Biology Reference
In-Depth Information
evade innate immune responses in three vector mosquito species of major medi-
cal importance” (
Bartholomay et al. 2010
).
4.9.4 Ribosomal Genes
The ribosome is the site in the cell where proteins are synthesized (
Frank 2000
).
The ribosome is a particle made of two subunits, each formed of ribosomal
RNAs (rRNAs) and ribosomal proteins. Protein synthesis is a serious business, and
it has to be done quickly and accurately. A typical protein takes
≈
15 seconds to
make (
Frank 2000
). Protein folding ensures the proper function of the protein,
and the folding relies on the location of particular amino acids, which can be
jeopardized by even a single point mutation. Typically, ribosomes have an error
rate of only 1 in 1000 to 10,000 amino acids. Ribosomes take up much of the
cell's mass and much of the cell's metabolism is devoted to making ribosomal
proteins and rRNAs. Ribosomes interact with mRNAs, initiation factors, and
transfer RNAs during protein synthesis;
>
120 macromolecular components are
needed to produce polypeptides in ribosomes (
Kaulenas 1985
).
Different arthropod species have different numbers of ribosomal genes located
in the nuclear chromosomes. For example,
Drosophila erecta
has 160 genes,
whereas
D. hydei
has
>
500 (
Berry 1985
). The fungus fly
Sciara coprophila
has
65-70 genes, one of the lowest numbers reported (
Kerrebrock et al. 1989
). Most
insect genomes have between 200 and 500 rRNA genes. The ribosomal genes of
Drosophila
are arranged into two clusters, with one cluster in the nucleolar orga-
nizer of each of the sex chromosomes (
Williams and Robbins 1992
). The 5.8S, 18S,
and 28S rRNAs are transcribed as a single unit that is then processed in the nucleus
to provide the separate subunit RNAs. Ribosomal genes comprise 2% of the total
genome and
≈
20% of the middle-repetitive sequences of
D. melanogaster
.
4.9.5 Silk Genes
Silk is important in the life history of many insects. Silk is produced in labial
glands, Malpighian tubules, and a variety of dermal glands (
Sutherland et al.
2010
). It seems that there are at least 23 different categories of silk-produc-
ing insects, possibly representing independent evolution in different lineages
of insects. However, silks have high levels of protein crystallinity and similar
amino-acid compositions, allowing silk to be produced in fine protein fibers.
Silk is used in cocoons by Lepidoptera; to produce an egg stalk and cocoons by
Neuroptera; to line tunnels by Ephemeroptera; to line tunnels and coat eggs
by Embiididae; for egg coverings and nests by Psocoptera; to produce cocoons
to protect against predators, high temperatures and low relative humid-
ity by Thysanoptera; to produce silk rafts to support eggs by water beetles
