3.4. Distinct tissue expression patterns of six SS genes in zebrafish
All six SS genes were functionally expressed in the zebrafish
tissues and exhibited different expression patterns (Fig. 7). The SS1,
SS2 and SS3 were mainly expressed in brain regions, e.g., the
telencephalon and hypothalamus, whereas the SS4 was mainly
expressed in peripheral tissues. SS5 was highly expressed in the
ovary with slightly lower levels in the optic tectum thalamus and
hind brain. SS6 was widely expressed in the central and peripheral
tissues.
Gene duplication is believed to be a primary source for the
evolution of novel functions (Ohno 1970; Zhang 2003). Functional
divergence between duplicates is required for the retention of the
duplicates in the genome (He and Zhang, 2005). The different
expression patterns of SS genes might indicate that each SS gene
has acquired different functions in certain tissues. After duplications,
the daughter genes are under relaxed constraint (Brunet et al., 2006)
and mutations in the regulatory regions of the genes might be
responsible for the observed distinct expression patterns of SS genes.
Furthermore, further studies are needed to investigate the physiological
function of each SS gene.