Direct Measures of Gene Flow

The establishment of new genets is either by immigration through spore dispersal or recruitment from spore sources within the population. Spore dispersal has the potential to connect populations over a large distance. The process of spore dispersal comprises several steps. First, the spores released into the turbulent air layer, beneath the basidiocarp. If the fruit body is in a forest, then spores will move in the relatively calm air within the canopy of the forest. The majority of spores settle within a few metres of the fruit body. Since sporocarps can produce thousands of spores, a few have the capacity to travel far. Models of the spore dispersal gradient in H. annosum suggest that occasionally a spore can disperse and germinate on a suitable resource 500 km away from its source (Stenlid, 1994b). This shows a potential for long-distance spread but it is not necessarily the actual situation for most populations. Spores have been detected 300-1,500 m from the closest known sporocarps in H. annosum (Moykkynen et al., 1997) and P. centrifuga (Norden and Larsson, 2000). Within a stand, dispersal seems not to be restricted and, although suitable logs are distributed in a clumped manner, the sporocarps are more randomly scattered among them (Edman and Jonsson, 2001).

In the atmosphere, spores are subject to UV radiation, and there can be 95% loss of vitality within 1-3 days (Kallio, 1973), but pigmented spores survive better than hyaline basidiospores (Burnett, 2003). Basidiospores are relatively small—5-10 p.m—and it has been suggested that their movement in air can be compared to that of smoke particles. Schlesinger et al. (2006) reported an altered mycoflora on particles carried by dust storms from the Sahara compared to the airborne fungal flora on a clear day. Mims and Mims (2004) showed that dispersal of fungal spores were correlated with smoke particles carried over the Mexican gulf.

Actual spore catches, using species-specific spore trapping techniques, show that spore deposition is highly dependent on spores released within the nearest 3 km of the traps. Interestingly, both in common and uncommon species, spore catches have been minimal > 20 km away from known distributions in controlled investigations (Moykkynen and Kontiokari, 2001; Edman et al., 2004a, 2004b). By using spore traps, James and Vilgalys (2001) were able to study the genotypes of wind-borne spores of S. commune, and the molecular variation provided no clear evidence for dispersal over large, aquatic barriers within the Caribbean region, but instead suggested that spore-trapping experiments primarily reflect the local, established population (James and Vilgalys, 2001). By contrast, Hallenberg and Kuffer (2001) reported on spore catches of the rare Peniophora aurantiaca, during

24 h exposure in the city of Goteborg, Sweden, where the closest known natural occurrence was ~1,000 km away!

Airborne spore dispersal from one continent to another may occasionally happen but the likelihood of surviving the journey and then landing on an appropriate resource makes this scenario very unlikely (Fries, 1987). Cosmopolitan species normally show a distinct genetic structure (James et al., 1999), have intersterility barriers among isolates from different continents (Burnett, 2003), or have clear differentiation of ecological niche (Hallenberg and Larsson, 1992). The conclusion must be that airborne spore establishment from one continent to another of wood-decay basidiomycetes is close to zero.

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