Evidence for homoploid speciation in Phytophthora alni supports taxonomic reclassification in this species complex
Résumé
Alder decline has been a problem along European watercourses since the early 1990's. Hybridization was identified as the main cause of this emerging disease. Indeed, the causal agent, a soil-borne pathogen named [i]Phytophthora alni[/i] subsp. [i]alni[/i][i] (Paa)[/i] is the result of interspecific hybridization between two taxa, Phytophthora alni subsp. multiformis [i](Pam)[/i] and [i]Phytophthora alni[/i] subsp.[i] uniformis[/i] [i](Pau)[/i], initially identified as subspecies of[i] Paa[/i]. The aim of this work was to characterize the ploidy level within the [i]P. alni[/i] complex that is presently poorly understood. For that, we used two complementary approaches for a set of 31 isolates of [i]Paa, Pam[/i] and[i] Pau[/i]: i) quantification of allele copy number of three single-copy nuclear genes using allele-specific real-time PCR and ii) comparison of the genome size estimated by flow cytometry. Relative quantification of alleles of the three single-copy genes showed that the copy number of a given allele in [i]Paa[/i] was systematically half that of its parents [i]Pau[/i] or [i]Pam[/i]. Moreover, DNA content estimated by flow cytometry in [i]Paa[/i] was equal to half the sum of those in [i]Pam[/i] and [i]Pau[/i]. Our results therefore suggest that the hybrid [i]Paa[/i] is an allotriploid species, containing half of the genome of each of its parents [i]Pam[/i] and [i]Pau[/i], which in turn are considered to be allotetraploid and diploid, respectively. [i]Paa[/i] thus results from a homoploid speciation process. Based on published data and on results from this study, a new formal taxonomic name is proposed for the three taxa [i]Paa[/i], [i]Pam[/i] and [i]Pau[/i] which are raised to species status and renamed [i]P. ×alni, P. ×multiformis[/i] and[i] P. uniformis[/i], respectively.
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