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is stressed by heat or cold, the hidden variants are expressed and selection then
could lead to the continued expression of these traits, providing a mechanism
for promoting evolutionary change in an otherwise entrenched developmen-
tal process. The protein may act as a “capacitor for morphological evolution”
( Rutherford and Lindquist 1998 ).
The small hsp gene family includes genes encoding hsp22, hsp23, hsp26, and
hsp27 proteins, all of which are expressed at several developmental stages.
4.9.2 Histone Genes
Five histone-gene families code for the basic proteins contained in eukaryotic
chromosomes. The basic unit of chromosomes, the nucleosome, is composed
of 146bp of DNA coiled around a histone octamer, two molecules each of his-
tone H2A, H2B, H3, and H4 (see Figure 3.1). Linking two nucleosomes is a small
stretch of DNA to which the fifth histone, H1, is bound. The histone genes share
regulatory sequences and are coordinately expressed. In some species, there
are tissue- or stage-specific gene sets. In Drosophila , the histone genes are tan-
demly repeated and closely linked. The histone genes of the midge Chironomus
thummi are different from those found in D. melanogaster ( Hankeln and
Schmidt 1991 ).
Histone genes typically lack introns. It is thought that introns were eliminated
because these genes must be expressed efficiently and rapidly during develop-
ment. Having histone genes organized in a tandem-repeat structure also ensures
that there will be equivalent amounts of the five proteins produced.
There is a 10-fold difference in copy numbers of histone genes in three species
of Drosophila ( melanogaster , hydei , and hawaiiensis ) ( Fitch et al. 1990 ). D. mela-
nogaster has 100 tandemly arranged histone genes, far more genes than would
be required for the maximal rate of transcription during development. By con-
trast, D. hydei has five and D. hawaiiensis has 20 tandem histone-repeat copies
per haploid genome. In D. melanogaster , the histone genes are located adja-
cent to a heterochromatic region of chromosome 2, whereas they are located
in euchromatic regions in the other two species. This difference suggests there
are more histone genes in D. melanogaster to compensate for the fact that the
genes are less active because they are located near heterochromatin.
4.9.3 Immune-Response Genes
Insects defend themselves against bacteria, viruses, fungi, and parasitoids with
both cellular and humoral immune responses ( Gillespie et  al. 1997, Khush and
Lemaitre 2000, Carton and Nappi 2001, Lazzaro and Rolff 2011 ). The first line
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