Chromosome inversion polymorphism in Drosophila pseudoobscura

    D. pseudoobscura is a wild relative of D. melanogaster. The Standard (ST) arrangement of the third chromosome has a characteristic polytene banding pattern (A-H). 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). The inversion types are characteristic of particular geographic populations.