Rhythms play an important role in the precise spatiotemporal regulation of biological processes during development and patterning of embryos. We here investigate the rhythmic contractions of the yolk during early development of the goldfish Carassius auratus . We quantify these contractions and record robust and persistent rhythmic yolk movements that are not seen in closely-related species (common carp Cyprinus carpio and zebrafish Danio rerio ). We report that yolk contractions are an intrinsic emergent property of the egg, i.e. goldfish eggs are independently irritable / excitable. These contractions do not require sperm entry / fertilization nor cell division, and they notably emerge at a precise time — suggesting that goldfish eggs are able to measure elapsed time from what we infer to be egg activation. We further show that these rhythmic contractions persist even in yolk in isolation. As the yolk itself is known to confer critical cues for early dorsoventral (DV) patterning of teleost embryos, we hypothesize that its contractions in goldfish may influence the patterning process of this species. Indeed, we find that embryos of the naturally more ventralized twin-tail goldfish strain Oranda display altered yolk contraction dynamics (i.e. faster contractions). We also present that the period of yolk contractions is independent of ChdS , a key gene involved in DV pattening and linked to the twin-tail phenotype, but is instead a trait that is maternal in origin. We aim to uncover whether the yolk contractions happening during early development of domesticated goldfish are the licensing process which permit the emergence of novel patterning phenotypes naturally-observed in this species (e.g. twin-tail and dorsal-finless strains) and which instead have not been found among closely-related species (e.g. common carp) whose yolks do not contract.
This manuscript is here published as a living communication (as described in Gnaiger (2021)). The authors intend to share findings when they are available, encourage feedback and discussion, and invite knowledge exchange and collaboration.