Seppo Nokkala

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.

Species and habitat races in the chrysomelid Galerucella nymphaeae species complex in northern Europe

The taxonomically controversial Galerucella nymphaeae species complex (Coleoptera, Chrysomelidae) contains at least two distinct species in northern Europe, G. nymphaeae (L.) consuming Nuphar luteum, and G. sagittariae (Gyll.), living mainly on Comarum palustre in southern Finland and on Rubus chamaemorus in northern Finland. By analyzing chromosomal polymorphisms in sympatric populations of G. nymphaeae and G. sagittariae, it was shown that the two species do not hybridize in nature. Chorion polypeptide analysis revealed fixed genetic differences between the two species. However, differences between species must be small, since hybridization is easily induced in the laboratory and F1 hybrids are fertile. Larvae of G. sagittariae cannot survive on Nuphar, and those of G. nymphaeae survive very poorly on Comarum and Rubus. Adults of G. sagittariae from Comarum or Rubus do not choose between these two food plants in laboratory conditions regarding dwelling, egg laying or feeding, but their avoidance of Nuphar is complete. Galerucella nymphaeae clearly prefers Nuphar, but may occasionally also sit, lay eggs or feed on terrestrial plants. It is suggested that G. nymphaeae and G. sagittariae represent sibling species and that the two forms of G. sagittariae, one feeding on Comarum and the other one on Rubus, are habitat races adapted to semiaquatic and terrestrial habitats, respectively. Adaptation to different habitats has created an effective premating barrier to gene flow, supporting the possibility that a sympatric mode of divergence has been involved in habitat race formation and speciation in the species complex.

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.

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.

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.

Patterns of chromosome banding in the sawfly family Tenthredinidae (Hymenoptera, Symphyta)

Abstract Karyotypes of five species belonging to the largest sawfly family Tenthredinidae(Tenthredo arcuata Forster, 1771, T. ferruginea Schrank, 1776, T. mesomela Linné, 1758, T. velox Fabricius, 1798, and Rhogogaster viridis (Linné, 1758) were studied using standard Giemsa staining, AgNOR- and C-banding techniques as well as fluorochrome Chromomycin A3 (CMA3) and DAPI. The species studied share the same chromosome number, n=10 andFN=40, in the diploid karyotype while they differ in the number of subtelocentric chromosomes. Number and location of NORs and amount of C-heterochromatin in the karyotypes vary between the species. C-banded chromosomes of T. velox, T arcuata and Rh. viridis subjected to DAPI and CMA3 staining were found to have no distinct AT-rich DNA clusters but possess groupings of GC base pairs concentrated in the NOR bearing regions. The data presented are the first to show genome organization in terms of the chromosomal banding patterns for the Tenthredinidae.

Holocentric Chromosomes of Psocids (Insecta, Psocoptera) Analysed by C-banding, Silver Impregnation and Sequence Specific Fluorochromes CMA3 and DAPI*

The pattern of nucleolus attachment and C-heterochromatin distribution and molecular composition in the karyotypes of psocid species Psococerastis gibbosa (2n = 16+X), Blaste conspurcata (2n = 16+X) and Amphipsocus japonicus (2n = 14+neo-XY) were studied by C-banding, silver impregnation and sequence specific fluorochromes CMA3 and DAPI. Every species was found to have a single nucleolus in male meiosis. In P. gibbosa the nucleolus is attached to an autosomal bivalent; in B. conspurcata to the X-chromosome; in A. japonicus to the neo-XY bivalent. The species show a rather small amount of constitutive heterochromatin, C-blocks demonstrating telomeric localization with rare exceptions. P. gibbosa is characterized by a polymorphism for C-blocks occurrence and distribution. In the autosomes of this species, C-heterochromatin consists of AT-rich DNA except for the nucleolus organizing region, which is also GC-rich; the X-chromosome shows both AT- and GC-rich clusters. In A. japonicus and B. conspurcata, C-heterochromatin of the autosomes and sex chromosomes consists of both GC-rich and AT-rich DNA clusters, which are largely co-localized.

The absence of chiasma terminalization and inverted meiosis in males and females of Myrmus miriformis Fn. (Corizidae, Heteroptera)

Terminalization of chiasmata and orientation of bivalents were analysed in males and females of Myrmus miriformis Fn. (Corizidae, Heteroptera). In spermatogenesis, the frequencies of the largest autosome bivalent with medial, subterminal and terminal chiasmata were compared at diplotene, early and late diakinesis and metaphase I. The bivalent displayed a single subterminal or medial chiasma in nearly 90 per cent of cells at diplotene. The frequency of such bivalents was still over 80 per cent at metaphase I, indicating the absence of complete chiasma terminalization. It is possible that chiasmata do not terminalize at all. In oogenesis, all bivalents at metaphase I displayed subterminal or medial chiasmata, indicating that terminalization is also absent in females. In both males and females all bivalents co-orientated at metaphase I, the telomeres of homologous chromosomes being orientated towards opposite spindle poles. Hence, they underwent prereduction in meiosis; no evidence for an inverted meiosis could be found.