Creation of diversity in the animal virus world by inter-species and intra-species recombinations: lessons learned from poultry viruses

The biological diversity within viruses is one of the largest found in all other forms of nature. Many mechanisms contribute to virus diversity and include incorporating genetic material from the host, recombination between viruses belonging to the same or to a different family, and even recombination between viruses normally infecting different hosts. In particular, avian viruses can utilize all three of these mechanisms to generate new viruses. It is well documented that recombinations can occur between Marek's disease virus (MDV), an oncogenic herpesvirus, fowlpox virus (FPV), and various avian retroviruses. In addition, chicken infectious anemia virus (CIAV), a circovirus, was created by several inter-family recombination events, which occurred between plant and animal viruses. The circovirus represents the ancestral creation of a recombination between a plant DNA virus (nanovirus) and a mammalian RNA virus (calicivirus), through a transition of RNA to DNA made by an endogenous mammalian retrovirus. The present review will discuss the current knowledge on recombination events that have occurred between avian herpesviruses and retroviruses following dual infections in vitro and in vivo. In addition, we will discuss recombinations between fowlpox viruses and the avian retrovirus reticuloendotheliosis (REV). Finally, the review will address the creation of CIAV and how it evolved from recombinations between a plant virus and an animal virus.