138 Anne LEGRE  VE 1 , Philippe DELFOSSE 2 and Henri MARAITE 1  Unite  de Phytopathologie, Universite  Catholique de Louvain, Place Croix du Sud 2 Bte 3, B-1348 Louvain-la-Neuve, Belgium.  International Crops Research Institute for the Semi-Arid Tropics (ICRISAT ), BP 12 404, Niamey, Niger. E-mail : legrevefymy.ucl.ac.be Received 22 March 2001 ; accepted 20 October 2001. A region of the nuclear ribosomal DNA containing the internal transcribed spacer 1 (ITS1), the 58S DNA and the internal transcribed spacer 2 (ITS2) was sequenced in 12 Polymyxa graminis and P. betae isolates, with particular emphasis on P. graminis from peanut clump-infested areas of the Indian subcontinent and West-Africa. Four different sequences were obtained from the seven isolates on sorghum or pearl millet, which differed from four sequences previously published for Polymyxa species and obtained for P. graminis isolates on barley, oat and wheat originating from temperate areas in Europe and America (two distinct sequences), for isolates on rice from Colombia and for P. betae isolates on sugar beet from several origins. The sequence variations concerned mainly the composition and length of ITS1 and ITS2 regions. Phylogenetic trees built with the distinct sequences currently known for Polymyxa spp. using parsimony, maximum likelihood and neighbour-joining methods separated P. betae from P. graminis. Within P. graminis, the hierarchy of the clustering partially matched the host range and geographical origin of the isolates. These results confirm the great diversity within P. graminis that has already been observed in host range and temperature requirements studies, and provide new arguments for considering several taxa within the species. On the basis of the ecological requirements and rDNA sequences of distinct P. graminis isolates, five special forms are proposed : P. graminis f. sp. temperata, P. graminis f. sp. tepida, P. graminis f. sp. subtropicalis, P. graminis f. sp. tropicalis and P. graminis f. sp. colombiana. INTRODUCTION Polymyxa (Plasmodiophoromycota) groups obligate endoparasites of plant roots, well known as vectors of phytoviruses. Two species that are morphologically similar, Polymyxa graminis (Ledingham 1939) and Polymyxa betae (Keskin 1964), have been described. They are distinguished by their separate host ranges (Barr 1979), P. graminis growing on Gramineae and P. betae on Chenopodiaceae and related plant families, such as Amaranthaceae, Portulaceae and Caryo- phyllaceae (Barr & Asher 1992). Host range studies have revealed a high level of diversity among isolates of each species. Several special forms were proposed for P. betae which are restricted to distinct families : e.g. P. betae f. sp. betae which develops on Chenopodiaceae (Barr 1979), P. betae f. sp. amaranthi which infects only Amaranthus retroflexus (Barr 1979) and P. betae f. sp. portulacae which is restricted to Portulaca oleracea and P. grandiflora (Abe & Ui 1986). Furthermore, studies performed with isolates on Chenopodiaceae from distinct origins or from the same soil revealed the existence of various biotypes with particular host ranges within P. betae f. sp. betae (Abe & Ui 1986, Barr & Asher 1992). The diversity within P. graminis appeared to be even more complex. This species is ubiquitous and is currently considered to be the vector of several rod-shaped or filamentous viruses in temperate and tropical areas on cereals and on groundnut. Isolates from various origins, whether or not they were associated with the trans- mission of viruses on barley, wheat, rice, oat, sorghum or groundnut, were all considered to be P. graminis because they infect and produce profuse quantities of sporosori on graminaceous species. Nevertheless, host range studies demonstrated a marked diversity in the host specificity of P. graminis isolates from various origins (Adams & Jacquier 1994, Barr 1979, Bastin, Boute & Maraite 1989, Legre  ve et al. 1996, 2000, Morales et al. 1999). Differences in host susceptibility range and multiplication rate on infected plants were shown not only between isolates from distinct plants adapted to specific climate regimes (e.g. Indian isolates on sorghum versus European isolates on barley), but also between isolates obtained from a similar plant but originating from distinct areas (e.g. among isolates on Mycol. Res. 106 (2) : 138–147 (February 2002).  The British Mycological Society DOI : 10.1017S0953756201005391 Printed in the United Kingdom. Phylogenetic analysis of Polymyxa species based on nuclear 58S and internal transcribed spacers ribosomal DNA sequences