Sex-Chromosome Inversions in Chironomus

Generally, the only guide to the sex of a larva of Chironomus is the structure of its gonads; the giant chromosomes are normally the same in male and female. However, there are occasional exceptions. In C. tentans there are two inversions, each in a different chromosome, which are inherited from male to male, and so must be in the Y-chromosomes (Beermann, 1955). These results were obtained using insects from Germany and Sweden. As no larvae were found in which these particular inversions occurred together, and as no homozygotes for either inversion were ever seen, my own observations (Acton 1956) suggest that the same may be true in England. Except perhaps for a population in Sweden, neither of the inversions is found in all males, so that many are not distinguishable from females by their giant chromosomes. In contrast, every male larva of C. annalarius was found to be heterozygous for a complex inversion (Beermann, 1955); again the samples were from Germany and Sweden. It is important that the inheritance of these inversions should be determined in larvae from other countries to see whether geographical isolation has allowed the populations to diverge in respect of their sex-chromosomes. I have sexed a total of 105 C. annularius larvae from widely separated localities in Britain, and of these 46 were males; with one doubtful exception, all the males were heterozygous for the inversion described by Beermann. One of the 59 females was heterozygous for a different inversion in very much the same part of the chromosome as the one just mentioned, but none of the other females had an inversion in this chromosome. Thus over a wide area all larvae of C. annularius which have been sexed have an inversion in the Y-chromosome; as would be expected, neither Beermann nor I have seen homozygotes for this inversion. I have studied three other species of the subgenus, but two of them are not yet certainly identified; these are C. cingulatus and C. plumosus. Though I have examined only a few larvae of the former species, these were enough to show that no inversion is found in all males. This point is made because parts of the banding-pattern of the chromosomes seem to be similar to parts in C. annularius, and in addition the imagines are almost indistinguishable. As there is a reason for supposing that the two species are closely related, the comparison is of some value.