Viral Genetics and Evolution

Viral Genetics and Evolution
In nature,viruses undergo an infinitely long series of replication cycles as they are transmitted from host to host.During this process,spontaneous mutants are continually generated,some of which will have different biological properties than the parent virus from which they arise.The in vivo environment brings pressures to bear which favor the selection of certain of these biological variants,primarily because of their preferential ability to be transmitted serially.Properties important in the survival and evolutionary progression of various viruses in nature include:
1.The capacity to replicate rapidly.In many instances,the most virulent strains of a virus replicate faster than more temperate strains.However,if replication is too rapid,it can be self-defeating－extremely rapid viral growth may not allow time enough for transmission before the host is removed by death or severe illness.
2.The capacity to replicate to high liter.A very high vertebrate host viremia titer is employed as a survival mechanism by arthropod-borne viruses to favor infection of the next arthropod.The same viruses produce very high titers in the salivary glands of their arthropod hosts so as to favor infection of the next vertebrate host.Such very high virus liters can be associated with silent infections in some natural vertebrate hosts (e.g.,reservoir avian hosts),but in vertebrate hosts the evolution of this capacity is most often associated with severe,even fatal,illness,
3.The capacity to replicate in certain key tissues.This quality is often important for the completion of the viral transmission cycle.For example,the evolution of viral tropisms and the employment of specific host cell receptors define many disease patterns.Further,the evolution of the capacity to grow in immunologically sequestered sites (such as brain or kidney) or in cells of the immune system itself provides great survival advantage.

病毒的遗传和进化
在自然界中,病毒在从宿主到宿主的转移过程中进行着一系列无限的复制循环.在这个过程中,会持续不断地产生自然突变,其中一部分突变会产生与该病毒分裂而来的亲本病毒不同的生物特性.病毒的内环境会对某些生物变种进行特定选择施加压力,主要是因为它们更易于被连续传播的特点.自然界各种病毒存活及进化过程的重要特质包括：
1,快速复制的能力.在许多情况下,病毒中毒性最大的品系复制的比那些较温和的品系要快.然而,如果复制太快,也会弄巧成拙——过快的病毒增长会导致在宿主死亡或染上重病之前没有足够的时间让病毒转移（到新宿主）.
2,复制达到高浓度的能力.病毒血症滴度非常高的脊椎动物宿主被作为通过节肢动物传播的病毒的存活机制,去促成下一个节肢动物的感染.相同的病毒在它们的节肢动物宿主的唾液腺中达到非常高的滴度从而促成下一个脊椎动物宿主的感染.这样高浓度的病毒可以悄无声息地感染一些自然界中的脊椎动物宿主（例如,水库中的禽类宿主）,但是这种能力的演化往往对脊椎动物宿主影响非常严重,甚至是致命的.
3,在某些重要器官组织中复制的能力.这项特质往往在病毒的传播周期竞争中占重要地位.例如,病毒向性的演化和具体被（病毒）占据的宿主细胞受体决定了许多疾病的类型.进一步讲,（病毒）在免疫隔离区（例如大脑或肾脏）或免疫细胞内生长能力的进步为它们自身提供了更好的生存优势.