A noninvasive photoactivatable split-Cre recombinase system for genome engineering in zebrafish.

The cyclic recombinase (Cre)/loxP recombination system is a powerful technique for in vivo cell labeling and tracking. However, achieving high spatiotemporal precision in cell tracking using this system is challenging due to the requirement for reliable tissue-specific promoters. In contrast, light-inducible systems offer superior regional confinement, tunability, and non-invasiveness compared to conventional lineage-tracing methods. Here, we took advantage of the unique strengths of the zebrafish to develop an easy-to-use highly efficient, genetically encoded, magnets-based, light-inducible transgenic Cre/loxP system. We demonstrate that our system does not exhibit phototoxicity or leakiness in the dark, and it enables efficient and robust Cre/loxP recombination in various tissues and cell types at different developmental stages through noninvasive illumination with blue light. Our newly developed tool is expected to open novel opportunities for light-controlled tracking of cell fate and migration in vivo.

Definitive hematopoiesis is dispensable to sustain erythrocytes and macrophages during zebrafish ontogeny.

In all organisms studied, from flies to humans, blood cells emerge in several sequential waves and from distinct hematopoietic origins. However, the relative contribution of these ontogenetically distinct hematopoietic waves to embryonic blood lineages and to tissue regeneration during development is yet elusive. Here, using a lineage-specific "switch and trace" strategy in the zebrafish embryo, we report that the definitive hematopoietic progeny barely contributes to erythrocytes and macrophages during early development. Lineage tracing further shows that ontogenetically distinct macrophages exhibit differential recruitment to the site of injury based on the developmental stage of the organism. We further demonstrate that primitive macrophages can solely maintain tissue regeneration during early larval developmental stages after selective ablation of definitive macrophages. Our findings highlight that the sequential emergence of hematopoietic waves in embryos ensures the abundance of blood cells required for tissue homeostasis and integrity during development.