**6. Conclusion**

Myelopoiesis is an orchestration of a multitude of growth factors and transcription factors that control cell fate decisions and differentiation along a chosen cell lineage. It is evident that there exists some functional redundancy in the action of myelopoietic growth factors, most likely put in place to ensure the production of these critical innate immune cells. Stud‐ ies have focused on examining the regulation of myelopoiesis in the mouse model system, and have only just begun in the teleost model system.

The divergence of teleosts and mammals occurred approximately 400-450 Mya, thus teleosts represent one of the most basal groups of vertebrates [326]. Comparison of soluble factors and their receptors in teleosts and mammals show retention of many of important hemato‐ poietic growth factors and receptors, including PDGFR [250, 251, 327], c-KIT [128-130], FLT3 (accession number DQ317446), CSF-1R [20, 250-254], GCSFR [292, 293], and their ligands PDGF [328], KIT ligand [128, 130, 131], CSF-1 [246, 249], and GCSF [290-292], although FLK2, the ligand to FLT3, has not yet been reported. However, it appears that teleosts do not possess the key myeloid growth factors IL-3 and GM-CSF, and their cognate receptors. In addition, teleosts possess all of the TF families required for hematopoiesis in higher verte‐ brates, reviewed in [28, 30]. Based on studies performed to date, the regulation of hemato‐ poiesis is largely similar between mammals and teleosts. However, teleosts often possess a number of gene duplications for many of the soluble factors, receptors, and to some extent, transcription factors as a result of a teleost-specific whole genome duplication predicted to have occurred approximately 350 Mya, and is believed to be responsible for the radiation of teleosts [329, 330]. Many of these teleost genes are rapidly evolving, often undergoing subfunctionalization or neo-functionalization making the identification of teleost orthologues difficult. By developing an understanding of the soluble mediators, receptors, and the intra‐ cellular machinery that govern teleost myelopoiesis, we may be better equipped to develop strategies to promote host defense against pathogens, particularly in aquaculture in which fish are predisposed to infection.
