| Summary: | The Antarctic icefishes (Channichthyidae) are the only vertebrate taxon whose species do not produce red blood cells, thereby providing a natural mutant model to study the regulators of blood development and disease. To identify novel regulators of erythropoiesis, I compared RNA-Seq transcriptomes from red- and white-blooded notothenioids. I find that both icefishes and their sister taxon, the dragonfishes (Bathydraconidae), model beta-spectrin mutated spherocytic anemia. Icefishes appear to have evolved morph-biased changes in expression of hematopoietic regulatory genes, including down-regulation of the histone acetyltransferase p300 and overexpression of histone deacetylase 1b. In icefishes, I characterize a frameshift mutation that truncates the P300-binding domain of Hemogen, an important erythroid transcription factor. Tol2 and CRISPR/Cas9-generated transgenic zebrafish lines reveal that hemogen is expressed in hematopoietic, renal, neural, and reproductive tissues. I find that two conserved non-coding elements differentially contribute to hemogen expression in primitive and definitive waves of hematopoiesis. CRISPR-generated mutant zebrafish lines, which replicate the C-terminal mutation in icefish hemogen, show severe anemia and growth defects. Furthermore, I show that the function of zebrafish Hemogen is dependent on acidic residues within the TAD. Thus, Antarctic icefishes evolved an intricate system for repression of erythropoiesis that is caused in part by the loss of Hemogen function.
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