Determination of Wolbachia genome size by pulsed-field gel electrophoresis

Genome sizes of six different Wolbachia strains from insect and nematode hosts have been determined by pulsed-field gel electrophoresis of purified DNA both before and after digestion with rare-cutting restriction endonucleases. Enzymes Sma I, Apa I, Asc I, and Fse I cleaved the studied Wolbachia strains at a small number of sites and were used for the determination of the genome sizes of w MelPop, w Mel, and w MelCS (each 1.36 Mb), w Ri (1.66 Mb), w Bma (1.1 Mb), and w Dim (0.95 Mb). The Wolbachia genomes studied were all much smaller than the genomes of free-living bacteria such as Escherichia coli (4.7 Mb), as is typical for obligate intracellular bacteria. There was considerable genome size variability among Wolbachia strains, especially between the more parasitic A group Wolbachia infections of insects and the mutualistic C and D group infections of nematodes. The studies described here found no evidence for extrachromosomal plasmid DNA in any of the strains examined. They also indicated that the Wolbachia genome is circular.