Abstract. Species of Pentatomidae are cytogenetically characterized by the presence of holokinetic chromosomes, a pre-reductional type of meiosis, and a great constancy not only in chromosome number (2n = 14 in 85% of the 250 species analyzed) but also in the sex chromosome determining system (XY/XX).Edessa meditabunda and E. rufomarginata males have 2n=14=12 + XY, and both species present small telomeric positively heteropycnotic bands which are DAPI and CMA bright. In E. meditabunda the NOR region is clearly revealed at the telomeric region of the largest autosomal pair by silver staining and CMA banding. Meiotic behaviour of both species follows the general pat tern of the order: autosomes divide pre-reductionally, sex chromosomes are achiasmatic and divide postreductionally, and at both metaphase plates the autosomes become arranged in a circle with the sex chromosomes lying at its center. In E. meditabunda, how ever, the larger sex chromosome is generally observed at metaphase I forming part of the ring of autosomal bivalents. Bivalents with two chiasmata are frequently observed in E. meditabunda and E. rufomarginata', mean chiasma frequency (6.45 and 6.26, respec tively) differ significantly between both species, but differences between populations within each species are not significant.The metaphase plate arrangement of autosomes and sex chromosomes is rather constant in Heteroptera. However, our results in E. meditabunda together with previous reports in other species of the order led us to suggest that the metaphase plate arrangement is more liable to variation at the first meiotic division than at the second one, when it is almost constant. The presence of ring bivalents in both species here analyzed constitutes further evidence against the previous statement of only one chiasma per bivalent in Heteroptera.

C-banding was carried out on Belostoma elegans (2n = 26 + X1X2Y ) (o'), B. micantulum (2n = 14 + XY) (~) and B. oxyurum (2n = 6 + XY) (o') (Belostomatidae, Heteroptera). The C-bands always have a telomeric localization and no interstitial bands were detected. An inverse relationship between chromosome size and chromosome number exists, and besides, an inverse relationship between chromosome size and the size of the Cbands was observed. The DNA content was determined in all three species. B. elegans has a C content of 1.55 + 0.06 pg, B. micantulum has 0.88 + 0.04 pg and B. oxyurum had 0.53 + 0.04 pg.Considering the male meiotic characteristics, the chromosome complement and the results of C-banding and DNA content, the karyotype of B. oxyurum probably originated through autosomal fusions. The karyotype of B. micantulum and B. elegans could have originated through autosomal fusions or fragmentations respectively; with the information available up to now it is not possible to discard any of the two pathways.

Heteropteran chromosomes are holokinetic; during mitosis, sister chromatids segregate parallel to each other but, during meiosis, kinetic activity is restricted to one pair of telomeric regions. This meiotic behaviour has been corroborated for all rod bivalents. For ring bivalents, we have previously proposed that one of the two chiasmata releases first, and a telokinetic activity is also achieved. In the present work we analyse the meiotic behaviour of ring bivalents in Pachylis argentinus (Coreidae) and Nezara viridula (Pentatomidae) and we describe for the first time the chromosome complement and male meiosis of the former (2n = 12 + 2m + X0, pre-reduction of the X). Both species possess a large chromosome pair with a secondary constriction which is a nucleolus organizer region as revealed by in-situ hybridization. Here we propose a new mode of segregation for ring bivalents: when the chromosome pair bears a secondary constriction, it is not essential that one of the chiasmata releases first since these regions or repetitive DNA sequences adjacent to them become functional as alternative sites for microtubule attachment and they undertake chromosome segregation to the poles during anaphase I.

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.

The neo-X and neo-Y sex chromosomes of Dysdercus albofasciatus represent a unique model for the study of early stages of sex chromosome evolution since they retained the ability to pair and recombine, in contrast to sex chromosomes in most Heteroptera. Here we examined structure, molecular differentiation, and meiotic behaviour of the D. albofasciatus neo-sex chromosomes. Two related species with the ancestral X0 system, D. chaquensis and D. ruficollis, were used for a comparison. In D. albofasciatus, 2 nucleolar organizer regions (NORs) were identified on the neo-X chromosome using fluorescence in situ hybridization (FISH) with an rDNA probe, whereas a single NOR was found on an autosomal pair in the other 2 species. Genomic in situ hybridization (GISH) differentiated a part of the original X in the neo-X chromosome but not the neo-Y chromosome. The same segment of the neo-X chromosome was identified by Zoo-FISH with a chromosome painting probe derived from the X chromosome of D. ruficollis, indicating that this part is conserved between the species. Immunostaining against the cohesin subunit SMC3 revealed that only terminal regions of the D. albofasciatus neo-Xneo-Y bivalent pair and form a synaptonemal complex, which is in keeping with the occurrence of terminal chiasmata, whereas the interstitial region forms a large loop indicating the absence of homology. These results support the hypothesis that the neo-X chromosome evolved by insertion of the original X chromosome into 1 NOR-bearing autosome in an ancestor carrying the X0 system. As a consequence, the homologue of this NOR-autosome became the neo-Y chromosome. A subsequent inversion followed by transposition of the NOR located on the neo-Y onto the neo-X chromosome resulted in the present neo-sex chromosome system in D. albofasciatus.

The suborder Heteroptera constitutes one of the most important insect groups because most species are plants feeders and cause damage on many plants of economic importance. One of the most important cytogenetic characteristics of Heteroptera is the holokinetic nature of the chromosomes. One particular feature of some species of Pentatomidae is the regular presence of an abnormal meiosis in one testicular lobe (harlequin lobe). From the 28 species cytogenetically analysed from Argentine material, 21 present the diploid number 2n ¼ 14, four species present a reduced number (2n ¼ 12) and another three species possess an increased diploid number (2n ¼ 16); among all these only three present an harlequin lobe. In the present work, a bibliographic review of the chromosome number and sex determining system of 294 species and subspecies belonging to 121 genera within the subfamilies Asopinae, Discocephalinae, Edessinae, Pentatominae, Phyllocephalinae and Podopinae is presented. The male diploid numbers range from six to 27 with a mode in 14 chromosomes; this last diploid number is present in 85% of the species. The sex chromosome determining system is XY/XX except in three species: Macropygium reticulare (Fabricius, 1803), Rhytidolomia senilis (Say, 1832) and Thyanta calceata (Say, 1832) which present derived sex chromosome systems. Furthermore, the cytogenetic relationships with the other families of Pentatomoidea are discussed.

Abstract.Heterochromatin is one of the most dynamic components in the genome of species. Previous studies on the heterochromatin content and distribution in Heteroptera (insects with holokinetic chromosomes) have shown that the species belonging to the family Coreidae are interesting model organisms since they show very diverse C bands patterns. In the present work, we analyzed the C-band pattern in individuals of Holhymenia rubiginosa from different populations collected in different years. This species has the diploid karyotype 2n = 27/28 = 24 + 2m + X0/XX (male/female). C-bands are terminally, subterminally or interstitially located on 10-17 chromosomes and a remarkable heterochromatin heteromorphism is observed in the meiotic bivalents: in the presence/absence of bands, in the size of bands and number of bands. A heteromorphism is also inferred in the number of ribosomal genes from the difference in the fluorescent in situ hybridization signals between NOR-homologues. Chiasmata are generally located opposite to conspicuous C-bands, but in some bivalents chiasmata are also observed in close proximity to C-bands. Considering the striking variation in heterochromatin content between individuals and populations it is suggested that heterochromatin should be selectively neutral in H. rubiginosa.

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