Chromosome inversion polymorphism in Drosophila pseudoobscura

    D. pseudoobscura is a wild relative of the common laboratory fruit fly, D. melanogaster. The Standard (ST) arrangement of the third chromosome is characterized by a particular polytene banding pattern, due to alternating euchromatic (light) and heterochromatic (dark) gene regions. A paracentric inversion involving about a third of the chromosome produces the Arrowhead (AR) arrangement. Chromosome pairing in meiosis requires formation of a characteristic inversion loop in order to bring homologous gene segments (region C-G) together. Individuals with two different third chromosomes are called heterokaryons (by analogy with "heterozygotes"; individuals with identical chromosomes are homokaryons). Other chromosome polymorphisms (such as Chiricahua [CH]) involve multiple inversions, and inversions-within-inversions, resulting in complicated loops (below).

    Th Dobzhansky's decades-long study of polytene patterns in Drosophila was the first direct evidence of extensive genetic polymorphism in a wild species. He was able to link pattern frequencies with adaptive changes in wild populations, including altitude and seasonal changes in temperature and aridity.