Meiotic behavior and karyotypic variation in Acledra (Pentatomidae, Heteroptera).

Acledra comprises 15 taxonomically identified species, most of which are crop pests. This is the first cytogenetic study of species of this genus. Acledra kinbergii and A. modesta showed the modal number of the Pentatomidae (2n = 14 = 12 + XY), while A. bonariensis had a reduced complement (2n = 12 = 10 + XY), with a markedly larger autosomal pair. Meiotic behavior follows the general pattern of the family; the autosomes divide pre-reductionally, the sex chromosomes are achiasmatic and divide post-reductionally, and at metaphase II the autosomes show a ring-shaped configuration with the pseudobivalent at the center. However, the configuration at metaphase I varies; A. modesta shows the typical arrangement (ring of bivalents with the sex chromosomes lying at its center). In A. kinbergii, the sex chromosomes are part of the ring or only the Y chromosome is at the center. In A. bonariensis, the ring arrangement is not well defined. There are also differences at the diffuse stage; chromatin strands of different width are observed in A. bonariensis and A. modesta, whereas bivalents do not entirely lose their identity in A. kinbergii. In A. bonariensis, the reduced complement may have originated from the fusion of the two larger non-homologous autosomes, which could characterize the ancestral karyotype of this genus. The presence of secondary constrictions in the larger pair of A. modesta and A. bonariensis may support this hypothesis. Since secondary constrictions are uncommon in the holokinetic chromosomes of heteropterans, their presence in these species may indicate that it is a plesiomorphic character of the genus.

Behaviour of ring bivalents in holokinetic systems: alternative sites of spindle attachment in Pachylis argentinus and Nezara viridula (Heteroptera).

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.

Meiotic studies in Dysdercus Guérin Meneville 1831 (Heteroptera: Pyrrhocoridae). I. Neo-XY in Dysdercus albofasciatus Berg 1878, a new sex chromosome determining system in Heteroptera.

The genus Dysdercus Guérin Méneville 1831 represents the only taxon within the family Pyrrhocoridae in the New World. Based on morphological features, it has been suggested that American species derived from immigrants from the Old World, most probably from the Ethiopian Region. So far, 10 species from Dysdercus, including six species from the Old World and four species from the Neotropical Region have been cytogenetically analyzed. As is characteristic of Heteroptera, they possess holokinetic chromosomes and a prereductional type of meiosis. While the X1X20 sex chromosome system has been reported in all cytologically analyzed species of Dysdercus from the Old World, the system X0 has been found in all but one species from the New World, regardless of the number of autosomes in the complement. In the present study the male meiosis of D. albofasciatus Berg 1878 was studied in specimens from four different populations from Argentina. The diploid chromosome number was found to be 2n = 10 + neo-XY. The neo-X shows at each subterminal region a positively heteropycnotic and DAPI-bright segment which corresponds to the ancestral X-chromosome. The origin of this neo-XY system involved, most probably, a subterminal insertion of the ancestral X chromosome in an autosome, followed by a large inversion, which included part of the original X chromosome.

Comparative meiotic studies in Triatoma sordida (Stål) and T. guasayana Wygodzinsky & Abalos (Reduviidae, Heteroptera).

Triatoma sordida and T. guasayana are competent Trypanosoma cruzi vectors, with overlapping distribution areas in Argentina. Both species are morphologically similar, and their immature stages are hard to discriminate. Cytogenetic studies in the genus Triatoma reveal scarce karyotypic variations, being 2n = 20 + XY the most frequent diploid number in males. In the present work the meiotic behaviour of different Argentinian populations of T. sordida and T. guasayana has been analyzed; the meiotic karyotype of both species has also been compared. The species differ in total chromosome area and in the relative area of the sex chromosomes. These meiotic karyotypic differences constitute an additional tool for the taxonomic characterization of T. sordida and T. guasayana. The analysis of an interpopulation hybrid of T. sordida (Brazil x Argentina) reveals a regular meiotic behaviour; despite the presence of heteromorphic bivalents. Our observations support the hypothesis that karyotype variations through the gain or loss of heterochromatin can not be considered as a primary mechanism of reproductive isolation in Triatoma.

Comparative meiotic studies in Triatoma sordida (Stal) and T. guasayana Wygodzinsky & Abalos (Reduviidae, Heteroptera)

Triatoma sordida and T. guasayana are competent Trypanosoma cruzi vectors, with overlapping distribution areas in Argentina. Both species are morphologically similar, and their immature stages are hard to discriminate. Cytogenetic studies in the genus Triatoma reveal scarce karyotypic variations, being 2n=20 + XY the most frequent diploid number in males. In the present work the meiotic behaviour of different Argentinian populations of T. sordida and T. guasayana has been analyzed; the meiotic karyotype of both species has also been compared. The species differ in total chromosome area and the relative area of the sex chromosomes. These meiotic karyotypic differences constitute an additional tool for the taxonomic characterization of T. sordida and T. guasayana. The analysis of an interpopulation hybrid of T. sordida (Brazil x Argentina) reveals a regular meiotic behaviour, despite the presence of heteromorphic bivalents. Our observations support the hypothesis that karyotype variatons through the gain or loss of heterochromatin can not be considered as a primary mechanism of reproductive isolation in Triatoma.