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Lewis, E., (1978) proposed a combinatorial model that assumes each insect seg-
ment is specified by a unique combination of homeotic genes that are expressed
in that particular segment. Thus, the fewest number of homeotic genes would
be required in thoracic segment 2, which would be the prototypical segment,
and progressively more genes would be active in the more-posterior segments.
Although this model has been modified, it provided a useful conceptual frame-
work for investigating Drosophila development.
Homeotic genes have some unusual characteristics. First, several homeo-
tic genes seem to be very large relative to most other genes in Drosophila . For
example, the Antennapedia + primary gene transcript is 100kb long and the
Ultrabithorax + transcript is 75kb. However, after the introns are spliced out,
the remaining sequences are only a few kilobases. Many of the exons in homeo-
tic genes seem to encode protein domains with distinct structural or enzymatic
functions. As a result, alternative splicing patterns in large genes, such as the
Antennapedia + and bithorax + gene complexes, may allow organisms to adapt
one basic protein structure to different, but related, developmental uses. By
adding or subtracting functional protein domains encoded by optional exons,
the structural and enzymatic properties of the homeotic gene product can be
modified and the ability of the protein to interact with other cellular compo-
nents can be altered as development proceeds.
4.14.3 Insect Metamorphosis
Insects may be ametabolous (Protura, Diplura, Thysanura, Collembola), hemime-
tabolous (Ephemeroptera, Odonata, Mantodea, Phasmida, Isoptera, Blattaria,
Plecoptera, Dermaptera, Embioptera, Psocoptera, Mallophaga, Anoplura,
Hemiptera), or holometabolous (Neoptera, Megaloptera, Mecoptera, Coleoptera,
Trichoptera, Lepidoptera, Diptera, Siphonaptera, Hymenoptera). Ametabolous
insects change little in form, except adults have fully developed reproductive
organs. Hemimetabolous insects undergo a progressive change (egg nymphal
stages adult) and holometabolous insects undergo dramatic changes at the end
of the life cycle (egg larval stages pupa adult). Some entomologists further
subdivide the types of metamorphosis, but these are the main categories.
The fossil record indicates that hemimetabolous insects were present 300 mil-
lion years ago and that holometabolous insects appeared 280 million years ago
( Belles 2010 ). The changes in morphology during development of both hemi- and
holometabolous insects are regulated by molting hormone (which promotes molts)
and juvenile hormone (JH), which represses transformation into the adult stage.
These two hormones are regulated by many transcription factors, and microRNAs
affect expression of these transcription factors ( Belles 2011 ). Gomez-Orte and
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