**Acknowledgements**

Conversely, the over-expression of *irf8* mRNA in zebrafish resulted in an increase in macrophages by approximately 50% and a decrease in neutrophil numbers by about 40% [322]. These data are similar to those of the mammalian system and suggest a conserved role for IRF8 in determining macrophage over neutrophil cell lineage during primitive myelopoiesis. However, whether IRF8 plays the same role during definitive myelopoiesis

In addition to the previously described role of MAFB in HSCs and CMPs (see section 4.3.1), *MAFB* is highly expressed in monocytes and macrophages [164, 323] and has been shown to induce differentiation of myeloblasts into monocytes and macrophages [163, 307, 324, 325]. Furthermore, MAFB and c-MAF double knockout mice displayed differen‐ tiated macrophages that were capable of proliferating in response to CSF-1 in semi-solid and liquid culture [325]. Therefore, it appears that *MAFB* expression is sustained in mon‐ ocytes and macrophages in order to prevent proliferation in these terminally differentiat‐

Studies examining the role of MAFB in teleost myelopoiesis are limited. In the goldfish PKM system, a *mafb* transcript was identified and showed increasing mRNA levels with macro‐ phage development [19]. The increasing mRNA levels of *mafb* during macrophage differen‐ tiation are similar to what has been observed in mammalian systems and suggest that

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,

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

MAFB may play a role in teleost macrophage differentiation.

and have only just begun in the teleost model system.

in teleosts remains to be determined.

122 New Advances and Contributions to Fish Biology

*5.3.4. MafB*

ed cell populations.

**6. Conclusion**

This work was supported by a grant from Natural Sciences and Engineering Research Coun‐ cil of Canada (NSERC) to MB. BAK was supported by NSERC and Alberta Ingenuity Fund (AIF) doctoral scholarships, FK was supported by a Japan Society for the Promotion of Sci‐ ence (JSPS) research fellowship.
