The fourth chromosome of Chironomus tentans Malpighian tubules: an ultrastructural study

Polytene Chromosomes: 70 Years of Genetic Research

Polytene chromosomes were described in 1881 and since 1934 they have served as an outstanding model for a variety of genetic experiments. Using the polytene chromosomes, numerous biological phenomena were discovered. First the polytene chromosomes served as a model of the interphase chromosomes in general. In polytene chromosomes, condensed (bands), decondensed (interbands), genetically active (puffs), and silent (pericentric and intercalary heterochromatin as well as regions subject to position effect variegation) regions were found and their features were described in detail. Analysis of the general organization of replication and transcription at the cytological level has become possible using polytene chromosomes. In studies of sequential puff formation it was found for the first time that the steroid hormone (ecdysone) exerts its action through gene activation, and that the process of gene activation upon ecdysone proceeds as a cascade. Namely on the polytene chromosomes a new phenomenon of cellular stress response (heat shock) was discovered. Subsequently chromatin boundaries (insulators) were discovered to flank the heat shock puffs. Major progress in solving the problems of dosage compensation and position effect variegation phenomena was mainly related to studies on polytene chromosomes. This review summarizes the current status of studies of polytene chromosomes and of various phenomena described using this successful model.

Sequences of globin 6 gene alleles and linkage of globin 6 and 7B genes in the insect Chironomus thummi thummi

The purpose of this study was to isolate the stage-specifically expressed Chironomus thummi thummi globin-encoding gene (Gb) 6 (ctt-6). The final product of this gene is hemoglobin (Hb) CTT-VI, a protein that is phylogenetically most closely related to Hb CTT-VIIB. In the absence of chromosomal rearrangement, genes of immediate common ancestry should be closely linked. This was shown for a genomic clone containing the Gb ctt-3 and ctt-4 genes (encoding Hb CTT-III and CTT-IV, respectively), and another clone containing the Gb ctt-2 beta and ctt-9a genes (encoding Hb CTT-II beta and CTT-IXa, respectively). We report the isolation and sequence of two alleles of Gb ctt-6 found on independent genomic clones screened with a ctt-6 cDNA, and the predicted linkage of Gb ctt-6 and a ctt-7B gene. The latter, designated Gb ctt-7B9/5, is unusual in being a chimera of two previously reported Gb ctt-7B genes. The result of a partial gene conversion or an unequal crossover between oppositely oriented genes, the chimeric ctt-7B9/5 represents either an additional ctt-7B locus or a 7B haplotype.