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Vertebrate heart development requires spatiotemporal regulation of gene expression to specify cardiomyocytes, increase the cardiomyocyte population through proliferation, and to establish and maintain atrial and ventricular cardiac chamber identities. The evolutionarily conserved chromatin factor Gon4-like (Gon4l), encoded by the zebrafish ugly duckling (udu) locus, has previously been implicated in cell proliferation, cell survival, and specification of mesoderm-derived tissues including blood and somites, but its role in heart formation has not been studied. Here we report two distinct roles of Gon4l/Udu in heart development: regulation of cell proliferation and maintenance of ventricular identity. We show that zygotic loss of udu expression causes a significant reduction in cardiomyocyte number at one day post fertilization that becomes exacerbated during later development. We present evidence that the cardiomyocyte deficiency in udu mutants results from reduced cell proliferation, unlike hematopoietic deficiencies attributed to TP53-dependent apoptosis. We also demonstrate that expression of the G1/S-phase cell cycle regulator, cyclin E2 (ccne2), is reduced in udu mutant hearts, and that the Gon4l protein associates with regulatory regions of the ccne2 gene during early embryogenesis. Furthermore, udu mutant hearts exhibit a decrease in the proportion of ventricular cardiomyocytes compared to atrial cardiomyocytes, concomitant with progressive reduction of nkx2.5 expression. We further demonstrate that udu and nkx2.5 interact to maintain the proportion of ventricular cardiomyocytes during development. However, we find that ectopic expression of nkx2.5 is not sufficient to restore ventricular chamber identity suggesting that Gon4l regulates cardiac chamber patterning via multiple pathways. Together, our findings define a novel role for zygotically-expressed Gon4l in coordinating cardiomyocyte proliferation and chamber identity maintenance during cardiac development.

Anteroposterior axis extension during vertebrate gastrulation requires cell proliferation, embryonic patterning, and morphogenesis to be spatiotemporally coordinated, but the underlying genetic mechanisms remain poorly understood. Here we define a role for the conserved chromatin factor Gon4l, encoded byugly duckling (udu),in coordinating tissue patterning and axis extension during zebrafish gastrulation. Although identified as a recessive enhancer of short axis phenotypes in planar cell polarity (PCP) mutants, we found that Gon4l functions in a genetically independent, partially overlapping fashion with PCP signaling to regulate mediolateral cell polarity underlying axis extension in part by promoting notochord boundary formation. We identified direct genomic targets of Gon4l and found that it acts as both a positive and negative regulator of gene expression, including limiting expression of the cell-cell and cell-matrix adhesion molecules EpCAM and Integrinα3b. Excessepcamoritga3bin wild-type gastrulae phenocopied notochord boundary defects ofudumutants, while downregulation ofitga3bsuppressed them. By promoting formation of this anteroposteriorly aligned boundary and associated cell polarity, Gon4l cooperates with PCP signaling to coordinate morphogenesis with the anteroposterior embryonic axis.