The viroid: Biological oddity or evolutionary fossil?

Abstract

Publisher Summary This chapter discusses the possible evolutionary significance of viroids as well as speculates as to their place within the biological universe. Recognition of the fundamental disparity between viruses and viroids became possible only after the establishment of certain basic principles of virology and molecular biology. These principles helped create an intellectual milieu in which the existence of free nucleic acid pathogens could not a priori be ruled out. A few plant virologists and molecular biologists recognized the potential significance of the viroid, not only for plant pathology but also for cell and molecular biology and decided to become active in the field. This led to healthy competition and a substantial acceleration of research. Comparative examination of viroid sequences reveals that some variants may have originated as a result of point mutations or sequence duplications/deletions. Others apparently originated because of recombination phenomena among different viroids. As is the case with plant viruses, viroid entry into host cells is a passive process. The required penetration of the cell wall is achieved by either appropriate wounding or direct injection into cells by insects or other vectors.