Subchromosomal Organization In Insects

Euchromatin and heterochromatin can be distinguished in insects in various ways. Euchromatin contains the active genes, and heterochromatin, contains mainly repetitious, transcriptionally inactive DNA. Heterochromatic segments of the chromosomes can be observed in meiosis because of their high degree of condensation during first prophase (Fig. 1). Heterochromatin may also be detected by hybridization in situ of repetitous DNA sequences, such as satellite DNA, to the chromosomes. The DNA of heterochromatin is replicated later in the S phase of the cell cycle than the DNA of the euchromatin. Examples of DNA replication that is both late to start and late to finish has been seen in the B chromosomes of C. terminifera, and in the sex chromosomes of the common earwig, Forficula auricularia. The C-banding technique (described shortly) can also be used to stain heterochromatic segments.

In most cases the heterochromatin of insects is constitutive (i.e., in a permanent state), but in some insects with peculiar life cycles, such as the Cecidomyidae, individual chromosomes or sets of them may be made facultatively heterochromatic before being eliminated from the soma or the germ line of one of the sexes.

Chromosome banding in insects is largely limited to C-banding (Fig. 2b), originally named because the repetitious DNA proximal to the centromeres was stained. The position of the C bands is often procentric in insects, but these bands can be procentric, terminal, or interstitially distributed on the chromosome arms in different races of the same species, as in the grasshopper Caledia captiva.

The results of treating insect chromosomes with trypsin or other reagents that induce the narrow G bands, which are distributed all over the chromosomes of vertebrates, are disappointing in insects. The dark G bands correspond to the chromomeres, regions of the chromosomes that are contracted during meiotic prophase, and since insect chromosomes display chromomeres during meiosis, it is surprising that they do not show typical G bands. The bands revealed by trypsin treatment of the B chromosomes of C. terminifera seem to be exceptional, and they are a reflection of the C-banding patterns of these chromosomes (Fig. 2).

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