SummaryWe record the occurrence of male recombination, detected in cytological preparations of testes in 2 Drosophila willistoni population samples. Comparative analysis of meiotic figures in imaginal discs of testes and the salivary gland polytene chromosomes of male third instar larvae were employed. We observed anaphase bridges, suggesting the occurrence of chiasmata and fragmented chromatids involving the second chromosomal pair and the heterozygous inversions IILF and IILDϩE segregating in the same individuals. This was observed in non-stressed larvae maintained at physiological temperature, and opens a wide field to study the factors that regulate crossing-over in natural populations of the highly polymorphic D. willistoni. We also observed what appears to be crossing-over in the tip of chromosomal arm IIL, in a sole male heterozygote for the inversion IILH of the G3 natural population. Key words Drosophila willistoni, Meiotic chromosomes, Male recombination.Male recombination is an uncommon phenomenon in Drosophila (Morgan 1912(Morgan , 1914. This absence of recombination in males is probably a consequence of mechanisms that improve the exploration of paracentric inversion polymorphism in the great majority of Drosophila species around the world. It seems to be advantageous for Drosophila to be polymorphic for paracentric inversions, since heterozygous larvae usually present high values for several fitness components (reviews in Sperlich and Pfriem 1986, Krimbas andPowell 1992). In females, the strategy used to prevent recombination between chromosomal sections involved in paracentric inversions was the dislocation of the achromatic spindle position towards one of the extremities of the oocyte (Hinton and Lucchesi 1960). This preferential orientation of the division apparatus promotes the expulsion of one nonrecombined chromatid (with the standard or the inverted order) as the first polar body, and the compulsory expulsion of the dicentric chromatid resultant of a putative recombination inside the inverted segment as the second polar body. Consequently, this results in the maintenance of a balanced chromatid as the functional nucleus of the oocyte, since the acentric fragment generated by the same recombination process will be lost in the cytoplasm.The refinement of this mechanism, accompanied by the increasing repression of male recombination in Drosophila, is certainly the result of strong selective pressures operating along the time of evolutionary diversification of these insects. Nevertheless, some Drosophila species tolerate a certain degree of recombination in males. This is the case of D. ananassae, in which crossing-over in male meiosis seems to be a common event (Kikkawa 1937, Moriwaki 1937, Matsuda et al. 1983, Goñi 1988 and genetic factors influencing this process were discovered in widespread natural populations (reviews in Matsuda et al. 1993.In D. willistoni, the highly adaptive chromosomal polymorphism for paracentric inversions
We examined the chromosome set of the aphid species Sitobion avenae, Schizaphis graminum and Methopolophium dirhodum by means of conventional staining and C, NOR, AluI and HaeIII banding methods. These species are considered important pests to several plants of economic interest in Brazil. No variation was observed in the number of chromosomes of S. avenae, whereas there was intraspecific variation in the other two species. Interspecific differences in the response to the banding treatments were observed. Whereas these techniques allowed the identification of several S. graminum chromosome pairs, only the AluI treatment was capable of inducing differential staining in the M. dirhodum chromosomes and no clear patterns emerged when the S. avenae preparations were treated
Por meio de coloração convencional e de métodos de bandamento C, NOR, AluI e HaeIII, foi feita uma tentativa de caracterizar o cariótipo e a variação no número cromossômico das espécies de afídeos Sitobion avenae, Schizaphis graminum e Methopolophium dirhodum, que são considerados como pragas importantes para várias plantas de interesse econômico no Brasil. Não foi encontrada variação no número cromossômico de S. avenae, enquanto que as outras duas espécies apresentaram variação numérica intra-específica. Diferenças interespecíficas quanto à resposta aos tratamentos de bandamento foram observadas. Através dos métodos utilizados, foi possível a identificação de vários pares cromossômicos de S. graminum, mas só o tratamento com AluI foi capaz de induzir coloração diferencial nos cromossomos de M. dirhodum, enquanto que nenhum padrão claro de bandamento apareceu nos preparados de S. avena
Some modifications were made to the methodology of Imai et al. (Jpn. J. Genet. 63: 159-185, 1988) for cytogenetic analysis of the leaf-cutting ants Atta sexdens piriventris and Acromyrmex heyeri (Hymenoptera, Formicidae), shortening preparation time and improving chromosomal preparations. The brain ganglia of prepupae were dissected in a 0.0025% hypotonic solution of colchicine, placed on a glass slide on a cold plate (4 ± 1oC) for 20 min. The material was fixed directly on the cold slide (with cold fixative I), macerated with a histological needle and fixed again with fixative I, followed by fixatives II and III, all of them cold. The slide was flame-dried right after the use of fixative III, and it was allowed to air-dry at room temperature for 2 h. The resulting metaphases presented less contracted chromosomes, with separated and well defined sister chromatids at a high frequency, when the material was processed in the manner described and stained with 3% Giemsa in phosphate buffer (pH 6.8) for 15 min.
Objetivando uma melhor análise citogenética das formigas cortadeiras Atta sexdens piriventris e Acromyrmex heyeri, algumas modificações foram feitas no sentido de otimizar a metodologia de Imai et al. (Jpn. J. Genet. 63: 159-185, 1988), tendo-se conseguido a diminuição do tempo de preparo do material e melhor qualidade da preparação. O gânglio cerebral de pré-pupa foi dissecado em solução de colchicina hipotônica 0,0025% e colocado sobre lâmina de vidro (nova e previamente limpa para ser corada com Giemsa) com colchicina hipotônica. A lâmina foi colocada sobre placa de gelo (4 ± 1oC) por 20 min. O material foi fixado diretamente na lâmina (com fixador I gelado), macerado com agulha histológica e fixado novamente com fixador I, seguido dos fixadores II e III, todos gelados. A lâmina foi rapidamente flambada após a última fixação e foi deixada secar à temperatura ambiente por 2 h. As metáfases resultantes apresentaram, com maior freqüência, cromossomos menos contraídos, com cromátides irmãs separadas e bem definidas, quando o material foi processado como descrito acima e corado com solução de Giemsa 3% em tampão fosfato pH 6,8, por 15 min
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