§ Theodor-Boveri-Institut für Biowissenschaften - Genetik, Am Hubland, 97074 Würzburg, FRG; # Institut für Zoologie, Universitätsstr. 31, 93047 Regensburg, FRG
Mushroom bodies (MBs) play a critical role in the processing and
storage of chemosensory information. They consist of approx. 2500
Kenyon cells sending out a compact bundle of thin parallel fibers, the
MB neuropil. This is subdivided into calyx, peduncle and the alpha-,
beta- and gamma- lobes. It forms during embryogenesis, grows in the
larva and, except for a small bundle, is replaced by new fibers during
metamorphosis.
We are analyzing the mushroom body miniature (mbm) gene of Drosophila
melanogaster(1) to study the developmental process of MB-formation.
In the original EMS-induced mutant allele mbm1 Kenyon cell fibers
degenerate in 3rd instar larvae and are not replaced during
metamorphosis leading to miniature MBs. In addition there are defects
in spontaneous olfactory behavior and in olfactory as well as visual
learning.
The gene is uncovered by the deficiencies Df(2L)netPMF and Df(2L)al
but not by the Df(2L)R70 thus putting at least part of the mbm gene in
an approx. 100kb region (chromomeres 21BC), previously cloned by
M.Noll, Zürich. A P-element insertion (P[21BC]) is located about 7kb
proximal to the Df(2L)al breakpoint. These flies show a weak mushroom
body defect which is amplified in P[21BC]/mbm1 flies. Furthermore, the
line cn bw sp, from which most likely the mbm1 mutation was derived,
shows a weak mbm phenotype. Additionally, both the mbm1 and cn bw sp
lines have an insertion of about 13kb, located near the P-element
insertion site.
There are five different transcription units in the putative mbm
region. One is the phospholipase C gene (plc21) cloned by Shortridge
et al. 1991(2). It was demonstrated that mbm and plc21 overlap to a
large extent raising the possibility that these two genes were
identical. This hypothesis was refuted by a gamma-ray mutation which
is a putative plc21 null allele exhibiting no mushroom body defect.
A second unit, about 20kb distal to the Df(2L)R70 breakpoint, is
abundantly expressed throughout development. The attempt to rescue the
MB phenotype of mbm1 using a genomic construct including this
transcription unit was unsuccessful.
Proximal to this gene two transcripts with weak expression in adult
heads were detected. However, flies with a gamma-ray-induced
translocation breakpoint within this transcription unit show no mbm
phenotype, suggesting that these transcripts do not represent the mbm
gene.
The main candidates are two further genes mapping near the P-element
and the 13kb insertion of the mbm1 and cn bw sp lines. Two other
gamma-ray mutations affecting these genes exhibit a strong mbm
phenotype. One of these genes, expressed throughout development and
seemingly enriched in adult heads, extends over the Df(2L)al
breakpoint and is homologous to the p85 subunit of PI3 kinase in
vertebrates. The other one, mapping inside Df(2L)al is homologous to
serin proteases and mainly expressed in 3rd instar larvae.
We currently examine within the mbm region further gamma-ray and
P-element mutations which should enable us to identify the mbm gene.