Biochemical Variation in Atlantic and Pacific Coast Populations of the American Shad

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Proceedings of a Workshop on American Shad

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American shad are native to the Atlantic Coast of North American and were introduced to the Pacific Coast by stockings from several eastern rivers approximately 100 years ago. We are presently undertaking a biochemical genetic analysis of populations spanning the genetic divergence between them. Eight systems have been examined for polymorphic genetic traits using electrophoretic techniques. The only consistently scorable polymorphic locus was have discovered to date is the EST-1 locus in blood plasma. There are 3 alleles at this locus. The frequency of the most common allele varies inversely with latitude in a linear fashion in East Coast populations. The same allele is in high frequency at the extremes of the range (north and south) on the West Coast but drops in frequency in the middle of the range. The allele exhibiting intermediate electrophoretic mobility is absent in southern populations on the East Coast and rare in the northern ones, however it exists at moderate frequencies in all West Coast populations. This is explained in terms of Founder Effect at introduction. Genotypic frequencies of all Atlantic Coast populations do not differ significantly from those expected at Hardy-Weinberg equilibrium. Three of the four Pacific Coast populations studied show large and significant heterozygote deficiencies, apparently due to the presence of a null allele not present on the East Coast. We have interpreted variation at this locus as adaptive to unpredictable temperature regimes occurring during the spawning migration in the northern east coast rivers and in Oregon coastal streams.

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