Duplicate mitf Genes in Zebrafish: Complementary Expression and Conservation of Melanogenic Potential

Abstract

Mutations in the zebrafish nacre/mitfa gene, expressed in all embryonic melanogenic cells, perturb only neural crest melanocytes, suggesting redundancy of mitfa with another gene in the zebrafish retinal pigment epithelium (RPE). Here, we describe a second zebrafish mitf gene, mitfb, which may fulfill this role. The proteins encoded by the two zebrafish mitf genes appear homologous to distinct isoforms generated by alternately spliced mRNAs of the single mammalian Mitf gene, suggesting specialization of the two zebrafish genes following a duplication event. Consistent with this hypothesis, expression of mitfa and mitfb is partially overlapping. mitfb is coexpressed with mitfa in the RPE at an appropriate time to compensate for loss of mitfa function in the nacre mutant but is not expressed in neural crest melanoblasts. Additionally, mitfb is expressed in the epiphysis and olfactory bulb where mitfa is not, and where Mitf expression has not previously been reported in other species. mitfb, but not a zebrafish ortholog of the closely related gene tfe3, can rescue neural crest melanophore development in nacre/mitfa mutant embryos when expressed via the mitfa promoter. These data suggest that mitfa and mitfb together may recapitulate the expression and functions of a single ancestral Mitf gene, and that mitfb may serve additional novel functions.

Genome evolution and biodiversity in teleost fish

“…In this case, both copies are preserved, since the presence of both is necessary to perform the original function of the ancestral single-copy gene. In teleosts, the evolution of numerous gene duplicates is consistent with the subfunctionalization model (eg Lister et al, 2001;Serluca et al, 2001;Altschmied et al, 2002;McClintock et al, 2002;Cresko et al, 2003;Yu et al, 2003;Amores et al, 2004). For example, mammals and birds have a unique microphthalmia-associated transcription factor gene mitf, from which different isoforms are expressed through the use of different promoter sequences and alternative exons.…”

“…Hence, the two mitf genes are required together to perform the functions of the unique mitf gene in mammals and birds. This partitioning of functions is associated with the degeneration of isoform-specific exons and regulatory sequences (Lister et al, 2001;Altschmied et al, 2002).…”

“…In the rare examples like mitfa/mitfb or sox9a/sox9b, for which gene duplicate expression and function have been examined in divergent fish species, the major partitioning of ancestral gene functions appears to be ancient (Lister et al, 2001;Altschmied et al, 2002;Cresko et al, 2003). This is manifested by paralog-specific subfunctions conserved in divergent fishes.…”

Zebrafish: A model system for the study of eye genetics

“…Twenty-two hours post fertilization, invagination of the central portions of this eye-mass and formation of the optic lumen contribute to the formation of an optic cup (Table I; Dowling, 1994, 1999)). The inner layer continues to proliferate and produces the neural retina, whereas the outer layer gives rise to the retinal pigment epithelium (RPE), likely through the action of mitf expression (Lister et al, 2001). The positioning of the optic stalk is regulated by the expression of the Pax2 and Pax6 genes (Macdonald et al, 1995).…”

Duplication and divergence of fgf8 functions in teleost development and evolution

“…In this context, nonfunctionalization, subfunction partitioning, and neofunctionalization, occurring independently in separate lineages, could lead to different functions, differential subfunction partitioning, relaxation of pleiotropic constraints, and speciation (Lynch and Force, 2000;Taylor et al, 2001;Postlethwait et al, 2004;Force et al, 2004Force et al, , 2005. Empirical studies have shown great support for the molecular mechanisms of the subfunctionalization model (Lister et al, 2001;Altschmied et al, 2002;Cresko et al, 2003;Liu et al, 2005).…”