EMS screens : from mutagenesis to screening and mapping.

The success of Drosophila as a genetic model organism is based on the efficient generation, recovery, and identification of new mutations. Various agents have been used to induce de novo DNA lesions. However, the use of mutagenic alkylating agents, especially ethyl methanesulfonate (EMS), has become a standard approach for mutagenesis that has been succesfully used in the classic forward genetic screens that have defined the field of developmental genetics, as well as in many alternative screening schemes that have since been developed. In this chapter, a basic EMS mutagenesis protocol is introduced, and examples for the fly crossing schemes used in several different types of screen are presented. In addition, some new genome sequence-based approaches are discussed that have alleviated the notoriously difficult molecular mapping of EMS induced point mutations. Together these protocols should allow researchers as yet unfamiliar with Drosophila genetics to take advantage of all the benefits of this mutagenesis method, which include its wide and largely unbiased coverage of the genome, the high mutation frequency, and the variety of null, hypomorphic, conditional (e.g., temperature sensitive), or domain specific mutations that can be caused by EMS treatment.