STRUCTURE OF THE HYBRID ZONE IN CALEDIA-CAPTIVA

The “Moreton” and “Torresian” races of Caledia captiva are parapatrically distributed in south-east Queensland. Although no obvious morphological differences exist between these taxa, there is a very high level of karyotypic divergence. An analysis of the structure of the contact zone has revealed that the transition from one race to the other occurs very abruptly, with approximately 65 per cent of the change in racial frequencies occurring in a 200 m interval. The total width of the zone of hybridisation is 1 k and within this area, mating occurs randomly within and between the races. Analyses of the karyotypic data have established that there is rapid elimination of“Moreton” autosomes on the “Torresian” side of the zone, but that “Torresian” autosomes are retained on the “Moreton” side. Further, the patterns of chromosomal gametic disequilibria differ on the two sides of the zone, providing evidence of asymmetrical hybrid breakdown and supporting the hypothesis of one-way introgression of autosomes from the “Torresian” into the “Moreton” race. A deterministic model examines the effect of hybrid breakdown in a contact zone. © 1979 The Genetical Society of Great Britain.