The Clopteryx-splendens-cline in southwestern France, analysed by quantitative wingspot analysis (Zygoptera: Calopterygidae)

It is confirmed that the reproductive barreer between Calopteryx xanthostoma and the C.-”splendens r”-complex N of it is not imperious. Considerable gene flow occurs between both groups. However, the dine is not symmetrical: S of the contact zone. C. xanthostoma is homogeneous, whereas N of it, a genetic gradient, expressed in continuous variation of the apical extent of the 6 wingspot, extends over many hundreds of km. The possible existence of a third genetic component, of eastern origin, cannot be ruled out. Also, it is presently impossible to decide in which direction introgression proceeds. While the Adour R. basin is inhabited by xanthostoma only, some pure xanthostoma populations on the southern tributaries of the Garonne basin are as probable to be pioneers as relicts. — Interbasin crossings, the very base of gene flow, proceed primarily via source areas of higher order tributaries, not via watershed crossings in lowland areas. The Calopteryx populations of different tributaries to a same major river may, therefore, be more different among themselves than populations of adjacent high-order streams belonging to different drainage basins.