W corr ld c apochromat objective

Wholemount images were acquired with an Olympus DP70 camera on a Leica MZ16F dissecting microscope and confocal images were acquired in ZEN on a Zeiss LSM Exciter with a 40×/1.1 W Corr LD C-Apochromat objective for fixed specimens. For live time-lapse imaging with a Zeiss 20×/1.0 W Plan-Apochromat, fluorescent embryos were mounted in 0.8-1% low melting point agarose (LMPA) in embryo medium (EM) containing 160 mg/L tricaine as an anaesthetic (Westerfield, 2000 ) in a Petri dish and covered with EM. Z-spacing was generally set to optimal for 1 Airy unit. Tiff stacks were exported to Volocity 4.2-6.3 (Perkin Elmer) for further analysis and are shown as maximum intensity projections unless otherwise stated. EYFP and mCer persisted well during repeated scanning, whereas tdTomato faded faster.

Images were obtained at room temperature on a Zeiss microscope (Axiovert 200M) equipped with LD A‐plan 20×/0.85 ph1, 10×/0.30ph1 and 40×/.075ph1 objectives or on a Zeiss Exciter laser scanning microscope (LSM) with 40×/1.1 W Corr LD C‐Apochromat objective (Carl Zeiss Ltd (https://www.zeiss.co.uk/), Cambridge, United Kingdom). Images were quantified with image J software (NIH). Neonatal MyHC labeling was quantified with R software. Values are mean ± SEM with a Student's t test (paired or unpaired as appropriate) or ANOVA with a post‐hoc Tukey's test for >2 groups.