Cxcr1 acts stem cell nonautonomously in parabiotic zebrafish. (a–e) Uninjected kdrl:GFP embryos were fused to kdrl:GFP embryos injected with DNA encoding hsp70l:cxcr1 or empty vector (Control), and gene expression was induced at 36 and 48 hpf. Fluorescent blue dextran was used to mark the injected halves of each pair. (a-d) Low magnification views showing a representative parabiotic from this experiment in transmitted light (a), green channel (b), blue channel (c), and green/blue overlay. Bars, 500 µm. (e) The fold change in CHT volume (injected:uninjected) is plotted for Control and CXCR1 groups (P = 0.012, Student’s t test; n = 4 for Control; n = 10 for CXCR1). The experiment was repeated twice with similar results; combined results are shown. (f–i) In these parabiotic animals, donor halves are Runx1:mCherry transgenics and recipient halves are casper injected with DNA encoding hsp70l:GFP or hsp70l:cxcr1 followed by heat shock induction at 36 and 48 hpf. (f–h) Representative transmitted light and fluorescence images of these parabiotics. The CHT of the donor and recipient animal is boxed in red and shown in fluorescence in panels g and h. Bars, 100 µm. (i) Expression of cxcr1 in the recipient niche favored HSPC engraftment there over colonization of the donor autologous niche (red circles, P = 0.019, paired Student’s t test). There was no difference between donors and recipients in the GFP group (green circles). Overall HSPC numbers were also increased in donors and recipients by recipient expression of cxcr1 (donor GFP vs. CXCR1, P = 0.045, Student’s t test; recipient GFP vs. CXCR1, P = 0.007, Student’s t test). n = 6 for GFP control; n = 5 for CXCR1. The experiment was repeated twice with similar results; combined results are shown.