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THE INVERSION POLYMORPHISM OF KEYACRIS SCURRA AND THE ADAPTIVE TOPOGRAPHY

White (1956) described two chromosome inversion polymorphisms in the grasshopper Moraba scurra which is now known as Keyacris scurra (Key, 1965). The CD chromosome may be standard in sequence (herein designated as St) or may carry the Blundell inversion (Bi). The EF chromosome may carry the Tidbinbilla inversion (Td) as an alternative to the standard sequence (St'). The results of an intensive study of this system at a number of locations in eastern Australia between 1956 and 1962 were reported and analysed by Lewontin and White (1960) and White et al. (1963). Wright (1949) defined an "adaptive surface" for the prediction of frequency changes and equilibrium compositions for multilocus polymorphisms. Lewontin and White (1960) used a similarly conceived two-dimensional "adaptive topography" to represent the interactive effects of the two chromosome inversions in K. scurra on the viability of males. Their application of the technique remains the most convincing demonstration of its practical relevance to predictions involving more than one polymorphism. Moran (quoted in White et al., 1963) pointed out, however, that the method of calculating genotype viabilities using the current genotype frequencies must lead to the actual inversion frequencies representing the composition of a population in either stable or unstable equilibrium. The majority of K. scurra populations studied had inversion frequencies which placed them at unstable equilibria, although the available evidence suggested that the inversion frequencies were quite stable (Lewontin and White, 1960). Subsequent attempts at a resolution of this apparent paradox have been directed at the alteration of the topographies so that the observed inversion frequencies correspond to peaks (stable equilibria) and not saddles (unstable equilibria) in the adaptive topography. This is accomplished by the relaxation of one or more of the assumptions that were implicit in the original calculations. For instance, while random mating was assumed it is quite possible that inbreeding has an effect on the topographies (Allard and Wehrhahn, 1964). There may also be an interaction between this inversion system and an as yet unrecognized genic or chromosomal factor. More especially there may be differences in the viabilities of males and females of a particular genotype (Turner, 1972). Further, White et al. (1963) recognized that there may be fertility differences between genotypes, that the selection coefficient may be frequency-dependent and that the year-to-year environmental changes may well cause temporal fluctuations in the viabilities of the genotypes. Turner (1972) calculated topographies which allow for the possibility of linkage disequilibrium between the inversions. Investigative efforts, after 1962, were directed at the determination of the practical importance of these factors. The grasshoppers were collected by M. J. D. White and 0. R. Byrne and were karyotyped by M. J. D. White and J. Cheney, who kindly provided the results for analysis. The data which are published for the first time in this paper extend the knowledge of the genetic history of the Murrumbateman population to 11 years. They also