Chromosomal polymorphism is described for natural populations of Chironomus pallidivittatus in both the Palearctic and Nearctic regions. The Palearctic populations studied exhibit 24 banding sequences, whereas 10 banding sequences have been recorded from Nearctic C. pallidivittatus. In total, 29 sequences and 37 genotypic combinations have been found. Of the 29 sequences known, only 5 are Holarctic (common to both the Nearctic and Palearctic), 19 are exclusively Palearctic, and 5 are Nearctic. The karyotype of Nearctic C. pallidivittatus is characterized by specific, homozygous Nearctic sequences in arms B and G and fixed Holarctic inversion sequences in the other arms. Only two chromosome arms in C. pallidivittatus, but all seven arms in the sibling species Chironomus tentans, differ between Palearctic and Nearctic forms by the presence of unique, homozygous sequences in the Nearctic karyotype. This indicates a great difference in the cytogenetic histories of these closely related species; much less karyotypic divergence between continents has occurred in C. pallidivittatus than in C. tentans. The cytogenetic distance between Palearctic and Nearctic populations of C. tentans is higher (DN = 1.62) than in C. pallidivittatus (DN = 0.27). Thus, Palearctic and Nearctic C. tentans should be regarded as sibling species, but Palearctic and NearcticC. pallidivittatus are best viewed as strongly divergent races of the same species. A photomap of polytene chromosomes of C. pallidivittatus is presented in which banding sequences are mapped by using C. tentans as a standard.
A comparative study of chromosomal polymorphism in certain south american species of Drosophila