Rapid chromosomal assignment of medaka mutants by bulked segregant analysis.

Genetic screens in medaka are leading to the identification of an increasing number of unique mutant phenotypes. However, so far only a few genes responsible for these phenotypes have been characterized. Furthermore, no protocols using a systematic positional cloning strategy have been developed to determine the implicated genes. The PCR-based bulked segregant analysis is a fast and reliable tool to accomplish the initial steps of the positional cloning of a mutation. Here we describe the selection of a panel of genetic markers that, evenly distributed over the 24 chromosomes of medaka, provide a full coverage of the compact medaka genome (800 Mb) when used in bulked segregant analysis. The reference panel, which consists of 48 EST-derived markers, is anchored to a collection of more than 2000 polymorphic markers, thus facilitating a rapid transition from chromosomal assignment to fine mapping of the mutants. More importantly, since most of the genetic screens have been performed in the inbred Cab strain (derived from the Southern population), the selection of markers included in this panel was intended to optimize the recognition of polymorphisms between Cab and the polymorphic inbred mapping strain Kaga. Here we present a reliable mapping panel, confirmed both by the assignment of the locus responsible for the medaka mutation eyeless/Rx3 to chromosome 12, and by the analysis of its resolution power using representative markers.