Uniformity in the nonsynonymous substitution rates of embryonic beta-globin genes of several vertebrate species.

The nucleotide substitution rate in structural portions of the embryonic beta-globin genes of placental mammals is lower than that for the adult beta-globin genes. This difference occurs entirely within the class of substitutions that result in nonsynonymous (replacement) differences between these genes, and therefore represents a constraint on the structure of the mammalian embryonic beta-globin proteins relative to the adult proteins (Shapiro et al. 1983; Hardison 1984). A similar effect has also been observed in marsupial mammals (Koop and Goodman 1988). In an effort to determine whether the observed rates are evidence of a uniform degree of selective constraint on the embryonic beta-globin genes, analyses were performed that compared replacement substitution rates. The analyses reveal that embryonic beta-globin genes appear to have been fixing replacement substitutions at nearly the same average rate not only in placental and marsupial mammals but in avian and amphibian species as well. In contrast, the adult beta-globin genes from these organisms appear to have a more variable rate of replacement substitution with an especially low rate for birds. In the chicken (Gallus gallus), the adult beta-globin gene replacement substitution rate appears to be lower than the embryonic replacement substitution rate.