Biology Reference
In-Depth Information
cluster is found on the X chromosome, and one gene cluster is on the third chro-
mosome. Chorion proteins could not be synthesized quickly and in sufficiently
large quantities unless gene amplification occurred because each chorion-gene
cluster is represented only once in the haploid genome. A 20-fold amplifica-
tion of the chorion genes on the X chromosome and an 80-fold amplification of
the genes on chromosome 3 is found in follicle cells. Amplification is achieved
by replicating the DNA segments at multiple replication origins (
Heck and
Spradling 1990
). DNA amplification extends bidirectionally for a distance of up
to 40-50 kb to produce a multiforked “onion-skin” structure that contains mul-
tiple copies of DNA containing the chorion genes (
Figure 4.1
).
Gene amplification also occurs in the chorion genes of the Mediterranean
fruit fly
Ceratitis capitata
. The overall organization of the cluster is similar to
that of
Drosophila
, with the same four genes maintained in tandem, in the same
order, and with similar spacing (
Konsolaki et al. 1990
). Despite the divergence of
Drosophila
and
Ceratitis
family lineages,
≈
120 million years ago, there is high con-
servation in coding sequences and regulatory properties of their chorion genes.
Silk moth chorion proteins are produced over a longer time and involve larger
numbers of genes that have probably arisen by
gene duplication
. More than 100
Figure 4.1
Amplification of the
Drosophila
chorion genes in follicle cells. The first three rounds of
DNA replication at the 66D locus on chromosome 3 are shown. The three small arrows represent
three well-characterized chorion genes in this cluster. The polarity of a fourth chorion gene and the
precise location of the origin are unknown. The boundaries of the amplified DNA are much larger
than the chorion protein transcription units within it.
