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An Analysis of Genetic Structure in the Monarch Butterfly, Danaus plexippus L.
Walter F. Eanes and Richard K. Koehn
Vol. 32, No. 4 (Dec., 1978), pp. 784-797
Published by: Society for the Study of Evolution
Stable URL: http://www.jstor.org/stable/2407494
Page Count: 14
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Thirty geographic samples of the monarch butterfly, Danaus plexippus L., were collected in the summer and fall of 1973, 1974, and 1975, in the eastern-central U.S. and Canada. Using Wright's F-statistics and electrophoretic alleles at six enzyme-loci as genetic markers, an analysis of the genetic structure of the population as a whole was carried out. It was assumed a priori that the monarch population undergoes changes in population structure during the spring repopulation of the northern range, the relatively sedentary summer buildup in population size, and lastly the fall migration to the Gulf states and Mexico. These changes should be detectable in the genetic structure. In non-migratory collections, common alleles at several loci show statistically significant allele frequency variation among collections (FST > 0). The sexes differ persistently in allele frequency at two of the loci (Got and Ndh). Among-sample variances are reduced in the migratory collections, and none of the alleles show significant differentiation (FST = 0). Thus, migration swamps genetic differentiation generated prior to, and during, the less vagile summer period. Inbreeding coefficients FIS) computed for each of the individual samples show persistent heterozygote excesses at three loci (Ndh, Got, and Mdh, FIS < 0), these may be explained in part by the differences in allele frequency observed between the sexes at two of the loci. The total population inbreeding coefficient, FIT, remains the same in migratory as in non-migratory collections. The observed decrease in the average FST with the concomitant increase in average FIS in the migratory collections is probably a reflection of the Wahlund effect due to natural mixing of subpopulations. Using estimates of monarch dispersal abilities for this period (Urquhart, 1960) in the context of Wright's isolation-by-distance model, it may be argued that the effective spatial densities need be unrealistically low (about one individual every 340 square mi) in order to generate stochastically the differentiation observed for these markers. But because the population structure of the monarch is undoubtedly more complex than the isolation-by-distance model and because the possibility of a 'founder effect' during the spring recruitment cannot be ruled out, this conclusion must be judged with caution. Evidence that natural selection could directly or indirectly influence the allele frequencies at some loci comes from the observations of significant differences in allele frequency between sexes at two loci, and of a correlation between allozyme heterozygosity and the deviation of individual body and spot size from mean size.
Evolution © 1978 Society for the Study of Evolution