Fig. 6. phyl is expressed in FCs and both loss-of-function and overexpression cause specific muscle defects. (A-F) Confocal images of rP298 embryos in which phyl expression has been detected by fluorescent in situ hybridization (A,D; green), and FCs have been detected using the ß-Gal reporter (B,E; red). Both channels are shown merged in C and F. phyl expression in the somatic (arrowheads; C,F) and visceral (bent arrow; F) mesoderm closely followed the ß-Gal expression (C,F), indicating that both signals originated from FCs. Based on this level of analysis, phyl is expressed only in a subset of FCs. Phenotypic analysis of phyl mutant embryos and embryos overexpressing phyl revealed specific morphological defects in the muscle pattern. Lateral views of anti-Mhc-stained late stage 16 embryos (G,I,K) and ventrolateral views of anti-Krstained late stage 12 embryos (H,J,L) of the genotypes wildtype (G,H), phyl²/Df(2R)Trix (I,J) and twi-Gal4; Dmef2-Gal4 driving UAS-phyl (K,L). The same symbols are used to designate the same muscles in G-L. A diagrammatic representation is included with muscle loss indicated in red and morphological abnormalites indicated in blue. Null mutations in phyl reiterated some defects found in ast mutant embryos, such as losses of the LL1 (bracket, I) and DO4 (black arrow, I) muscles. In addition to muscle losses, the final muscle morphology in these embryos was compromised. Overexpression of phyl throughout the mesoderm lead to specific morphological problems (K). The LL1 and DO4 muscles were usually present in these embryos, but LT4 (arrowheads, K) frequently showed an abnormal shape. These muscles contained more than one nucleus, indicating that a fusion block was not responsible for the observed defect. Similar problems were found in the DT1 (bent arrow, K) and SBM (not shown) muscles. Loss of phyl function interfered with Kr expression in the LL1 muscle FC. The LL1 FC was absent in some hemisegments (black bent arrow, J) or showed reduced Kr expression in others (asterisk, J), whereas other FCs, such as LT2-4 and VA1, were present. These data suggest that the muscle losses detected stem from defects in initiation or maintenance of muscle FC determinants such as Kr. Conversely, phyl overexpression in the mesoderm (L) did not reproducibly affect Kr expression in LT4 FCs and, therefore, the morphological defects found in the final LT4 muscle must stem from a later interference of Phyl during the morphogenetic process.