Different patterns of strand asymmetry have been documented in a variety of 
prokaryotic genomes as well as mitochondrial genomes. Because different 
replication mechanisms often lead to different patterns of strand asymmetry, 
much can be learned of replication mechanisms by examining strand asymmetry. 
Here I summarize the diverse patterns of strand asymmetry among different 
taxonomic groups to suggest that (1) the single-origin replication may not be 
universal among bacterial species as the endosymbionts Wigglesworthia 
glossinidia, Wolbachia species, cyanobacterium Synechocystis 6803 and Mycoplasma 
pulmonis genomes all exhibit strand asymmetry patterns consistent with the 
multiple origins of replication, (2) different replication origins in some 
archaeal genomes leave quite different patterns of strand asymmetry, suggesting 
that different replication origins in the same genome may be differentially 
used, (3) mitochondrial genomes from representative vertebrate species share one 
strand asymmetry pattern consistent with the strand-displacement replication 
documented in mammalian mtDNA, suggesting that the mtDNA replication mechanism 
in mammals may be shared among all vertebrate species, and (4) mitochondrial 
genomes from primitive forms of metazoans such as the sponge and hydra 
(representing Porifera and Cnidaria, respectively), as well as those from 
plants, have strand asymmetry patterns similar to single-origin or multi-origin 
replications observed in prokaryotes and are drastically different from 
mitochondrial genomes from other metazoans. This may explain why sponge and 
hydra mitochondrial genomes, as well as plant mitochondrial genomes, evolves 
much slower than those from other metazoans.