| Summary: | Spiders (Arachnida: Araneae) are important polyphagous predators which limit pest numbers in arable crops. To utilise them fully as part of an IPM (integrated pest management) programme a greater understanding of basic population dynamics and ecology is required. Two species common to UK arable land: <I>Lepthyphantes tenuis</I> (Blackwall) and <I>Enoplognatha ovata</I> (Clerck) were studied. These species exhibit different rates of dispersal; the former a highly dispersive ballooning spider, the latter a more sedentary species - permitting the effect of dispersal on patterns of genetic variation to be examined. RAPD data were generated via 5 decamer primers and used to determine variation between and within populations at a local scale (<100 km). Individuals within a species showed a high degree of similarity calculated via the Nei and Li similarity coefficient (<I>E. ovata</I> >83% and <I>L. tenuis</I> >54%). Despite this high level of similarity, PCO (Principle Co-Ordinate) analysis detected a pattern within the similarity matrix which could be related to the geographical separation of the populations. A less obvious structure was detected by PCO analysis of <I>L. tenuis</I> samples. Further statistical analysis revealed that F<SUB>st</SUB> estimates were higher, and heterozygosity levels lower, within <I>E. ovata</I> populations than <I>L. tenuis</I> populations emphasising the effect of dispersal and gene flow on reducing population structure. PCR-RFLP of a rDNA fragment containing ITS1 and 2 was carried out as an initial screen of variability with populations of <I>L. tenuis</I> from England, Scotland and New Zealand, and <I>E. ovata</I> populations from Dundee, Edinburgh and Elgin. Near identical RFLP patterns were found within a species. To investigate variation at the finest level, multiple ITS1 clones from one individual from each site were sequenced in both species. A total sequence variation of 2.7% was recorded across 10 <I>E. ovata</I> clones and 1.5% across 10 <I>L. tenuis</I> clones.
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