Submitted to: Plant Pathology European Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: August 16, 1998
Publication Date: N/A
Technical Abstract: Sympatric speciation occurs when species form without geographical separation. Although the theory of sympatric speciation has been well developed for more than 30 years, there are few good examples in the literature, and whether it occurs at all is highly controversial. A change in host specificity could initiate sympatric speciation for pathogens that complete their life cycles on their hosts. Many plant-pathogenic fungi ar host specific, complete their life cycles on their hosts, and thus might have arisen by sympatric speciation. An analysis of host specificities, degree of reproductive isolation, and genetic incompatibilities in F1 interspecific hybrids, revealed evidence for probable recent sympatric speciation between Phytophthora infestans and P. mirabilis in central Mexico. The two species are host specific, sympatric, and reproductively isolated. Differences between P. infestans and P. mirabilis were found for rmitochondrial DNA, isozymes, DNA fingerprints and RAPDs. Cluster analyses provided clear separation between the species. Gene flow analyses revealed that P. infestans and P. mirabilis are as reproductively isolated from each other as they are from the other species in Phytophthora Group IV. Extensive efforts to develop F2 progenies from F1 interspecific hybrids failed. They clearly are different species. However, they are so closely related that the sequences of their ribosomal DNA Internal Transcribed Spacer II regions are identical. Because their hosts are very different (one is in the Nyctaginaceae, the other in the Solanaceae), gradual divergence during allopatric speciation seems highly unlikely. The only likely explanation for the data is that these two species arose by recent, sympatric speciation that was driven by a change in host specificity.