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Genotypic and phenotypic variation in populations of Daphnia pulex Krepp, Susan Rose

Abstract

Because of their reproductive pattern, parthenogenetic organisms may have limited genetic variation and may rely on alternative mechanisms other than genetic diversity for maintaining phenotypic variability and adapting to the environment. This hypothesis was tested by measuring genotypic and phenotypic variation in several populations of Daphnia pulex, an apomictic, parthenogenetic cladoceran. Genotypic variation measured by starch gel electrophoresis indicated 0% variable loci in 3 species of Daphnia in the lower mainland around Vancouver, B.C. and 3855 polymorphic loci in Near Roundup, a pond in the Interior of the province near Williams Lake. Differences in environmental conditions and electrophoretic patterns provide a rationale for comparing phenotypic variation in 3 electrophoretically, physically and geographically similar ponds, P2, P4, and P5, and in an electrophoretically polymorphic population (NR) which are physically and geographically distinct. Means and variances of 5 morphological and 1 to 6 reproductive characters were compared within and among clones in each population and among populations and indicated the following: 1) There were significant differences in means for most characters among clones and among populations regardless of electrophoretic similarity or dissimilarity among clones or populations, 2) There was greater intraclonal variation than interclonal variation in all populations for all characters, 3) there was significantly greater total variance, intraclonal variance, and interclonal variance in P2 than in HR, and 4) variances were partitioned equally within and among clones in P2 whereas the greatest % variation in NB was within clones. These data suggest an inverse relation of genetic and phenetic variability in these populations of Daphnia and suggest that P2 and NR are examples of adaptation by individual and populational homeostasis. P2 individuals which are electrophoretically monomorphic may rely on extreme phenotypic plasticity in order to adapt to the environment. NS Daphnia may also rely on phenotypic plasticity to a lesser extent as demonstrated by the large % variation within clones, however, the relatively small absolute variance and the electrophoretic variation may indicate adaptation by genetic changes in the population. These possible strategies have been further interpreted relative to selection and temporal stability of the environment. Phenotypic plasticity and lack of electrophoretic variation in Daphnia and in other parthenogenetic and inbreeding organisms suggest that these organisms are not dependent on genetic changes in the population to survive. There is however evidence of genetic and phenetic variation in parthenogenetic organisms comparable to variation in sexually reproducing organisms and this suggests that genetic variation is not necessarily constrained by the mode of reproduction.

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