A study of the differences between widely separated populations of Chironomus (= Tendipes) tentans (Diptera)

Abstract

Chironomus tentans occurs in Europe and Canada and must at some time have migrated from one continent to the other. The banding pattern of the giant chromosomes is inherited directly and cannot be affected by environmental conditions: it is thus a particularly favourable character by which to judge the amount of evolutionary divergence. Populations of C. tentans in Europe have fundamentally identical banding patterns, and in general the inversions which are superimposed on these basic patterns differ only in frequency from one area to another. However, the banding patterns of Canadian C. tentans differ from the European much more than those of the European or Canadian larvae differ among themselves. It is possible to make hybrids between imagines from the two continents and there is little, or no, reduction in fertility. The basic banding pattern of chromosome 1 in Canada is occasionally like the European, except that it has a nucleolar organizer: apart from two inversions, none of those in Canada occurs in Europe and vice versa. No banding pattern of chromosome 2 in Canada is very like the European and the differences (inversions) in this chromosome could be considered to be at the 'full-species level'. Chromosome 3 differs from the European, but not by inversions: it has an additional band and has no nucleolar organizer. There is an inversion of chromosome 3 which occurs on both sides of the Atlantic. Chromosome 4 has the same basic pattern in Canada and Europe, and also has an inversion which is the same in both regions. As with the other three chromosomes there are inversions which occur in Canada only. These findings suggest that there can be little migration between Canada and Europe. The populations on the two continents may even now belong to different races; and perhaps to incipient species.