Pulsed field gel electrophoresis as a method for examining phylogenetic relationships between organisms; Its application to the genus Phytophthora

Abstract

Pulsed field gel electrophoresis separates chromosomal-sized pieces of DNA in agarose gels and enables karyotype, genome size and genetic maps to be established for organisms where conventional sexual genetics are difficult. This technique has been applied to two isolates of Phytophthora megasperma. In both isolates (numbers 53 and 63), nine chromosomal DNA bands ranging in size from about 1.4 to 4 million base pairs, were separated; the largest band probably consists of several larger DNAs which were unresolved under all conditions tested. Cytological studies by others have shown that isolate 53 has twice the chromosome complement of isolate 63. Since pulsed field gel electrophoresis indicates that chromosomal DNA in both isolates are identical in size distribution, it is likely the ploidy of this isolate is double that of 63.

These two isolates can be distinguished by restriction fragment length polymorphisms in the nuclear ribosomal DNA repeat unit. In both isolates the length of this repeat unit is 11 kilobase pairs.