Hierarchical analyses of genetic differentiation in a hybrid zone of Sorexaraneus (Insectivora : Soricidae)

Microsatellites are used to unravel the fine-scale genetic structure of a h ybrid zone between chromosome races Valais and Cordon of the common shrew ( Sorex araneus) located in the French Alps. A total of 269 individuals colle cted between 1992 and 1995 was typed for seven microsatellite loci. A modif ied version of the classical multiple correspondence analysis is carried ou t. This analysis clearly shows the dichotomy between the two races. Several approaches are used to study genetic structuring. Gene flow is clearly red uced between these chromosome races and is estimated at one migrant every t wo generations using X-statistics and one migrant per generation using F-st atistics. Hierarchical F- and R-statistics are compared and their efficienc y to detect inter- and intraracial patterns of divergence is discussed. Wit hin-race genetic structuring is significant, but remains weak. F-ST display s similar values on both sides of the hybrid zone, although no environmenta l barriers are found on the Cordon side, whereas the Valais side is divided by several mountain rivers. We introduce the exact G-test to microsatellit e data which proved to be a powerful test to detect genetic differentiation within as well as among races. The genetic background of karyotypic hybrid s was compared with the genetic background of pure parental forms using a C RT-MCA. Our results indicate that, without knowledge of the karyotypes, we would not have been able to distinguish these hybrids from karyotypically p ure samples.