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unresolved (
Aksoy 2000
). Now, we know that at least three different microor-
ganisms are present: the primary (P) symbiont
Wigglesworthia glossinidia
is an
intracellular symbiont residing in specialized epithelial cells that form a special
U-shaped organ (bacteriome) in the anterior gut. The secondary gut symbiont
Sodalis glossinidius
is present in midgut cells. The third,
Wolbachia
, is found in
reproductive tissues. Tsetse females are viviparous, retaining each egg within her
uterus where it hatches. The larva matures there and is born as a fully devel-
oped third-instar larva. During its intrauterine life, the larva receives nutri-
ents and both of the gut symbionts from its mother via milk-gland secretions;
the
Wolbachia
are transmitted transovarially. Efforts to eliminate tsetse symbi-
onts with antibiotics result in retarded growth and a decrease in egg produc-
tion. Because it is impossible to eliminate only one at a time, it is difficult to
decipher the role each plays. However, the gut symbionts supply B-complex vita-
mins, and
Sodalis
also produces a chitinase that seems responsible for increasing
the susceptibility of its host to the sleeping sickness trypanosome (
Aksoy 2000
).
Analysis of the
Wigglesworthia
and
Sodalis
genomes indicate that they each
form a distinct lineage in the Proteobacteria. Molecular analyses suggest that a
tsetse ancestor was infected with a
Wigglesworthia
and from this ancestral pair
evolved the tsetse species and
Wigglesworthia
strains existing today. No evi-
dence was found for horizontal transfer of
Wigglesworthia
symbionts between
tsetse species.
Sodalis
infections might represent recent independent acquisitions
by each tsetse species or multiple horizontal transfers between tsetse species.
4.13 Insect Development
Studies of
Drosophila melanogaster
have provided much of what we know
about the genetics of development in insects (
Lawrence 1992, Wilkins 1993,
Klingler 1994, Powell 1997, Gilbert 2000, Otto 2000
), although that is begin-
ning to change. Extensive analyses of insect development became feasible with
the tools of molecular genetics and thousands of papers have been published
on the molecular genetics of development in
D. melanogaster
. Review articles
and topics have been published on this rapidly advancing ield (
Lawrence 1992,
Wilkins 1993
). A complete discussion of insect development is beyond the scope
of this chapter. However, the following provides a brief outline of
D. melanogas-
ter
embryonic development that will be useful in understanding sex determina-
tion, behavior, and
P
-element-mediated transformation (Chapters 9-11).
4.13.1 Oocyte Formation in
D. melanogaster
A substantial amount of development of the insect embryo is determined in
the oocyte, before oocyte (n) and sperm (n) pronuclei fuse to form an embryo
