Xenopus hybrids provide insight into cell and organism size control

Determining how size is controlled is a fundamental question in biology that is poorly understood at the organismal, cellular and subcellular levels. The Xenopus species, X. laevis and X. tropicalis differ in size at all three of these levels. Despite these differences, fertilization of X. laevis eggs with X. tropicalis sperm gives rise to viable hybrid animals that are intermediate in size. Hybrid and X. laevis embryos initially start from the same size egg and develop at the same rate, but hybrid animals were smaller by the tailbud stage, and reduced nuclear to cell size ratios were observed shortly after zygotic genome activation (ZGA), indicating that differential gene expression may contribute to size differences. Transcriptome analysis at the onset of ZGA identified twelve transcription factors paternally expressed in hybrids. A screen of these X. tropicalis factors by expression in X. laevis embryos revealed that Hes7 and Ventx2 were sufficient to significantly reduce X. laevis organismal size by the tailbud stage, although nuclear to cell size ratios were not significantly affected. Together, these results suggest that transcriptional regulation contributes to biological size control in Xenopus. Collect mRNA from whole embryos; three biological replicates were analyzed