Biology Reference
In-Depth Information
selection with increasing concentrations of toxin. Gene amplification results in
the production of more protein per unit time.
Gene amplification may be important to economic entomologists because
some aphids and mosquitoes that are resistant to insecticides have amplified
resistance genes, as described in Section 4.11.2, although the definition of gene
amplification in these examples differ from that described for mammalian cells.
Gene duplication
involves copying a gene multiple times; the copies may
be maintained on the same chromosome in tandem array or be transferred to
other chromosomes over evolutionary time.
4.11.1 Chorion Genes in
Drosophila
and Moths
Both gene amplification and gene duplication occur in the chorion genes of
Drosophila
and the moths
Bombyx mori
and
Antheraea polyphemus
(
Kafatos
1981, Eickbush and Burke 1985, Kafatos et al. 1986, Orr-Weaver 1991, Carminati
et al. 1992, Lecanidou and Papantonis 2010
). Analyses of these chorion genes
resulted in significant advances in knowledge of the mechanisms of gene regu-
lation and development.
In both
Drosophila
and silk moths, the egg is produced in the ovary and con-
sists of follicles composed of three cell types: 1) the oocyte, 2) a small number of
nutritive nurse cells connected to the oocyte, and 3) follicular epithelial cells that
surround the oocyte and nurse cells. There are
≈
1000 follicular epithelial cells per
follicle in
Drosophila
and up to 10,000/oocyte in silk moths. These cells synthesize
a complex mixture of proteins and secrete them onto the surface of the oocyte to
form the outer covering, or
chorion
. The chorion protects the embryo after fertil-
ization and oviposition, preventing desiccation, yet enabling respiration to occur.
Drosophila
and
B. mori
chorions are quite different. The
Drosophila
chorion
is comparatively simple, with an endochorion and exochorion composed of six
major and 14 minor proteins that are produced over
≈
5 hours. In silk moths, the
number of genes and the time devoted to producing the chorion is much greater.
There are three gene families in the wild silk moth
Antheraea polyphemus
, and
the same three families, plus two others, in the domesticated silk moth
B. mori
.
Approximately 100 chorion proteins are produced during a period of
≈
51 hours
in silk moths (
Kafatos 1981
).
Moth and fly chorion genes are organized differently. In
Drosophila
, large
amounts of the chorion proteins are produced in a relatively short time by
amplification
of the chorion genes (
Lu et al. 2001
). In
Drosophila
, two chorion-
gene clusters, each 5-10kb, encode tandemly oriented chorion genes. One gene
