Biology Reference
In-Depth Information
mother, but coordinates provided by the mother also determine the position
and organization of adult structures (
Couso and Gonzalez-Gaitan 1993
). After
emergence as an adult, the insect mates and their progeny begin this develop-
mental cycle again.
4.14 Dissecting Development with
D. melanogaster
Mutants
The study of development in
Drosophila
has depended upon the availability
of mutants so that the process can be dissected; this process is called
reverse
genetics
. In fact, in discussing development in
Drosophila
, the genes influenc-
ing development are called by names that reflect their mutant form.
Nusslein-
Volhard and Wieschaus (1980)
began a systematic program of mutagenizing
Drosophila
females to obtain many developmental mutants in insect embryos. In
addition, E. B. Lewis discovered many useful developmental mutants in his pio-
neering work on
Drosophila
development involving the bithorax homeotic gene
complex. Christiane Nusslein-Volhard, Eric Wieschaus, and Edward Lewis jointly
were awarded the 1995 Nobel Prize in Medicine for their pioneering research on
development.
The term
homeotic
was coined to describe the replacement of one part of
the body by a serially homologous part.
Lewis (1978)
developed the hypothesis
that families of structurally related genes control the specification of the insect
body plan because insect bodies are metameric (composed of serially repeating
units or body segments). The body segments differentiate into specific structures
according to their position. Likewise, the appendages in each major body seg-
ment develop into appropriate structures, with antennae located on the head,
legs on the thoracic segments, and wings on the second thoracic segment.
However, this normal pattern is disrupted by a number of homeotic mutants.
Drosophila
embryos seem to go through two phases in their development.
During the first phase, many genes seem to encode transcription factors or
nuclear proteins, suggesting that a cascade of transcriptional factors regulate
other genes in development. There is a successive subdivision of the embryo into
smaller and smaller domains that is accomplished by the differential and combi-
natorial action of transcription factors. The first phase is completed by the time
cells and are fully formed at the end of blastoderm.
The second phase begins after the formation of the cellular blastoderm and
consists of elaborating the information provided from reference points that
have been deposited along the dorsal-ventral and anterior-posterior axes. This
requires the communication of information between cells by intercellular signal
molecules.
