Allozyme frequencies in western European populations of Asellus aquaticus (L.) Isopoda and their association with water pollution

A study was made of allozyme variation in western European populations of Asellus aquaticus. Two enzymes were examined glucose phosphate isomerase (CPI) and phosphoglucomutase (PGM).
The populations show significant latitudinal clincs for GPI and PGM with the GPI-2 and PGM-2 alleles increasing in more southerly latitudes. While GPI-2 frequencies in British populations are consistent with the overall clinc, considered separately they show a reverse cline. This appears to be the result of a strong association of GPI-2 frequencies with water pollution combined with the geographic distribution of the polluted sites.
The association of high GPI-2 frequencies with high levels of water pollution is present in two independent sets of populations in Britain, and is more specifically associated with low levels of environmental oxygen. This latter association also occurs independently in temporal studies made at three locations. However, an additional temporal association of high GPI-2 frequencies with high temperatures also exists, consistent with the observed latitudinal cline. A study of adult mortality by oxygen deprivation at different temperatures suggests that both temperature and oxygen concentrations may act selectively with the GPI 2-2 genotype to be favoured under certain conditions. In contrast to GPI, PGM allozymes do not appear to be associated with pollution related variables.
The findings of the study point to selection acting on the two loci examined, though most convincingly for GPI. However, it remains to be demonstrated that selection is acting directly on the loci, and if so, how it operates to produce the patterns of allozyme variation observed in nature.