Major morphological effects of a regulatory gene: Pgm1-t in rainbow trout

We have investigated the morphological effects of a genetic locus, Pgm1- t,that affects the expression of a phosphoglucomutase locus (Pgm1) in liverof rainbow trout (Salmo gairdneri). We have previously shown that embryoswith liver Pgm1 expression hatch earlier than those without liver Pgm1expression. We predicted that this difference in developmental rate shouldcause a reduction in meristic counts in the more rapidly developing fishwith liver Pgm1 expression. Eight meristic (countable) characters in ninefull-sib groups segregating for the presence or absence of liver Pgm1expression are in agreement with this prediction. In eight of the ninefamilies, there is a significant difference in the multivariatedistribution of the eight meristic counts between full sibs with andwithout liver Pgm1 expression. This separation in multivariate space isbased on a tendency for lower meristic counts in fish with liver Pgm1expression. The magnitude of these morphological differences is similar tothat between two subspecies of cutthroat trout (Salmo clarki) that showsubstantial genetic divergence at structural loci encoding enzymes (Nei's D= 0.34). These data support the view that small changes in thedevelopmental process caused by genetic differences at regulatory genes canhave large effects on morphology.