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

Population genetic structure of the circum-Mediterranean caddisfly Mesophyl ax aspersus (Trichoptera, Limnephilidae) on the Canary Islands was investig ated by studying allozyme variation at nine putative loci in five populatio ns. Genetic variability, population structure and gene flow were compared w ith data in the literature for continental taxa to assess the effect of iso lation of island populations on the genetic structure. Larvae were collecte d from streams on the islands or Tenerife (one population), La Gomera (two populations in the same catchment) and La Palma (two populations in differe nt catchments). Genetic variability within Populations was high relative to that recorded previously for continental Trichoptera, e.g. mean heterozygo sity 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 signi ficant within-population structuring (mean F-IS = 0.098). Populations from the same catchment or island were no more similar than populations from dif ferent islands, which suggests that occasional long-distance dispersal, bot h between and within islands, is the predominant influence on the populatio n structure. This dispersal ability has contributed to the colonization of most permanent streams on the Canary Islands by M. aspersus.