Mighty mites

Natural History, Dec, 2004 by Robert H. Cruickshank

In "The Sex Lives of Scales" [9/04], Benjamin B. Normark describes the bizarre sex lives of scale insects. Mites constitute another group of animals with an equally beguiling variety of genetic systems. In mites, arrhenotoky (in which fertilized eggs produce female offspring and unfertilized eggs produce haploid male offspring) has arisen, independently; at least nine times, and paternal genome elimination (PGE) at least once.

Mr. Normark mentions the pioneering work of Sally Hughes-Schrader. In 1931 she hypothesized that PGE is a necessary step in the evolution of arrhenotoky. Recent molecular analyses of mites support her hypothesis, indicating that at least some of the arrhenotokes arose from ancestors with PGE. That raises an intriguing question: Why has PGE been retained in scale insects and in mites? The simple answer is that no one knows; as Mr. Normark points out, however, it could be due to the influence of symbiotic bacteria. Perhaps a few of those biologists "subtle enough to resist the allure of prettier creatures" should consider investigating the mites.

Robert H. Cruickshank

Lincoln University

Canterbury, New Zealand