Biology Reference
In-Depth Information
(Hydrophilidae); to produce cocoons, nests and webs by Hymenoptera; to pro-
duce cocoons by Siphonaptera; and to produce underwater prey-capture nets by
Trichoptera (
Craig 1997, Sutherland et al. 2010
). The best-studied are the silks
produced by the Lepidoptera, including the silk moth
Bombyx mori
.
Silks are composed of one or more proteins called
fibroins,
, proteins composed
of several simple amino-acid sequences in reiterated arrays (
Craig 1997, Sezutsu
and Yukuhiro 2000
). The silk gland provides a model system for cell biologists
and molecular geneticists to study gene regulation and development. Silk-gland
cells of
B. mori
are polyploid (up to 20-fold), which may explain how silk moth lar-
vae produce huge amounts of silk proteins within a short time (5-6 days) before
pupating. The cells from the posterior silk gland produce fibroin, and those from
the middle part of the silk gland store fibroin and
sericin
, a mixture of four to six
hot water-soluble polypeptides. Sericin binds strands of raw silk fibers together.
Silk proteins have an unusual amino-acid composition, with a predominance of
glycine and alanine in fibroin, and serine in sericin (
Prudhomme et al. 1985
).
Although
Drosophila melanogaster
does not produce silk, a silk-encoding
gene (
P25
) of
Bombyx mori
was expressed in the anterior salivary gland after
flies received the moth silk gene by
P
element-mediated transformation (
Bello
and Couble 1990
; for a discussion of
P
element-mediated transformation, see
Chapter 9). The
P25 B. mori
gene was appropriately expressed in the fly larval
salivary glands, indicating that
Drosophila
salivary glands can recognize
Bombyx
silk protein-coding sequences and control their expression, despite the evolu-
tionary divergence of flies and moths
>
250 million years ago.
4.9.6 Transfer RNA Genes
More than 90 transfer RNAs (tRNAs) have been identified during
Drosophila
development that are encoded by at least 670 genes, which can be divided into
60 separate groups. One to 18 tRNA genes are contained in each of 30 chromo-
somal sites, but there are no tandem repeats.
4.9.7 Vitellogenin Genes
Yolk proteins provide embryos with nutrients essential for growth within the
egg. Most are phosphoglycoproteins, and they provide a source of amino acids,
phosphate, lipids, and carbohydrates. The major yolk proteins are derived from
vitellogenins
that are produced by the fat body and secreted for uptake by
maturing oocytes. Vitellogenin-gene structure and regulation have been studied
in
Locusta migratoria
, the tobacco hornworm
Manduca sexta
,
Bombyx mori
, the
boll weevil
Anthonomus grandis grandis
, the Mediterranean fruit fly
Ceratitis
