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

Genome and cell size are strongly correlated across species1-6 and influence physiological traits like devel-opmental 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 ancestral frog 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 levels.16 Paradoxically, the rare, critically endangered dodecaploid (12N = 108) Xenopus longipes (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, whereas nuclear size correlated with genome size during embryogen-esis, resulting in different N/C ratios in blastulae prior to gastrulation. At the subcellular level, nuclear size correlated more strongly with genome size, whereas mitotic spindle size scaled with cell size. Our cross -spe-cies 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.

Automated summary

Genome size and cell size in frogs are strongly correlated, and this correlation affects developmental rate. However, it is unclear how the relationship between cell size and ploidy is established during embryonic development. By studying different polyploid frogs, researchers discovered that cell size is primarily determined by egg size, while nuclear size is correlated with genome size. At the subcellular level, nuclear size is more strongly correlated with genome size, while mitotic spindle size scales with cell size. These findings demonstrate the relationship between cell size and ploidy in frogs and reveal different size scaling mechanisms during embryogenesis, suggesting that Xenopus development is consistent across a wide range of genome and egg sizes.