Population structure and dispersal in the Canary Island caddisfly Mesophylax aspersus (Trichoptera, Limnephilidae).

Population genetic structure of the circum-Mediterranean caddisfly Mesophylax aspersus (Trichoptera, Limnephilidae) on the Canary Islands was investigated by studying allozyme variation at nine putative loci in five populations. Genetic variability, population structure and gene flow were compared with data in the literature for continental taxa to assess the effect of isolation of island populations on the genetic structure. Larvae were collected from streams on the islands of Tenerife (one population), La Gomera (two populations in the same catchment) and La Palma (two populations in different catchments). Genetic variability within populations was high relative to that recorded previously for continental Trichoptera, e.g. mean heterozygosity was 0.119--0.336 (0.035--0.15 in continental taxa). Highly significant population structuring was observed (mean F(ST)=0.250), and there was significant within-population structuring (mean F(IS)=0.098). Populations from the same catchment or island were no more similar than populations from different islands, which suggests that occasional long-distance dispersal, both between and within islands, is the predominant influence on the population structure. This dispersal ability has contributed to the colonization of most permanent streams on the Canary Islands by M. aspersus.