Cytogenetics of the Viatica group of morabine grasshoppers II.Kangaroo Island populations

Abstract

Three members of the viatica group of morabine grasshoppers occur on Kangaroo Island: the 19- and 17-chromosome races of "viatica" and the XY race of "P24". Their distributions are parapatric and fit in well with the respective mainland distributions, assuming a former land connection. Confrontations of these taxa in all three possible combinations occur; two of them have been located, each at one point, and the reproductive situations studied. The zone of overlap, or "tension zone", between P24(XY) and viatica17 was found to be 200-300 m wide and to contain representatives of both forms (as diagnosed by the karyotype), as well as a substantial proportion of cytologically hybrid individuals. Good evidence was secured for a much more extended reciprocal introgression of genes controlling the form of the male cercus and certain anatomical dimensions. The tension zone between P24(XY) and viatica19 seems to be even narrower, but, in spite of close contact between the two forms, no cytologically hybrid individuals were found and no evidence of intergradation in anatomical features; it is concluded that they mate freely with each other, but that the product of these unions is inviable. The apparent reproductive isolation of P24(XY) and viatica19 in their zone of overlap on Kangaroo Island gives support to the view that the accretion of genetic differences along tension zones may lead ultimately to full speciation. However, it is questionable whether P24(XY) and viatica19 themselves should be regarded as distinct species, since on the mainland they form the terminal members of a geographical series of forms, all adjacent members of which almost certainly interbreed.