Population Genetics of Mexican Drosophila VI. Cytogenetic Aspects of the Inversion Polymorphism in Drosophila pseudoobscura

Abstract

Cytogenetic aspects of chromosomal polymorphism in Drosophila pseudoobscura populations from Mexico are summarized. Eight new inversions of the third chromosome are described and guides to their identification are provided. An updated phylogenetic tree of inversions in D. pseudoobscura is given, and the central role of the Tree Line gene arrangement is emphasized. We consider the possibility that Tree Line is the ancestral gene arrangement in the D. pseudoobscura-D. persimilis-D. miranda triplex of sibling species. The distribution of breakpoints and lengths of inversions are considered. Naturally-occurring inversion breakpoints appear to be uniformly spread over the third chromosome. Short inversions, involving less than 20% of the chromosome, are usually rare and endemic. Among longer inversions there is no relationship between cytological length and success in terms of geographic distribution and frequency The remarkable persistence of rare gene arrangements underscores the way D. pseudoobscura populations store genetic variability.