Dodecaploid Xenopus longipes provides insight into the emergence of size scaling relationships during development

SUMMARY Genome and cell size are strongly correlated across species 1–6 and influence physiological traits like developmental rate 7–12 . Although size scaling features such as the nuclear-cytoplasmic (N/C) ratio are precisely maintained in adult tissues 13 , it is unclear when during embryonic development size scaling relationships are established. Frogs of the genus Xenopus provide a model to investigate this question, since 29 extant Xenopus species vary in ploidy from 2 to 12 copies (n) of the genome, ranging from 20 to 108 chromosomes 14,15 . The most widely studied species, X. laevis (4n=36) and X. tropicalis (2n=20), scale at all levels from body size to cellular and subcellular 16 . Paradoxically, the rare, critically endangered dodecaploid (2n=108) X. longipes is a small frog 15,17 . We observed that despite some morphological differences, X. longipes and X. laevis embryogenesis occurred with similar timing, with genome to cell size scaling emerging at the swimming tadpole stage. Across the three species, cell size was determined primarily by egg size, while nuclear size correlated with genome size during embryogenesis, resulting in different nuclear-cytoplasmic ratios at the mid-blastula transition. At the subcellular level, nuclear size correlated more strongly with genome size, whereas mitotic spindle size scaled with cell size. Our cross-species study indicates that scaling of cell size to ploidy is not due to abrupt changes in cell division timing, that different size scaling regimes occur during embryogenesis, and that the developmental program of Xenopus is remarkably consistent across a wide range of genome and egg sizes. ONE SENTENCE SUMMARY Comparison of size metrics in embryos from different ploidy Xenopus species, including the dodecaploid X. longipes , reveals distinct size scaling regimes during development.