Biology Reference
In-Depth Information
Chapter 15
Multicellular Systems
15.1 The Morphogenesis of Drosophila melanogaster
The morphogenetic processes of
Drosophila melanogaster
(fruit fly) and
Homo
sapiens
are homologous (i.e., similar) since they have been found to utilize closely
related sets of genes in highly conserved manner. Because of easy genetic
manipulations possible with insects relative to humans, most of our current knowl-
edge about the molecular basis of animal morphogenesis has come from researches
performed on
Drosophila
.
The life span of
Drosophila melanogaster
is about 30 days, and it takes 10 days
for a fertilized
Drosophila
egg to become an
adult
fly. After fertilization, the
Drosophila
zygote begins mitosis (i.e., nuclear division), but cytokinesis (i.e., divi-
sion of the cytoplasm) does not occur in the early stages of the embryo, resulting in
a multinucleate cell called a
syncytium
(also called
syncytial blastoderm
). Because
of the common cytoplasm shared by all the nuclei of the syncytium, morphogen
(i.e., diffusible molecules regulating morphogenesis) gradients play a key role in
controlling the pattern of transcription of individual nuclei. At the tenth nuclear
division, the nuclei migrate to the periphery of the embryo, and at the thirteenth
nuclear division, the 6,000-8,000 nuclei are partitioned into separate cells forming
the
cellular blastoderm
. The embryogenesis, that is, the process of a fertilized egg to
develop into an embryo, takes about 15 h in
Drosophila
.
L´cuyer et al. (2007) used the fluorescence
in situ
hybridization (FISH) tech-
nique to determine the localization in the embryo of approximately 25% of the
mRNA encoded in the
Drosophila
genome. They found that the majority of
sampled mRNAs (i.e., 71% of 3,370 genes, or 2,360 genes) are localized in the
early embryo as illustrated in Fig.
15.1
. Their study demonstrates that mRNA
localization in
Drosophila
embryo is heterogeneous and suggests that such a
distribution of mRNA in embryo may be a widespread biological phenomenon
playing a fundamental role in organizing cellular architecture.
The patterns of distribution of mRNA molecules in cells measured with
microarrays can be either spatial (Fig.
15.1
) (L
´
cuyer et al. 2007) or temporal
