Conservation of zebrafish epo and epor expression. Detection of transcripts for epo and epor during ontogeny (A) and in select adult tissues (B,C) by quantitative RT-PCR. Expression of epo peaks slightly at 36 hpf, followed by an increase between 4 and 8 dpf. A total of 2 waves of epor expression are detected, 1 peak at 24 hpf, and another between 4 and 8 dpf. Expression of epo is induced in the hypochromic mutant weh, with respect to wild-type siblings (D) as well as in adult hypoxia-treated hearts, with respect to untreated controls (E). The average transcript abundance is represented by the bar graphs with standard deviations. (F) Spatiotemporal expression of epor was evaluated during embryonic development by whole-mount in situ hybridization. Transcripts were detected in the ICM from 18 somites until the onset of circulation at 24 hpf. Circulating erythroid cells maintain expression of epor until 32 hpf. At 4 dpf, epor is weakly expressed in the heart (*), and by 14 dpf, epor is localized to the heart (*) and is strongly expressed in the head kidney (▾). Lateral views, anterior to the left, a dorsal view of the 14-dpf embryo is also shown.