Biology Reference
In-Depth Information
mitochondrial and nuclear genomes are involved in determination of the germ
line during embryonic development
.
Maternal-effect genes are most important during development of the egg
up to the blastoderm stage; before the blastoderm stage, the embryo has dis-
crete nuclei but no cell walls (it is syncytial). After blastoderm, genes inherited
by the zygote from both parents become dominant factors in determining
development. However, because development is an elaborative process and
the adult phenotype is a summation of the developmental effects accrued
over the life span of the individual, developmental events early in the life cycle
can significantly influence the phenotype at later stages. For example, mater-
nal effects have significant and diverse effects on insect life histories, includ-
ing incidence and intensity of diapause, production of sexual forms, wing
polyphenism, dispersal behavior, developmental time, growth rate, resistance
to chemicals and microbial infection, and survival. Some of these influences
are caused by maternal age and diet, but some are genetically determined
(
Mousseau and Dingle 1991
).
4.14.2 Zygotic-Segmentation Genes
During blastoderm, the embryo begins to develop a pattern of repeating body
segments. Zygotic genes of three types control segmentation: 1) pair-rule, 2)
gap, and 3) segment-polarity genes. Segmentation mutants found in
Drosophila
embryos initially were difficult to interpret because they did not affect what
appeared to be a single segment; usually they affected half of one “segment”
and the adjacent half of the next “segment.” Eventually, it was determined that
“true segments” are not reflected by the visible cuticular patterns of sclerites
and sutures; visible “segments” are, in fact,
parasegments
. There are 14 com-
plete parasegments in
D. melanogaster
that are defined early in development;
each is a precise set of cells.
Gap genes subdivide the embryo along the anterior-posterior axis because
they encode transcription factors that regulate the expression of the pair-rule
genes. Pair-rule genes divide the embryo into pairs of segments and encode
transcription factors that regulate the expression of the segment-polarity
genes. The segment-polarity genes establish the anterior-posterior axis of each
segment (
Figure 4.7
). Most segmentation mutants are lethal in the zygote,
but some gap genes have a maternal effect and are expressed during oogen-
esis. Four of the segmentation genes (
fushi tarazu
+
,
even-skipped
+
,
paired
+
,
and
engrailed
+
) contain a homeo box (see Section 4.14.2.4). Thus, these genes
encode DNA regulatory proteins or transcription factors that bind to specific
DNA or RNA sequences.
