Abstract
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. We observed that although hybrid and X. laevis embryogenesis initiates from the same sized zygote and proceeds synchronously through development, hybrid animals were smaller by the tailbud stage, and a change in the ratio of nuclear size to cell size was observed shortly after zygotic genome activation (ZGA), suggesting that differential gene expression contributes 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 significantly reduced X. laevis body length size by the tailbud stage, although nuclear to cell size scaling relationships were not affected as in the hybrid. Together, these results suggest that transcriptional regulation contributes to biological size control in Xenopus.
Highlights
Biological size control and scaling are important and fundamental features of living systems
Cross-fertilization of X. tropicalis eggs with X. laevis sperm produces hybrid embryos that die during zygotic genome activation (ZGA), the reverse cross of X. laevis eggs and X. tropicalis sperm results in viable hybrid embryos that possess genetic features of both X. laevis and X. tropicalis parents (Bürki, 1985; Lindsay et al, 2003; Narbonne et al, 2011; Elurbe et al, 2017; Gibeaux et al, 2018)
Development in the le × ts hybrid proceeded normally according to Nieuwkoop and Faber staging (Nieuwkoop and Faber, 1994) until the end of neurulation, and at a similar rate compared to wild type X. laevis embryos (Figure 1C, Supplementary Movie S1)
Summary
Biological size control and scaling are important and fundamental features of living systems. A number of factors involved in many different processes, such as Biological Scaling in Xenopus Hybrids growth, metabolism and protein synthesis, development, differentiation, and cell cycle regulation (Björklund et al, 2006) can influence cell size in a variety of organisms, from bacteria, to yeast, to Drosophila, to mammals (Marguerat and Bähler, 2012). Many of these genes are conserved and contribute to tissue and organ size in a variety of multicellular organisms, how they influence organism size, and how organism size feeds back to organ/tissue/cell size to attain homeostasis remains unclear
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