Genetic Variation and Phylogeny of Spongospora subterranea f.sp. subterranea Based on Ribosomal DNA Sequence Analysis

American Journal of Potato Research, Nov/Dec 2004 by Qu, Xinshun, Christ, Barbara J

ABSTRACT

The nuclear rDNA regions of the two internal transcribed spacers (ITS1 and ITS2) and 5.8S rRNA gene from 52 field isolates of Spongospora subterranea f.sp. subterranea obtained from the British Isles and North America were polymerase chain reaction-amplified, sequenced, and assessed for genetic variation. Two genetically distinct groups (I and II) were identified based on the ITS sequence diversity among the isolates, representing 34.6% and 65.4% of the isolates, respectively. British Isles isolates occurred in groups I and II, whereas North American isolates belonged only to group II. The British Isles groups of S. subterranea were associated with particular potato cultivars. The full-length small-subunit rRNA gene of S. subterranea was sequenced and analyzed by both neighbor-joining and parsimony methods to clarify the taxonomic position of this pathogen. The results of phylogenetic analysis showed that S. subterranea grouped together with other species of plasmodiphorids, and this group clustered with the phylum Cercozoa, an assemblage of filose and reticulose amoebae and phylogenetically related zooflagellates. The recognition of the existence of different genetic groups within S. subterranea will be important for the design of plant-breeding programs and in testing for plant resistance.

Accepted for publication 26 May 2004.

ADDITIONAL KEY WORDS: powdery scab

RESUMEN

Las regiones rADN nuclear de los dos espaciadores transcritos internes (ITS1 e ITS2) y el gen 5.8S rARN de 52 aislamientos de campo de Spongospora subterranea f.sp. subterranea obtenidos de las Islas Brit�nicas y de Norteam�rica fueron amplificadas por reacci�n en cadena de la polimerasa, secuenciadas y evaluadas para variaci�n gen�tica. Se identificaron dos grupos gen�ticamente diferentes (I y II) en base de la diversidad de frecuencia ITS entre los aislamientos, lo que representa el 34.6% y el 65.4% de los aislamientos respectivamente. En los aislamientos de las Islas Brit�nicas se encontr� los grupos I y II, mientras que los aislamientos de Norteam�rica pertenec�an s�lo al grupo II. Los grupos de S. subterranea de las Islas Brit�nicas estuvieron asociados con cultivares especiales de papa. El tama�o completo de la sub-unidad del gen rARN de S. subterranea fue secuenciado y analizado por los m�todos "neighbor-joining y parsimony," con el objeto de aclarar la posici�n taxon�mica de este pat�geno. El resultado del an�lisis filogen�tico demostr� que S. subterranea junto con otras especie de plasmodiophoridos est� relacionado con el phylum Cercozoa que incluye un conjunto de amebas filiformes y zooflagelados filogen�ticamente relacionados. El reconocimiento de la existencia de diferentes grupos gen�ticos dentro de S. subterranea ser� importante para el dise�o de programas de mejoramiento y pruebas de resistencia de la planta.

INTRODUCTION

Spongospora subterrcmea (Wallr.) Lagerh f.sp. subterranea Tomlinson, a soil-borne obligate biotroph, causes powdery scab in potatoes. Powdery scab seriously reduces tuber quality and marketability and is a major concern to potato growers worldwide. In some potato-production regions, S. subtermnea is also important as the vector of potato mop-top virus (Jones and Harrison 1969), which causes a reduction in growth and internal tuber necroses and has recently been found in the United States (Lambert et al. 2003). Control of powdery scab of potato is difficult because of the persistence of the spore balls in soil or on tubers. There is no effective chemical or cultural treatment and all cultivars are susceptible to some degree (Harrison et al. 1997). Therefore, the development of resistant cultivars is now considered the most economical and efficient method for the control of powdery scab.

Successful breeding and effective deployment of durable plant resistance requires an understanding of pathogen diversity and the ways virulence evolves in pathogen populations. However, little information is available on the existence of physiological races, pathotypes, or genetic variation in populations of S. subterranea. Khrobrykh (1938) suggested that viruliferous and non-viruliferous "forms" of S. subterranea exist, but failed to find any conclusive evidence of specialization. Boning and Wallner (1938) claimed to have found a virulent form of the pathogen in Germany, but there was no evidence that it was particularly adapted to any variety of potato. The possibility of the existence of S. subterranea pathotypes was studied more intensively by W�rzer (1964) using 17 isolates from America and Europe. He noted that 17 isolates could be divided into two groups according to the diameter of resting spores, but found no conclusive evidence of biological specialization. In a review of powdery scab, Harrison et al. (1997) concluded that the existence of physiological races, or pathotypes, of S. subterranea has not, yet been demonstrated and that this requires further investigation. The differences in the susceptibility of potato cultivars to powdery scab have been demonstrated in many trials (Christ 2000; Christ and Petrunak 1997; Christ and Weidner 1988; Gans et al. 1987; Sica and Christ 2000a, 2000b), but it is not known whether the level of susceptibility is affected by different populations or pathotypes of S. subterranea. Bulman and Marshall (1998) assessed the genetic variation in ribosomal internal transcribed spacer (ITS1 and 2) sequences of Australasian and European S. sublerranea isolates and noted that no sequence variation was detected between any of the Australasian or European isolates with the exception of one from Inverness (Scotland) which was identical to two Peruvian samples. Considering the worldwide distribution of S. subterranea, more samples from different geographic locations are clearly required to assess overall S. subterranea variation.