A chromosomal cline in the grasshopper Podisma pedestris

The grasshopper Podisma pedestris includes two chromosomal races, which differ by a Robertsonian fusion involving the sex chromosome. The two races meet in a cline which runs for 100 km across the Alpes Maritimes in south-eastern France. An intensive study of the easternmost end of this cline shows that it is about 800 m wide; the cline is not smooth, containing substantial spikes in chromosome frequency which might be due to sampling drift. Though the cline seems narrow, it is wide compared with the dispersal rate of the insect; a selective force of only 0.5% would be enough to maintain the cline. It is difficult to determine the nature of this force, but some evidence comes from the position of the cline, and from the presence of coincident clines at other loci. An estimate of the distribution of Podisma has been made, and the cline seems to follow, for the most part, a region of low population density, suggesting that it is maintained by hybrid unfitness. However, in the one region where the cline is relatively free to move, the XY race bulges forwards more than would be expected if hybrids are unfit. The observation of severe inviability in crosses between the races, though it is not associated with the chromosomal difference, also indicates that this cline is the result of some sort of genetic incompatibility.