Muller's ratchet, the inevitable accumulation of deleterious mutations in 
asexual populations, has been proposed as a major factor in genome degradation 
of obligate symbiont organisms. Essentially, if left unchecked the ratchet will 
with certainty cause extinction due to the ever increasing mutational load. This 
paper examines the evolutionary fate of insect symbionts, using mathematical 
modelling to simulate the accumulation of deleterious mutations. We investigate 
the effects of a hierarchical two level population structure. Since each host 
contains its own subpopulation of symbionts, there will be a large number of 
small symbiont populations linked indirectly via selection on the host level. We 
show that although the separate subpopulations will accumulate deleterious 
mutations quickly, the symbiont population as a whole will be protected from 
extinction by selection acting on the hosts. As a consequence, the extent of 
genome degradation observed in present day symbionts is more likely to represent 
loss of functions that were (near-) neutral to the host, rather than a snap shot 
of a decline towards complete genetic collapse.