Cytogenetics of the viatica group morabine grasshoppers. I. The coastal species

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.