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The viatica group (Acridoidea : Eumastacidae : Morabinae) includes about eight presumptive species of grasshoppers. The group as a whole is distributed from the west coast of the Eyre Peninsula and the Flinders Ranges in South Australia through parts of western New South Wales and the mallee country of north-western Victoria. One species (viatica) extends throughout coastal Victoria as far east as the Bairnsdale area and also occurs in Tasmania. The present paper deals with the cytogenetics of what we call the "coastal" forms. Certain other inland members of the group which occur in the mallee country, in the Flinders Ranges, and in western New South Wales will be dealt with in a later paper. The primitive karyotype of the group has 2nB = 19, there being separate "A" and "B" acrocentric chromosomes, a "CD" metacentric and six pairs of smaller autosomes. The primitive sex chromosome mechanism was of the XO type, but three separate X-autosome fusions have occurred in the phylogeny of the group, giving rise to XY (B) races of originally XO (B) species. In two of these the neo-XY mechanisms have arisen through the usual process of centric fusion between an acrocentric X and an autosome; in the third case, however, the fusion was a tandem one, and the XY bivalent is consequently unique for orthopteroid insects in that it is postreductional, the X and Y segments separating at the second anaphase rather than at the first one. In addition to X-autosome fusions, a centric fusion between two autosomes is present in two taxa of the viatica group. In one of these a local race is homozygous for a translocation between the two largest chromosomes. A number of small pericentric inversions have reached fixation in certain populations, races and species. Hybrids between many of the species and races have been reared in the laboratory. Hybrid progenies consisted of males and females in approximately equal numbers and development of the gonads in both sexes was normal. The meiosis of the hybrids provides evidence as to the course of evolution of the group. It is concluded that the "coastal" species and races have been derived from an ancestral species, "proto-viatica" as a result of a number of chromosomal re-arrangements which have spread out from their points of origin, giving rise to the present-day mosaic pattern of geographic distribution. This type of speciation is discussed in relation to the well-known "allopatric" and "sympatric" models.

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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.

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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.

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