Ribosomal DNA analyses reveal greater sequence variation in Polymyxa species than previously thought and indicate the possibility of new ribotype-host-virus associations.

Polymyxa species transmit viruses to many important crops. They are poorly-understood obligate 29 parasites occupying a distinct position in the Tree of Life. To better understand the potential for 30 spread of Polymyxa-vectored diseases, ribosomal DNA was analysed from isolates covering a wide
31 range of geographic locations, virus associations and hosts. ITS2 structure analysis indicated that P. 32 graminis isolates could represent many species and there was more sequence variation within the 33 known subgroups (ribotypes) than previously described. In cereal crops and soils from temperate
34 climates Polymyxa isolates were usually ribotype I or II, but their host specificities or preferences 35 were unclear. For the first time, there was evidence that ribotype I (in addition to ribotype II) could 36 transmit SBWMV/ SBCMV. Different ribotypes often occurred together in the same soil or plant.
37 New hosts were identified for particular ribotypes, including the first detection of the sugar beet38 infecting P. betae, in wheat. Unexpectedly, ribotype III-like sequences, usually restricted to crops 39 in the tropics, were found in wheat from the USA. P. betae isolates showed limited variation (≤2%)
40 and the recent change in susceptibility of sugar beet varieties to BNYVV in the USA is unlikely to 41 be due to changes in P. betae.