Dorsal
Ventral
C3
than adopting the default chordotonal organ fate upon receipt of the inducing signal. Molecular differences that prime subsets of cells to adopt particular fates upon induction are termed prepatterns. Hence, the prime-and-respond model
proposes that s al prepatterns dorsal ectodermal cells to become oenocytes.
However, it can be assumed that other dorsal competence factors are also
required, given that sal expression alone is insufficient to promote oenocyte
induction upon EGFR activation. In addition to its role as an oenocyte prepattern
gene, the results from experiments varying the concentration of sal or EGFR
activity suggest that sal also increases the apparent threshold of EGFR signalling required to promote a response. This correlates with the high and prolonged expression of rho that is seen in C1 in comparison to the other abdominal primary COPs.
One of the earliest events in response to EGFR signalling is the upregulation of SAL, observed in the committed sickle-shaped oenocyte precursors. As previously discussed, a moderate level of SAL appears necessary
for this subsequent upregulation. Therefore sal provides a molecular link between
the oenocyte prepattern and the response to EGFR signalling. This may be similar to the relationship between sal and EGFR signalling during the development of the dorsal tracheal trunk. In this system, it has been reported that the initial transcription of sal is EGFR-independent, while EGFR signalling is required for the maintenance of sal expression (Chen et al., 1998).
The absence of svp and vvl expression in sal mutants (Figure 5.6A in this chapter; Figure 4B in Elstob et al., 2001; Figure 7H in Rusten et al., 2001) indicates
that both genes lie downstream of sal in the genetic hierarchy for oenocyte
other cells within sal territory. This suggests that high levels of SAL positively regulate the transcription of svp and vvl, while the moderate level of sal that primes the dorsal ectoderm has no input. However, the dorsal SAL domain does appear to repress secondary COP recruitment and thus low level SAL might negatively regulate the expression of genes specific to these neuronal precursors. A concentration dependent role has also been implied for SAL in the transcriptional regulation of knirps, during the formation of the L2 wing vein (de Celis and Barrio, 2000).
In summary, the prime-and-respond model has been proposed to explain the dual role of SAL during oenocyte induction. This model predicts that SAL functions in the dorsal ectoderm to prime cells with an oenocyte prepattern. An early EGFR response appears to be the upregulation of SAL itself, which is required for the expression of downstream oenocyte markers.
5.3.4 abdA and oenocyte specification
The abdominal Hox gene abdA is necessary for the formation of oenocytes and is
sufficient to promote ectopic clusters in the thoracic segments. In this latter
experiment, abdA was ubiquitously expressed using a heat-inducible promoter and
a dual heat-shock protocol (see Materials and Methods). The variable thoracic phenotype produced (see Figure 5.2) suggests that there is a critical time window during extended germband stages when ABDA can specify the oenocyte developmental pathway. A more precise single heat-shock regime would be necessary to determine this competence period more precisely. A similar approach could be used to establish the time at which endogenous abdA function is required for the formation of abdominal oenocytes. Using the heat-shock system, ABDA
could be added back into abdA mutants at specific developmental time points, and the embryos examined for the presence of oenocytes. Although diagrams often depict uniform expression of a Hox gene within a single segment this is generally not the case. Rather a Hox gene is expressed in specific cells within a segment at different times and levels (Castelli-Gair and Akam, 1995). Salser and Kenyon
(1996) showed the importance of this mosaic of Hox expression in Caenorhabditis
elegans, where the precise expression of a single Hox gene in a single lineage controls four different cell fate decisions. Similarly, spatiotemporally regulated
expression of abdA might be required to promote oenocyte induction in the
abdominal segments.
Oenocytes are absent in abdA mutants, and the abdominal IchS is replaced by a dorsal chordotonal array consisting of three organs (dchS), which is characteristic of thoracic segments (Heuer and Kaufman, 1992). Therefore, with
respect to both oenocytes and chordotonal organs, in abdA mutants, the abdomen
is transformed towards the identity of the thorax. In the wild type situation, the thoracic primary COPs express rho at a low level (zur Lage et al., 1997), probably explaining the lack of oenocyte induction and secondary COP recruitment in this region. I have shown that misexpressing a constitutively active form of the EGFR ligand (sSPI) mimicks the misexpression of abdA, as in both cases it is sufficient to
promote ectopic oenocyte formation in thoracic segments. This suggests that abdA
could promote oenocyte formation by influencing the duration and/or level of EGFR ligand production. In this scenario, abdA would play a non-autonomous role in the formation of oenocytes by controlling the induction signal generated in a single cell, the C l primary COP.