The embryonic vertebrate heart is composed of two major chambers, a ventricle and an atrium, each of which has a characteristic size, shape and functional capacity that contributes to efficient circulation. Chamber-specific gene expression programs are likely to regulate key aspects of chamber formation. Here, we demonstrate that epigenetic factors also have a significant influence on chamber morphogenesis. Specifically, we show that an atrium-specific contractility defect has a profound impact on ventricular development. We find that the zebrafish locus weak atrium encodes an atrium-specific myosin heavy chain that is required for atrial myofibrillar organization and contraction. Despite their atrial defects, weak atrium mutants can maintain circulation through ventricular contraction. However, the weak atrium mutant ventricle becomes unusually compact,exhibiting a thickened myocardial wall, a narrow lumen and changes in myocardial gene expression. As weak atrium/atrial myosin heavy chainis expressed only in the atrium, the ventricular phenotypes in weak atrium mutants represent a secondary response to atrial dysfunction. Thus, not only is cardiac form essential for cardiac function, but there also exists a reciprocal relationship in which function can influence form. These findings are relevant to our understanding of congenital defects in cardiac chamber morphogenesis.

Precise temporal and spatial control of transcription is a fundamental component of embryonic development. Regulation of transcription elongation can act as a rate-limiting step during mRNA synthesis. The mechanisms of stimulation and repression of transcription elongation during development are not yet understood. We have identified a class of zebrafish mutations (pandora, sk8 and s30) that cause multiple developmental defects, including discrete problems with pigmentation, tail outgrowth, ear formation and cardiac differentiation. We demonstrate that the pandora gene encodes a protein similar to Spt6, a proposed transcription elongation factor. Additionally, the sk8 and s30 mutations are null alleles of the foggy/spt5 locus, which encodes another transcription elongation factor. Through real-time RT-PCR analysis, we demonstrate that Spt6 and Spt5 are both required for efficient kinetics of hsp70 transcription in vivo. Altogether, our results suggest that Spt6 and Spt5 play essential roles of comparable importance for promoting transcription during embryogenesis. This study provides the first genetic evidence for parallel functions of Spt6 and Spt5 in metazoans and establishes a system for the future analysis of transcription elongation during development. Supplemental figure available on-line