Studies using medaka revealed that zic1/zic4 in the dorsal somites regulate the dorsal- specific external morphology of the trunk region. Importantly, establishment of a transgenic medaka line that reproduces the expression patterns of zic1/zic4 revealed that zic1/zic4 are exclusively expressed in the dorsal part of somite deriva- tives with a sharp and straight boundary at the level of the notochord, suggesting that the trunk region of medaka is compartmentalized into dorsal and ventral domains (Fig. 8.7) (Kawanishi et al. 2013). The dorsal domain defined by the
Dorsal-specific pigmentation patterns, body shape, fin shape
ventral module dorsal module
zic1/zic4 = selector genes NC
Fig. 8.7 A model for the modular organization of the medaka trunk region. The somite is compart- mentalized into dorsal and ventral domains with a sharp and straight ventral expression boundary of zic1/zic4 at the level of the notochord (NC). The dorsal domain could be a developmental mod- ule, and zic1/zic4 could serve as selector genes, regulating the global morphology of the dorsal trunk such as pigmentation patterns, body shape, and fin shape
155
zic1/zic4 expression could be a developmental module—a quasi-independent devel- opmental unit—because the loss of zic1/zic4 activity does not affect the ventral part of the trunk. Since zic1/zic4 globally determine the fates of various organs on the dorsal side, they serve as selector genes in the dorsal module. This function of zic1/
zic4 should be conserved among teleosts and potentially among vertebrates.
The modular organization of the animal body could promote diversification in morphology during evolution; one module can adopt a novel phenotype without affecting the others (Wagner et al. 2007). In general, vertebrates exhibit a variety of color patterns and structures on the dorsal side, whereas those on the ventral side are relatively conserved. This could be achieved through the modular organization and recruitment of various target genes of the selector gene, zic1/zic4, during adaptation to ever-changing environmental conditions (Kawanishi et al. 2013).
The specific functions of zic1/zic4 in the dorsal module are still largely unknown.
Roughly, there could be two functions: regulation of somite-derived tissue morphol- ogy and regulation of ectodermal tissue patterning mediated by signaling molecules produced by somite cells. The search for factors downstream of zic1/zic4 would lead to further understanding of morphogenesis in the vertebrate trunk region and human developmental defects such as spina bifida.
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