Distance matrices were computed from alignments with the distmat program in EMBOSS (http://bioweb2. pasteur.fr/docs/EMBOSS/embossdata.html) by using the Tamura distance correction. This method uses transition and transversion rates and takes into account the deviation of GC content from the expected value of 50%. Gap and ambiguous positions were ignored. Final values were then converted to similarity matrices by subtracting each value from 100. 