Comparison of Physical Maps of Ribosomal DNA Repeating Units in Pythium, Phytophthora and Apodachlya

SUMMARY: A satellite band in CsCl-bisbenzimide density gradients of Pythium diclinum DNA was found to consist of ribosomal DNA (rDNA) repeating units with a complexity of 10.3 kilobases (kb). Similar satellite bands were also obtained from a morphologically diverse selection of species from the Pythiaceae: Pythium torulosum (10.4 kb), Pythium irregulare (11.8 kb), Pythium anandrum (10.6 kb), Pythium paddicum (10.6 kb) and Phytophthora cryptogea (11.2 kb). Physical maps were constructed using seven endonuclease restriction enzymes and the maps were aligned on the basis of nine conserved restriction sites in a 6 kb region which was shown to be homologous to a DNA plasmid probe containing ribosomal genes from Neurospora crassa. A map of the rDNA region in Apodachlya pyrifera (10.1 kb), a species external to the Pythiaceae (Leptomitales), was also constructed to serve as a taxonomic reference. Percentage sequence divergence values indicated that P. diclinum and P. torulosum are closely related, but not identical. P. anandrum and P. irregulare also appeared closely related in spite of morphological differences. P. paddicum and Phytophthora cryptogea were the most outlying of the six species of Pythiaceae, but all six species formed a tight cluster when considered against A. pyrifera.