/. Embryol. exp. Morph. Vol. 28, 3, pp. 481-489, 1972 481 Printed in Great Britain Development of protein polymorphisms in redwing blackbirds By ALAN H. BRUSH 1 AND ALAN F. SCOTT 2 From the Regulatory Biology Section, Biological Sciences Group, University of Connecticut SUMMARY Breeding populations of the redwing blackbird, Agelaius phoeniceus, were studied to com- pare protein differences during development. Proteins which represented a minimum of ten loci were studied in egg white, embryonic, nestling and adult tissue. In starch-gel electro- phoresis at several conditions of pH only ovoglobulin, ovotransferrin and general esterases were polymorphic. Tissue specificity was observed in other isozymes. These data suggest that developmental sequences of proteins may be considered an adaptative response of the organism. INTRODUCTION The existence of protein polymorphisms in avian tissues is now well established (Manwell & Baker, 1970; Karig & Wilson, 1971; Segre, Richmond & Wiley, 1970). However, the function of this variability remains poorly understood (Buettner-Janusch, 1970). A few cases of geographic protein polymorphism have been described in birds (Milne & Robertson, 1965; Brush, 1968), inferring selec- tive advantage in certain environments, and in some cases, preliminary genetic analyses have been attempted (for review, see Ferguson, 1971; Sibley et ah, in preparation). Few birds have been studied as thoroughly as the domestic chicken, where extensive polymorphisms have been described in egg white (Baker, 1968) and serum (Baker, Croizier, Stratil & Manwell, 1970). An aspect of protein variability in avian populations which has been relatively neglected is the sequence in which these multiple molecular systems develop. Although foetal hemoglobins have been described in some detail for several species (Manwell, Baker & Betz, 1966; Shaughnessy, 1970; Borgese & Bertles, 1965; Bush & Townsend, 1971), many questions still remain regarding even this relatively simple system (Wilt, 1967). Other, more comprehensive, develop- mental studies on serum and other proteins include those on Gallus (Manwell & Betz, 1966) and the house sparrow (Bush, 1967; Bush & Siebert, 1968). In both of these cases there were limitations on the numbers and types of proteins and tissues studied and the number of populations sampled. 1 Author's address: Department of Biochemistry, University of California, Berkeley, California 94720, U.S.A. 2 Author's address: Regulatory Biology Section, Biological Sciences Group, University of Connecticut, Storrs, Connecticut 06268, U.S.A. 31 E M B 28