Origin of a new Phytophthora pathogen through interspecific hybridization

Abstract

Plant disease epidemics resulting from introductions of exotic fungal plant pathogens are a well known phenomenon. An associated risk-that accelerated pathogen evolution may be occurring as a consequence of genetic exchange between introduced, or introduced and resident, fungal pathogens-is largely unrecognized. This is, in part, because examples of natural, interspecific hybridization in fungi are very rare. Potential evolutionary developments range from the acquisition of new host specificities to emergence of entirely new pathogen taxa. We present evidence from cytological behavior, additive nucleotide bases in repetitive internal transcribed spacer regions of the rRNA-encoding DNA (rDNA), and amplified fragment length polymorphisms of total DNA that a new, aggressive Phytophthora pathogen of alder trees in Europe comprises a range of heteroploid-interspecific hybrids involving a Phytophthora cambivora-like species and an unknown taxon similar to Phytophthora fragariae. The hybrids' marked developmental instabilities, unusual morphological variability, and evidence for recombination in their internal transcribed spacer profiles indicates that they are of recent origin and that their evolution is continuing. The likelihood of such evolutionary events may be increasing as world trade in plants intensifies. However, routine diagnostic procedures currently in use are insufficiently sensitive to allow their detection.

Figure 1

(a–c) Colonies of alder phytophthoras. (a) Swedish variant, showing chimeric white fertile patches. (b) Standard alder Phytophthora with typical appressed colony. (c) Dutch variant, showing irregular fertile sectors. (d–f) Gametangial types. (d) Swedish variant showing smooth oogonium (above) and two-celled amphigynous antheridium (arrowed). (e) Standard alder Phytophthora, showing P. cambivora-like ornamented oogonium and two-celled amphigynous antheridium. (f) Netherlands variant showing extremely ornamented oogonium. (g–j) Diverse gametangial types representative of four different, single hyphal-tip subcultures from U.K. variant P841, including oogonium with paragynous antheridium (g, arrowed) and highly coralloid oogonium (h). (Bar = ≈10 mm.)

Figure 2

Location of polymorphisms in rDNA of alder Phytophthora isolates. (a) Arrangement within a single repeat of an rRNA gene (rDNA) of the rRNA subunits and spacer regions (ETS, external transcribed spacer; ITS1 and ITS2, internal transcribed spacers; IGS, intergenic spacer; 18S, 5.8S, 28S, and 5S subunits). (b) Arrangement of polymorphic sites within the 831-bp region covering the ITS1 spacer, the 5.8S subunit, and ITS2 spacer: the 5′ end of ITS1 spacer sequence has been designated as 1, and all other sites are numbered accordingly. At dimorphic sites (shaded regions) both bases are shown and boxed. Numbers alongside the boxes refer to the numbers of cloned PCR products containing these bases at these sites; totals for all three standard alder isolates are also shown. Along the bottom are the restriction enzymes used to examine each of the dimorphic sites.

Figure 3

Distribution of ITS sequence combinations across five alder Phytophthora isolates (three standard, one Dutch, and one Swedish variant) as determined by restriction digestion at six points across the cloned ITS PCR products. More ITS combinations are evident in the standard hybrid types than in the variants.

Figure 4

Dendrogram prepared from AFLP fingerprints of genomic DNA of the standard alder Phytophthora (five isolates); the Dutch (one), German (one), U.K. (one), and Swedish variants (two); P. cambivora (three); P. fragariae [var. fragariae (two) and var. rubi (two)]; and P. cinnamomi (one).