GENETIC VARIATION AND DIFFERENTIATION OF PROTEINS IN MAMMALS AND BIRDS: NEW AND REFINED ESTIMATES

The amount and distribution of genetic variation within and between species of mammals and birds was addressed from two perspectives: the amount of variation detected by conventional gel electrophoresis, and the bias of the proteins assayed by these techniques.

Geographic patterns of "hidden" genetic variation were examined in the deer mouse, Peromyscus maniculatus, a species previously found by conventional electrophoretic methodologies to be remarkably homogeneous. This biochemical uniformity was particularly surprising given the marked heterogeneity in morphology, ecology and karyotype observed across this species range. Of particular interest was whether any hidden variation existed within the two common electromorphs of glutamate oxatate transaminase-1 observed by conventional electrophoretic procedures to exist as basically a two-allele polymorphism throughout the North American range of P. maniculatus. Essentially no additional variation could be resolved within these electromorphs. Similarly, no scoreable hidden variation was detected for the essentially monomorphic malate dehydrogenase-1. In sharp contrast, however, remarkable amounts of hidden variation were uncovered for the previously highly polymorphic esterase-1. While it has not been established that all of the variation in esterase-1 is at the structural locus, the results of this study, taken in sum, suggest the geographic uniformity of the two-allele polymorphism at glutamate oxalate transminase-1 is incompatible with the predictions of selective neutrality.

Two-dimensional gel electrophoresis was used to analyze levels and patterns of genetic differentiation at an average of 189 abundant liver proteins between six species of wild mice representing levels of evolutionary divergence ranging from different subspecies to different families. The magnitude of protein divergence estimated was on the average only one-half that predicted by conventional one-dimensional electrophoretic procedures. Whether this discrepancy is due to differences in senstivities between the techniques or differences in the mean level of variation and divergence between the sets of proteins assayed by the two methods remains unresolved. The identical ranking of genetic distances by both techniques, however, does support the use of the simpler one-dimensional electrophoretic techniques to estimate relative levels of genetic divergence.

The conservative pattern of protein evolution observed among avian species by conventional starch gel electrophoresis was also investigated. Our ability to evaluate hypotheses to account for this pattern hinge critically upon the reality and generality of the low levels of differentiation observed. An examination of the North American thrushes and relatives for six protein loci, and representatives of 22 families in 10 avian orders for supernatant and mitochondrial malate dehydrogenase, using varied electrophoretic conditions suggests the conservatism is real. While new variation was unveiled, the total level of differentiation remained remarkably low when we consider the taxonomic diversity examined, particularly for the malate dehydrogenases for which representatives of nearly an entire class of vertebrates were examined. Similarly, an examination of abundent liver proteins by two-dimensional gel electrophoresis support the generality of low levels of protein divergence among birds relative to other vertebrates. Whether these low levels of protein divergence represent an actual deceleration of protein evolution or simply reflect a relatively recent age for most extant avian taxa remains unresolved.