A CHROMOSOMAL CLINE IN THE GRASSHOPPER PODISMA PEDESTRIS.

Abstract

Narrow clines, especially those which involve chromosomal differentiation, have excited a great deal of interest recently, primarily because they are suggestive of incipient speciation (see Endler, 1977; White, 1978). Hewitt (1975), following on the work of John and Hewitt (1970), and Hewitt and John (1972), described a narrow interface between two chromosome races of the alpine grasshopper, Podisma pedestris. Over most of its range, which extends over Siberia, the European Alps, the northern Appennines, and the Pyrenees, this insect has (as far as we can tell) an XO sex chromosome system. In the Alpes Maritimes, however, a Robertsonian fusion between an acrocentric autosome and the X chromosome has led to the fixation over a wide area of a neo-XY sex chromosome system. The two races meet in the high mountain ridge which runs from east to west along the FrenchItalian border, and on into France (Fig. 1). Six areas were found in which the two races approached each other closely. At four of these, mixed populations, which included heterozygous females, were found. A detailed study has been made of the easternmost end of the cline, near Col de Tende (Fig. 2). This paper presents the results of the intensive mapping of karyotypic frequencies in this region, and discusses the ways in which the chromosomal cline might be maintained.