THE STRUCTURE OF CONDENSED CHROMOSOMES IN MITOSIS AND MEIOSIS OF INSECTS

This article revolves around the structure of condensed mitotic and me iotic chromosomes in insects. In the first section, the potential of c ytological approaches in the field of chromosome biology is described. Emphasis is on immunolabeling, and transmission and scanning electron microscopy. In particular, the latter technology revealed a series of unusual components in association with the chromosomes, such as membr anes and non-chromatin material, which is presumably responsible for t he formation of achiasmatic bivalents. Virus-like particles were found scattered throughout the chromatin in a Lepidoptera species. This ass ociation is possibly responsible for the transmission of the particles into the next generation. Then, the cytology and the molecular make-u p of the key components of insect chromosomes are described. These are the centromeres, telomeres, and nucleoli. In any case, the situation in insects is compared briefly with that in mammals. The general struc ture of the centromeres in terms of centromere-specific repetitive DNA and proteins is similar in insects and mammals. This applies also to telomeres of most insect orders, but the chromosome ends of Diptera sp ecies differ from those in mammals. Fine structure observations raise also the possibility that insect nucleoli have a specific architecture . Chromosomal proteins-and emphasis is on histone acetylation-are addr essed in an individual section. Evidence is accumulating that histone H4 acetylation plays a role in dosage compensation and is a cytogeneti c marker of constitutive heterochromatin in insects. In the final sect ion, the characteristics of holokinetic chromosomes are listed. A seri es of insect orders, where direct or indirect evidence points to chrom osomes with relatively large centromeres, is presented. These are Lepi doptera, Trichoptera, Hemiptera, Homoptera, Odonata, and Dermaptera. C opyright (C) 1996 Elsevier Science Ltd.