One gene encodes the heavy chains for three different forms of IgY in the duck.

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