Measurement of nuclear DNA contents of Mexican isolates of Phytophthora infestans

The nuclear DNA contents of 145 isolates of Phytophthora infestans from the central highlands of Mexico (the centre of origin of this oomycete), northwestern Mexico, and northeastern Mexico were measured by quantifying the fluorescence from nuclei stained with 4,6-diamidino-2-phenyl-indole (DAPI). Isolates were collected over several different years. Isolates that had been repeatedly subcultured tended to have higher DNA contents than had isolates from storage. Therefore, the DNA contents were measured on isolates soon after isolation from the field or soon after retrieval from storage (under oil at 18 °C or cryogenically). In all three locations, P. infestans isolates mainly contained low quantities of DNA (consistent with presumed diploidy). However, a few isolates with larger quantities of DNA were also detected — primarily in populations from northern Mexico. There appeared to be no strong association between level of DNA content and mating type or year of collection.