Cytogenetics of the grasshopper Moraba scurra. 2. heterotic systems and their interaction.

Moraba scurra Rehn shows inversion polymorphism in two different chromosome pairs, ."CD" and "EF". Each of these systems produces a heterotic effect on male viability. The cytological polymorphism of the CD pair is present in almost all populations except in the south-western part of the distribution area of the species (Tumut, N.S.W., to Merton, Vic.), where only the Standard sequence is present. Some populations have the Standard CD much more frequent than the Blundell CD, while in other localities the relationship is reversed and in five colonies a third sequence, Molonglo, was found, either replacing Standard or coexisting with Standard and Blundell. The cytological polymorphism of the EF chromosome is distributed rather irregularly throughout the geographic area occupied by the species. Many populations have only the Standard type of EF chromosome and even in those which also contain the Tidbinbilla sequence the latter is always greatly outnumbered by Standard. In populations containing both cytological polymorphisms, these are apparently not combined at random, there being a genetic interaction between them, as far as the viability of the males is concerned. This interaction leads to a deficiency of Bl/Bl, St/St individuals and to a further deficiency of genotypes in which Tidbinbilla coexists with the Standard CD chromosome, either in the heterozygous or the homozygous condition. On the assumption that the overall selective values of these genotypes are generally similar to their effects on male viability, those populations which possess both heterotic mechanisms must have a complex balanced polymorphism, involving equilibria between the alternative gene sequences of the two different chromosome pairs, as well as between those of the same pair.