Influence of avian leukosis viral sequences on transmission to the egg and embryo.

Rous associated virus type-0 (RAV-0), a subgroup E replication-competent endogenous virus of chickens, is associated with a low efficiency of virus shedding into the egg albumen and failure to establish congenital transmission. In contrast, RAV-1, a subgroup A virus of exogenous origin, is efficiently shed into the albumen and readily infects the embryo. Among a series of in vitro constructed recombinants between RAV-0 and RAV-1, we have identified subgroup E recombinants that efficiently shed virus into the egg albumen but do not undergo efficient congenital transmission. The LTR region, subgroup-determining sequences in env, and sequences within a 375 bp Sacl-Xhol fragment at the 5' end of the genome each influenced the efficiency of virus shedding into the albumen. Egg inoculations with viruses differing only in env were used to confirm the low rate of congenital transmission of subgroup E viruses. These studies revealed that subgroup A envelope antigens are at least 100-fold more effective for the establishment of embryonic infection than subgroup E.