Population responses to contaminant exposure in marine animals: influences of genetic diversity measured as allozyme polymorphism.

Current understanding of the genetic and metabolic basis of relations between heterozygosity and animal performance under non-polluted conditions is relevent to interpreting apparently inconsistent findings concerning the relative advantage of allozyme polymorphism during contaminant exposure. Many interrelated factors which may influence and even compromise those relations include species (lifestyle, reproductive behaviour etc), lifestage, environmental influences and a variety of background genetic effects (limited parentage, null alleles, aneuploidy, genomic imprinting etc). Nevertheless, there is some promise that single-locus responses may be diagnostic for specific pollutants. In addition, limited evidence to date supports the a priori expectation that reduced energy requirements for maintenance metabolism may facilitate longer survival of multiple-locus heterozygotes during exposure to contaminants with toxic effects that result either in the reduced acquisition or availability of metabolizable energy, and/or a reduction in the efficiency with which metabolizable energy is used to fuel metabolic processes. More work is required to fully establish this trend in response to specific contaminant types, to assess any direct consequences of underlying differences in protein metabolism, and to resolve the interactive effects of contaminant mixtures. But the functional value of genetic variation within populations is confirmed. Reduced genetic diversity may not only compromise the capacity of an impacted population for genetic adaptation in the face of further environmental challenge, but may also result in increased energy requirements, lower production efficiency and reduced reproductive output. These metabolic consequences of reduced genetic polymorphism would further lower that population's potential for survival under lethal conditions of contaminant exposure, and also affect the genetic makeup of populations through differential reproduction under conditions of sublethal stress.