Every organism follows its own species-specific pace of development – fertilisation to birth takes 2-3 weeks in mouse, but 9 months in human. How is this timing defined and how is it regulated such that, within in individual, paired organs such as the kidneys or eyes develop at the same pace despite being physically separated? Jana Fuhrmann, Lazaro Centanin and colleagues here use transplantations between zebrafish and medaka – fish species separated by 250 million years of evolution – to explore this question, developing a system in which ectopic retinas formed exclusively from one species develop in a host of the other. They find that the ectopic retina develops according to the timing of the donor species – demonstrating an organ-intrinsic mechanism for the pace of development. However, there are interactions between host and donor tissues: ectopic retinas always develop close to the host retina, suggesting the donor cells may be responding to positional information in the host, and the donor neurons can project axons into the host optic tectum, implying they may recognise pathfinding cues. Intriguingly, an ectopic zebrafish embryo can induce medaka ectoderm to differentiate as lens, but the converse is not true – highlighting a potential temporal window of competence for lens differentiation. This system sets the stage for a more in-depth examination of both autonomous and non-intrinsic mechanisms regulating the timing of organ development.
