Fig. 4. Mapping and molecular characterization of tendrils mutations. (A) Meiotic and deficiency mapping of tendrils^13-8. Initial mapping placed tendrils^13-8 between roughoid (ru) and hairy (h) on chromosome 3 (top line). Markers used for mapping, FRT sites (triangles) and centromere (closed circle) are indicated. Second line shows ru-h interval with SNP markers used for mapping. Number of recombinant breakpoints in each SNP interval (out of 20 ru-h recombinants from first round of mapping) is shown below the line; arrows show direction of tendrils^13-8 relative to SNP markers. Third line shows 3L075-h interval along with the combined results of first and second rounds of mapping (235 ru-h recombinants). The structure of three deficiency chromosomes and complementation results with tendrils^13-8 are shown below the line. Gaps indicate minimal sizes of the deficiencies from published cytology. Fourth line shows genes (black boxes) in region between Df(3L)BSC13 proximal breakpoint and 3L078A. tendrils^13-8 failed to complement rhea¹ (gold box), which encodes talin. (B) Changes in talin-coding sequence in tendrils alleles. tendrils^13-8 alters a 3' splice site in the fourth intron (see C) that results in mis-splicing, which changes codon D268 and beyond (see panel C). Putative head and rod regions, FERM- and F-actin-binding domain (hatched rectangle), and F-actin-binding domains (stippled rectangles) are indicated (Brown et al., 2002). (C) Sequences of fourth intron splice junctions of talin gene in wild-type (tendrils⁺) and tendrils^13-8 mutant. tendrils^13-8 has a G>A mutation (asterisk) in the 3' splice site that leads to use of a cryptic 3' splice site seven nucleotides downstream, as determined by RT-PCR analysis of tendrils^13-8 RNA. (D) Diagram of talin dimer linking cell-surface integrin to actin cytoskeleton. Talin head binds to cytoplasmic domain of ß-integrin subunit; rod domain contains actin binding sites. Modified, with permission, from Calderwood and Ginsberg (Calderwood and Ginsberg, 2003). (E) Model of talin function in tracheal terminal cells. Talin associates with a ß-integrin at the basal surface of the terminal branches and stabilizes binding to the muscle. We speculate that talin also associates with the cytoskeleton and stabilizes lumen (Lu) position in the cell.