Fig. 1

Cardiac fusion defects in refuse-to-fuse (ref) mutants.

(A,B) Three-dimensional reconstructions depict wild-type (wt) and ref mutant hearts expressing the myocardial reporter transgene Tg(myl7:egfp) at 48 hpf. In contrast to the normal contours of the wt heart (A), the ref mutant heart (B) often displays a bifurcated, two-lobed ventricle and a misshapen atrium. See Figure 1—figure supplement 1 and Table 1 for more information on the range of cardiac phenotypes observed in ref mutants at 48 hpf. A: atrium; V: ventricle. (C–G) Dorsal (C–E) and ventral (F,G) views, anterior to the top, of wt (C,F) and ref (D,E,G) mutant embryos displaying the expression of myl7 at the 22 somite stage (22 s; C–E) and the localization of ZO-1 at the 18 somite stage (18 s; F,G). (C–E) In ref mutants at this stage, cardiomyocytes typically fail to fuse at the midline (D) or only fuse posteriorly (E). Note that the ref mutation is incompletely penetrant, although its penetrance is more evident at 20 s than at 48 hpf (Table 2), suggesting that some ref mutants recover as development proceeds. (F,G) ZO-1 localization highlights junctions forming within the maturing epithelium of the ALPM in both wt and ref mutant embryos. The ventral portion of the neural tube located at the midline is also visible. By 18 s, the wt ALPM (F) has initiated fusion at the midline, whereas the two sides of the ref mutant ALPM are still separate (G). Scale bars: 60 μm.