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Abteilung Mykologie: Prof. em. Dr. Gerhard Rambold

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Phylogeny and Character Evolution in Erysiphales



Powdery Mildews' Biology

The powdery mildews (Erysiphales, Ascomycota) are a cosmopolitan group of obligate phytopathogenic fungi on angiosperms. With an overall species number of about 650 the Erysiphales show a relatively high specificity of host plant choice. The occurrence of most species is restricted to individual genera or families of angiosperms.

Attacked plants show a characteristic white covering of leaves, stems or even fruits due to a network of fungal hyphae that grow on the host plants surface. The hyphal covering can be easily removed by wiping the surface manually because the hyphal growth is predominantly superficial. After forming appressoria some fungal hyphae penetrate the plant surface and develop haustoria inside the plant tissue to supply the fungal cells with nutrition.

The cleistothecial fruit-bodies of the powdery mildews develop between the fungal hyphae upon the host plants surface. They are more or less globose in shape and have characteristic complex appendages. The colour of the ascocarps ranges from bright yellow or orange when young to dark brown or black when mature and they open by means of vertical or equatorial slits. The cleistothecia contain one or several asci with colourless, 10–50 x 8–30 µm large ascospores.

In addition to cleistothecia asexual reproduction by conidia is very common within the Erysiphales.

Konrad Schieferdecker 1947, watercolour

Morphology

The Erysiphales show many morphological characters that can be useful in analyzing the phylogenetic realationships within this well-defined group of ascomycetous fungi. The most conspicuous character is the shape of cleistothecial appendages, but there are many others like the dorsiventral orientation of ascocarps, the structure of peridial cell layers or the number of asci per ascocarp. Nowadays the taxonomic value of anamorphic structures such as vegetative hyphae, conidiophores and conidia is also widely accepted.

Phylogeny

For a long time the relationships between Erysiphalean members have been concluded almost exclusively from the characters of fruit-body appendages. With the upcoming of molecular techniques for analysing the organism's DNA-sequences, this traditional concept of classification has been shaken up. Today it seems that some types of fruit-body attachments have evolved at least twice within the powdery mildews.

Whereas there are a lot of DNA-sequences of Asian or North American species available, there have been rather less molecular work done on the powdery mildews of Europe. For more reliable statements on the phylogeny of Erysiphales additional sequence data from European specimens is important. Furthermore the conducted studies are confined to sequence-alignments of the ribosomal genes (18S, 28S) and internal spacer regions (ITS). Checking of extra-ribosomal PCR-primer sites could be useful for detecting additional phylogenetically meaningful DNA-regions. In this way the existing results could be tested and the phylogenetic relationships of difficult species clusters possibly revealed.

Character Evolution

The specific occurrence of Erysiphales species on different angiosperm genera or families is analysed by recording the label data of large Erysiphales collections in the herbaria of München (M) and Halle (HAL) usingmodern database techniques. The parasite-host data are combined with geographical data, also obtained from the specimen labels, and processed with GIS applications. Thereby allowing different character specifications of Erysiphales species to be correlated with ecological parameters and insights into the process of character evolution attributable to ecological and coevolution effects can be attained.




References

  • Braun, U. 1987. A monograph of the Erysiphales (powdery mildews). – Beih. Nova Hedwigia 89: 1–700.
  • Braun, U. 1999. Some critical notes on the classification and the generic concept of the Erysiphales. – Schlechtendalia 3: 48–54.
  • Braun, U., Takamatsu, S. 2000. Phylogeny of Erysiphe, Microsphaera, Uncinula (Erysipheae) and Cystotheca, Podosphaera, Sphaerotheca (Cystotheceae) inferred from rDNA ITS sequences – some taxonomic consequences. – Schlechtendalia 4: 1–33.
  • Saenz, G. S.& Taylor, J. W. 1999. Phylogeny of the Erysiphales (powdery mildews) inferred from internal transcribed spacer ribosomal DNA sequences. – Canad. J. Bot. 77: 150–168.
  • Takamatsu, S., Hirata, T. & Sato, Y. 1998. Phylogenetic analysis and predicted secondary structures of the rDNA internal transcribed specers of the powdery mildew fungi (Erysiphaceae). – Mycoscience 39: 441–453.
  • Takamatsu, S., Hirata, T., Sato, Y. & Nomura, Y. 1999. Phylogenetic relationships of Microsphaera and Erysiphe section Erysiphe (powdery mildews) inferred from the rDNA ITS sequences. – Mycoscience 40: 259–268.
  • Takamatsu, S., Kano, Y. 2001. PCR primers useful for nucleotide sequencing of rDNA of the powdery mildew fungi. – Mycoscience 42: 135–139.

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