Biology Reference
In-Depth Information
structural proteins are in the chorion of the silk moth
A. polyphemus
, which has
an elaborate organization. In
B. mori
, the chorion genes are on chromosome 2
and consist of two segments that total
>
1000kb of DNA. Subsets of the genes
are expressed at different periods of choriogenesis (early, middle, late, very
late), with the early proteins associated with framework formation, middle pro-
teins with framework expansion, late proteins with densification, and very-late
proteins with surface sculpturing of the chorion
.
Silk moths have solved the problem of producing large amounts of protein
quickly by gene duplication. Silk moth chorion genes are found in multiple cop-
ies of divergently transcribed, coordinately expressed pairs (
Kafatos et al. 1986
).
For example, all members from each of two late-gene families are arranged in
15 pairs on a 140-kb segment. The members of each family have a high degree
of sequence homology, although they are not identical.
Concerted evolution
could be the mechanism for maintaining this homology (
Eickbush and Burke
1985
). Concerted evolution often occurs in multigene families and could be
maintained by two mechanisms: either unequal crossing over or gene conver-
sion. Unequal crossing over may occur between the two sister chromatids of a
chromosome during mitosis of a germ-line cell or between two homologous
chromosomes at meiosis. It is a reciprocal recombination that results in sequence
duplication in one chromatid or chromosome and a corresponding deletion in
the other chromatid or chromosome. As a result of unequal exchange, daughter
chromosomes become more homogeneous than the parental chromosomes. If
the process is repeated, the numbers of each variant repeat on a chromosome
will fluctuate with time, and eventually variant repeat will become dominant in
the family.
Gene conversion
involves recombination in which a DNA fragment
in one region is transferred to the corresponding place in another region so the
transferred region becomes identical (
Osada and Innan 2008
).
Despite the very different organization of chorion genes in
Drosophila
and
silk moths, silk moth chorion genes can function in
D. melanogaster
. Moth cho-
rion genes were cloned into
P
-element vectors and inserted into the
D. melano-
gaster
germ line (
Mitsialis and Kafatos 1985
). (For a description of the methods
used in inserting moth genes into
Drosophila
using a
P
element that has been
modified to carry exogenous genes, see Chapter 9). Analysis of RNAs from trans-
formed flies indicated that moth genes are expressed in an appropriate manner
in the correct sex, tissue, and time in
D. melanogaster
. Fly and moth lineages
diverged
>
250 million years ago, yet regulatory elements conferring sex, tis-
sue, and temporal specificity of gene expression must have been conserved.
Chorion gene promoter sequences from
Antheraea pernyi
and
A. polyphemus
