the Pliocene, when pairs of more closely related species diverged; and the most recent during the Pleistocene glaciations, when the separation of P. tesselatus and P. spumarius took place. The species status of all Philaenus species is confirmed except for P. tesselatus.

Achiasmate segregation of a B chromosome from the X chromosome in two species of psyllids (Psylloidea, Homoptera)

The segregation of a B chromosome from the X chromosome was studied in male meiosis in two psyllid species, Rhinocola aceris (L.) and Psylla foersteri (Flor.) (Psylloidea, Homoptera). The frequency of segregation was determined from cells at metaphase II. In R. aceris, the B chromosome was mitotically stable and segregated quite regularly from the X chromosome in four geographically distant populations, while it showed less regular, but preferential segregation in one population. This was attributed to the presence of B chromosome variants that differ in their ability to interact with the X chromosome in segregation. In P. foersteri, the B chromosome was mitotically unstable and segregated preferentially from the X chromosome in spermatocyte cysts, which displayed one B chromosome in every cell. Behaviour of the B chromosome and X chromosome univalents during meiotic prophase and at metaphase I in R. aceris, and during anaphase I in P. foersteri suggested that the regular segregation resulted from the incorporation of B chromosomes in achiasmate segregation mechanisms with the X chromosome in the place occupied by the Y chromosome in species with XY system. The regular segregation of a B chromosome from the X chromosome may obscure the distinction of a B chromosome and an achiasmate Y chromosome in some cases.

Holocentric chromosomes in meiosis. II. The modes of orientation and segregation of a trivalent

The modes of orientation and segregation of the sex chromosome trivalent X1X2Y in male meiosis of Cacopsylla mali (Psylloidea, Homoptera) were analysed. Males with an X1X2Y sex chromosome system coexist with males displaying a neo-XY system in populations of this species. The fusion chromosome resulting in the formation of a trivalent in meiosis originates from the fusion of an autosome with the neo-Y chromosome. In the majority of metaphase I cells (92.4%) the X1X2Y trivalent showed co-orientation; X1 and X2 chromosomes oriented towards one pole whereas the Y oriented towards the opposite pole. In the rest of the cells (7.6%) the trivalent with subterminal chiasmata was oriented parallel to the equatorial plane. From this orientation the trivalent produced triple chromatids joined together by undivided telomeric parts of chromosomes and hence by sister chromatid cohesion at anaphase I. In the majority of metaphase II cells the orientation of triple chromatids suggested the production of unbalanced gametes. However, in a small number of cells (1.7%) the trivalent showed co-orientation of X1X2 with Y. Both the first division and second division co-orientations, or 94.1% of divisions as a whole, were estimated to yield balanced gametes, containing either X1 and X2 chromosomes or Y chromosome. It was concluded that, since the triple chromatid contained undivided telomere regions at metaphase II, which divided at anaphase II, the orientation of the trivalent with its longitudinal axis parallel to the equatorial plane in metaphase I also represents co-orientation and results in pre-reduction. The existence of post-reductional behaviour of holocentric bivalents and multivalents is discussed.