Virus survival through inclusion of AMGs is governed by the laws of natural selection and has been made highly selective through co-evolution with their hosts. As such, the AMGs that confer a fitness advantage to the virus's ability to infect a host and reproduce will be more abundant. AMG abundance is largely dictated by the lifestyle of the virus, environmental conditions surrounding it, and host characteristics.
Lytic viruses also encompass a more diverse set of AMGs than
lysogenic viruses, in part due to their larger host range and higher infection frequency.
Temperate viruses, on the other hand, may employ AMGs to improve host fitness and virulence due to their often longer lifespan in the cell as a
prophage. Gene density in these viruses is higher when compared to their
lytic counterparts. Higher rates of HGT in
lysogenic viruses allows for more AMG transfer but also lowers overall gene diversity.
Environmental conditions Pathways utilizing nutrients found in low concentrations in the local environment are generally found in higher abundance in the virus. In
marine environments, AMGs can confer fitness advantages for both host and viruses under relatively nutrient-limited conditions compared to sediment and strong ultraviolet stress of water. In sedimentary environments, carbon and sulfur metabolism AMGs are typically more prevalent to outcompete other organisms for the abundant resources.
Host factors A virus's host range determines which host it can acquire AMGs from. Additionally, the abundance of a host surrounding a virus will affect its likelihood to acquire genes from the host. Virus populations increasingly occupy lytic lifestyles as bacterial production increases. As such, many AMGs characterized have been discovered in
phages of these host systems. == Identification ==