Chromosomal polymorphism in Chironomus

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Abstract
Excluding some which are very rare, seventeen different chromosome arrangements are found in populations of Chironomus tentans in the British Isles. In C. dorsalis there are nine chromosome arrangements which are found in nearly all populations. All the arrangements differ by inversions which are almost certainly paracentric. Only one larva in about 3000 of both species had a pericentric inversion, and this appears only just to include the centromere. In a sample containing mainly C. tentans larvae with a small proportion of the closely related C. pallidivittatus , I found one first-generation hybrid and three ‘partial’ hybrids. One of the chromosomes in the ‘full’ hybrid differed from the other of the pair by two inversions only, and the banding pattern of one of the arms of this chromosome is the same in the two species. There is an inversion in this part of the chromosome in C. tentans which is found in Germany and Sweden (Beermann 1955 a ) as well as in the British Isles. Beermann has seen the same inversion in a larva of C. pallidivittatus , so that this was very likely present in the common ancestor of these species. It is improbable that the two inversions can have had an independent origin, and it is interesting that in C. tentans (and perhaps C. pallidivittatus ) the inversion occurs in males only (Beermann 1955 b ). This inversion cannot be an indispensable part of the sex-determining mechanism, as I have found it in negligible numbers in some samples, and it never occurs at a high frequency. In C. tentans there are a number of other inversions which are often found in the same chromosome, so that there must be a selective advantage for certain combinations of inversions in the male only. In the Oxford area there are no less than four non-overlapping inversions found in coupling with the ‘male’ inversion; though only four such combinations were found without the ‘male’ inversion, forty-three were found with it. Beermann has found that there is another inversion in another chromosome which also is found only in males.
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