Slow evolution of transferrin and albumin in birds according to micro-complement fixation analysis

Rabbit antisera were prepared to purified ovotransferrin from chicken (order Galliformes) and red-winged blackbird (order Passeriformes) and to purified serum albumin from chicken and rhea (order Rheiformes). Quantitative microcomplement fixation was used to compare these proteins immunologically with those of representatives of all 27 orders of birds. The average interordinal immunological distances were 123 units for transferrin and 53 units for albumin. Extensive intraordinal comparisons of transferrin among 51 species within the order Galliformes and 33 species within the order Passeriformes were also carried out. Values ranging from 0-75 immunological distance units were found within each order. Rabbit antisera to purified alligator albumin were also prepared and shown to react with representatives of all 27 orders of birds, the average immunological distance being 166 units. When the data presented here are considered in relation to the fossil record of birds, it appears that transferrin and albumin have evolved more slowly in birds than in other vertebrates. If prevailing interpretations of the fossil record are correct, transferrin has evolved 2-4 times as fast in mammals and snakes as in birds, while serum albumin has evolved about 3 times as fast in mammals, iguanids, crocodilians, and frogs as in birds. Published immunological and sequence comparisons of lysozyme and cytochrome c are also consistent with a slower rate of evolution in birds than in other vertebrates. The implications of a general slowdown in the evolution of bird proteins are discussed.