Viroids are plant subviral pathogens whose genomes are constituted by a 
single-stranded and covalently closed small RNA molecule that does not encode 
for any protein. Despite this genomic simplicity, they are able of inducing 
devastating symptoms in susceptible plants. Most of the 29 described viroid 
species fold into a rodlike or quasi-rodlike structure, whereas a few of them 
fold as branched structures. The shape of these RNA structures is perhaps one of 
the most characteristic properties of viroids and sometimes is considered their 
only phenotype. Here we use RNA thermodynamic secondary structure prediction 
algorithms to compare the mutational robustness of all viroid species. After 
characterizing the statistical properties of the distribution of mutational 
effects on structure stability and the wideness of neutral neighborhood for each 
viroid species, we show an evolutionary trend toward increased structural 
robustness during viroid radiation, giving support to the adaptive value of 
robustness. Differences in robustness among the 2 viroid families can be 
explained by the larger fragility of branched structures compared with the 
rodlike ones. We also show that genomic redundancy can contribute to the 
robustness of these simple RNA genomes.