The genetic diversity of Ralstonia solanacearum causing bacterial wilt of tomato in Trinidad was assessed using the hierarchical phylotyping scheme and rep-PCR DNA fingerprinting. Seventy-one isolates were collected in 2003 on infected tomato crops in the four main vegetable cropping areas of Trinidad (North, Central, South-East and South). Two phylotypes were present, with phylotype II being much more prevalent (66%) than phylotype I (34%). Phylotype II strains consisted mainly of sequevar 7 in Central and South-East, and sequevar 35 in North, South-East and South. This is the first report of sequevar 7 outside southeastern USA. In contrast, no 'brown rot' (phylotype IIB ⁄ 1, race 3 biovar 2) or emerging strains of phylotype IIB ⁄ 4NPB were identified. Rep-PCR data were used to assess population genetic structure. No significant clustering by geographical distance was found, suggesting regular gene flow among cropping areas (via waterways, plant or soil). However, the population from Central was significantly differentiated from the others, containing only phylotype II ⁄ seq 7 strains, with a high degree of clonality, suggesting a possible recent introduction from abroad. The South population was less aggressive and more genetically diverse, suggesting horizontal gene transfers within the population, even among isolates of different phylotypes. Phylotype I and phylotype II populations differed slightly in clonality levels, with indications of more frequent recombination events within phylotype I populations. Possible factors influencing genetic diversity and distribution within the island are discussed.

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Thamnolia subuliformis (Ehrh.) Culb. is a sterile lichen-forming ascomycete with no known sexual or vegetative reproductive structures except fragments of thallus branches. The open tundra in northern Manitoba contains fragments of T. subuliformis randomly scattered over the landscape, giving the appearance that dispersal is common. This may provide a misleading impression of gene flow, but similar fragmentation is seen in seven other locally occurring species of lichen-forming fungi. The primary objective of this study was to examine the population structure of T. subuliformis using four multilocus markers (interspersed simple sequence repeats; ISSR) and a single locus marker (the presence/absence of a group I intron in the nuclear 18S rDNA locus). Significant population subdivision is reported for two different classifications of what constitutes a population. Thallus fragments of T. subuliformis may be intact and slow growing, having accumulated tissue over a long period of time, with low levels of gene flow occurring in localized areas. The presence of the group 1 intron in localized transects may suggest either that the intron was introduced recently and has not had sufficient time to disperse among populations, or that it has been lost in other locations.