ONE GENE ENCODES THE HEAVY-CHAINS FOR 3 DIFFERENT FORMS OF IGY IN THEDUCK

IgY, the major Ab of the duck (Anas platyrhynchos), exists in two secr eted forms and a transmembrane (TM) form. To investigate the genetic r elationships of the multiple IgY we cloned the gene encoding the IgY ( upsilon) heavy chains. The heavy chain of the smaller (5.7S) IgY, whic h lacks the third and fourth constant domains, results from the use of a unique terminal exon found in the intron between the second and thi rd C region exons. Alternate pre-mRNA processing pathways also produce a full-length v-chain and a TM form, each having four C region domain s. Although the number of secretory exons and the inferred positions o f intramolecular disulfide bonds indicate structural similarity betwee n IgY and IgE, the TM exons of duck IgY share high sequence identity a nd a similar pattern of RNA processing with those of IgG. These result s suggest that Ige and IgE may have diverged from an ancestral molecul e resembling IgY.