Production of avian leukosis virus particles in mammalian cells can be mediated by the interaction of the human immunodeficiency virus protein Rev and the Rev-responsive element.

In human immunodeficiency virus type 1-infected cells, the efficient expression of viral proteins from unspliced and singly spliced RNAs is dependent on two factors: the presence in the cell of the viral protein Rev and the presence in the viral RNA of the Rev-responsive element (RRE). We show here that the HIV-1 Rev/RRE system can increase the expression of avian leukosis virus (ALV) structural proteins in mammalian cells (D-17 canine osteosarcoma) and promote the release of mature ALV virions from these cells. In this system, the Rev/RRE interaction appears to facilitate the export of full-length unspliced ALV RNA from the nucleus to the cytoplasm, allowing increased production of the ALV structural proteins. Gag protein is produced in the cytoplasm of the ALV-transfected cells even in the absence of a Rev/RRE interaction. However, a functional Rev/RRE interaction increases the amount of Gag present intracellularly and, more strikingly, results in the release of mature ALV particles into the supernatant. RCAS virus containing an RRE is replication-competent in chicken embryo fibroblasts; however, we have been unable to determine whether the particles produced in D-17 cells are as infectious as the particles produced in chicken embryo fibroblasts.

The pre-S domain of the large viral envelope protein determines host range in avian hepatitis B viruses.

In addition to their well-recognized hepatotropism, all hepatitis B viruses (HBVs) display marked species specificity, growing poorly or not at all in species other than those closely related to their natural hosts. We have examined the molecular basis for this narrow host range, using duck HBV (DHBV) and heron HBV (HHBV) as a model system. HHBV virions will not infect ducks in vivo and infect cultured duck hepatocytes extremely inefficiently in vitro. Mutant HHBV genomes lacking all viral envelope proteins (HHBV env-) can be complemented in trans with DHBV envelope proteins; the resulting pseudotyped virions can efficiently infect duck hepatocytes. Further complementation analysis reveals that of the two viral surface proteins (L and S), it is the L protein that determines host range. Pseudotyping of HHBV env- with DHBV/HHBV chimeric envelope proteins reveals that replacement of as few as 69 amino acids of the pre-S domain of the HHBV L protein by their DHBV counterparts is sufficient to permit infection of duck hepatocytes. These studies indicate that the species-specificity of hepadnaviral infection is determined at the level of virus entry and is governed by the pre-S domain of the viral L protein.

Evidence from molecular systematics for decreased avian diversification in the pleistocene Epoch.

Pleistocene glaciations have been suggested as major events influencing speciation rates in vertebrates. Avian paleontological studies suggest that most extant species evolved in the Pleistocene Epoch and that species' durations decreased through the Pleistocene because of heightened speciation rates. Molecular systematic studies provide another data base for testing these predictions. In particular, rates of diversification can be determined from molecular phylogenetic trees. For example, an increasing rate of speciation (but constant extinction) requires shorter intervals between successive speciation events on a phylogenetic tree. Examination of the cumulative distribution of reconstructed speciation events in mtDNA phylogenies of 11 avian genera, however, reveals longer intervals between successive speciation events as the present time is approached, suggesting a decrease in net diversification rate through the Pleistocene Epoch. Thus, molecular systematic studies do not indicate a pulse of Pleistocene diversification in passerine birds but suggest, instead, that diversification rates were lower in the Pleistocene than for the preceding period. Documented habitat shifts likely led to the decreased rate of diversification, although from molecular evidence we cannot discern whether speciation rates decreased or extinction rates increased.