We have screened a collection of approximately 400 GAL4 enhancer trap lines for useful patterns of expression in the embryo, larval brain, imaginal discs, and ovary using a UAS‐lacZ reporter construct. Although similar patterns of expression have previously been reported in the original P[lacZ] enhancer trap screens, these lines are useful for directing ectopic expression of genes in discrete patterns during these stages. In addition, we have identified some unique patterns of expression that have not been previously reported. Dev. Dyn. 209:310–322, 1997. © 1997 Wiley‐Liss, Inc.

Highly ordered arrays of intramembrane particles are observed in freeze-fractured plasma membranes of the green alga Micrasterias denticulata during the synthesis of the secondary cell wall. The observable architecture of the complex consists primarily of a precise hexagonal array of from 3 to 175 rosettes, consisting of 6 particles each, which fracture with the Mace. The complexes are observed at the ends of impressions of cellulose fibrils . The distance between rows of rosettes is equal to the center-to-center distance between parallel cellulose fibrils of the secondary wall . Correlation of the structure of the complex with the pattern of deposition indicates that the size of a given fibril is proportional to the number of rosettes engaged in its formation . Vesicles containing hexagonal arrays of rosettes are found in the cytoplasm and can be observed in the process of fusing with the plasma membrane, suggesting that the complexes are first assembled in the cytoplasm and then incorporated into the plasma membrane, where they become active in fibril formation. Single rosettes appear to be responsible for the synthesis of microfibrils during primary wall growth . Similar rosettes have now been detected in a green alga, in fern protonemata, and in higher plant cells . This structure, therefore, probably represents a significant component of the cellulose synthesizing mechanism in a large variety of plant cells.KEY WORDS cellulose synthesis -plasma membrane " intramembrane complexes freeze-fracture " Micrasterias The plasma membrane of plant cells is generally believed to play a significant role in the synthesis of linear fl-1,4-glucan polymers and in the assembly of these chains into microfibrils of cellulose . J . CELL BIOLOGY

We have isolated three classes of monoclonal antibodies against Drosophila cell-surface antigens that are expressed with positional specificity in imaginal discs. Comparison of immunofluorescence patterns with the wing-disc fate map reveals that expression of the antigens is not directly related to the specific type of cuticular structure that a cell will make upon differentiation but depends on the position of the cell in the undifferentiated disc epithelium. On mature wing discs, each class of position-specific (PS) antibody binds nonuniformly with respect to the dorso-ventral compartment boundary, with PS1 antibodies binding primarily to dorsal cells and PS2 antibodies, to ventral cells. Antibodies of the different PS classes extract similar but nonidentical sets of large glycoproteins from cell lysates, and antibodies of the most general class, PS3, recognize the PS1 and PS2 antigens in addition to PS3-specific components. Thus, the distributions and molecular characteristics of the PS antigens suggest that the molecules are structurally and functionally related to one another.Much of the adult Drosophila epidermis and some internal structures derive, at metamorphosis, from imaginal discs, which are present in larvae as convoluted inpocketings from the larval hypoderm. [Imaginal disc morphology and development are reviewed by Poodry (1).] In mature third-instar larvae each imaginal disc, although still largely undifferentiated, is stably determined to make a particular region of the adult (2, 3), and the cells within a particular part of a disc are specified to make a particular adult structure (4) (see Fig. 1). This precise specification of pattern within the disc appears to depend on intercellular interactions (6, 7). Clonal analysis has shown that discs are subdivided into compartments, defined by the observation that clones of genetically marked cells do not cross a compartment boundary into the territory of neighboring compartments (8). Genetic studies indicate that at least some of these compartments are units of gene action in the development of the epidermis (9, 10), although the relationship between compartments and the precise specification of pattern elements is not clear. We have studied the molecular basis of early disc development by using monoclonal antibodies to detect cell-surface antigens with heterogeneous distributions in larval discs before overt differentiation occurs. We previously described (5) the distribution of one such antigen, recognized by monoclonal antibody DK.1A4. Comparison of this antigen's distribution with a cell-lineage analysis of the mature wing imaginal disc (which makes the adult wing and associated mesothorax) showed that expression of the antigen correlated with the dorso-ventral lineage restriction (the border between dorsal and ventral compartments) in the epithelium. It was also clear that the expression of the antigen was not tightly linked to the type of adult structure that a disc cell was specified to differentiate, but seemed rather to depend on ...

The integrin family of cell surface receptors is strongly conserved in higher animals, but the evolutionary history of integrins is obscure. We have identified and sequenced cDNAs encoding integrin ␤ subunits from a coral (phylum Cnidaria) and a sponge (Porifera), indicating that these proteins existed in the earliest stages of metazoan evolution. The coral ␤ Cn1 and, especially, the sponge ␤ Po1 sequences are the most divergent of the ''␤ 1 -class'' integrins and share a number of features not found in any other vertebrate or invertebrate integrins. Perhaps the greatest difference from other ␤ subunits is found in the third and fourth repeats of the cysteine-rich stalk, where the generally conserved spacings between cysteines are highly variable, but not similar, in ␤ Cn1 and ␤ Po1 . Alternatively spliced cDNAs, containing a stop codon about midway through the full-length translated sequence, were isolated from the sponge library. These cDNAs appear to define a boundary between functional domains, as they would encode a protein that includes the globular ligand-binding head but would be missing the stalk, transmembrane, and cytoplasmic domains. These and other sequence comparisons with vertebrate integrins are discussed with respect to models of integrin structure and function.