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Gene flow in a cline Amathes glareosa Esp. and its melanic F. Edda Staud. (Lep.) in Shetland

1969; Springer Nature; Volume: 24; Issue: 1 Linguagem: Inglês

10.1038/hdy.1969.1

1365-2540

H B D Kettlewell, R. J. Berry,

Genetic diversity and population structure

THE Caradrinid moth Amathes glareosa Esp. has a distinct melanic form (edda) in Shetland.This form decreases in frequency from 97 per cent, in the north of the 70-mile-long group of islands to about one per cent. in the south (Kettlewell and Berry, 1961) (fig.I).The difference between the two forms is controlled by a single gene, and in North Shetland (Unst) the black form seems to have near-complete dominance.However, slightly lighter forms are sometimes encountered in the wild population, although we found it impossible to differentiate these from the darker forms with accuracy.The obvious assumption is that these lighter moths are heterozygous for the edda gene.Despite the difficulty of scoring, we can state confidently that under 15 per cent, of 12,81 8f.edda from Unst were classifiable as being of the lighter form.In this area the Hardy-Weinberg expectation for the frequency of heterozygotes is 28 per cent.By contrast in South Shetland, in the Orkneys and in Fair Isle where f. edda occurs at a low frequency, many of the specimens are paler than any found on Unst, the majority are light and none of the darkest ones are as black as those from northernmost Shetland.This can be accounted for by either incomplete dominance in the Southern populations in contrast to the North Shetland one or alternatively by a different gene-complex which produces a paler insect here in both the f.edda genotypes.It must be emphasised that never is there any difficulty in distinguishingf.edc(a from f. typica which is the only form occurring throughout the rest of Britain.We have described the occurrence and intensity of the dine inf.edda in three papers (Kettlewell, 1961a, b;Kettlewell and Berry, 1961) based on field-work in Shetland in 1959 and 1960.This paper describes work done in 1961 and 1962 with the particular object of investigating an apparent barrier to gene-flow in the centre of the dine.In 1960 we showed that the frequency off.edda over most of the north of the Shetland Mainland (= themain island) was 5 0-60 per cent.; in the South Mainland, all the populations sampled contained less than five per cent. of the melanie.In the intervening Central Mainland the phenotype frequency decreased by 50 per cent, over a distance of about 15 miles.The simplest explanation for this state of affairs is a barrier to gene-How separating popu lations living under different ecological conditions.Now in Shetland A. glareosa is caught in numbers in two very different * Strictly speaking the north and south borders of the Tingwall Valley are south-east and north-west facing slopes.However as the main axis (and populations of A. glareosa) of the Shetland Mainland runs north and south, we have written throughout of the Tingwall Valley as if it ran due east and west.