The dynamics of B chromosomes in populations of the Australian plague locust, Chortoicetes terminifera (Walker)

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Abstract
The B chromosomes of C. terminifera possess an accumulation mechanism. B chromosomes were transmitted at a rate of 0.766 in 1 B(.tau.) .times. 0 B(.male.) crosses and 0.477 in 0 B(.tau.) 1 .times. B(.male.) crosses. In crosses where the female had a B chromosome, there were significant differences in transmission rates between pods, but these were not related to differences in G-banding patterns of the B chromosomes involved. In crosses where either the male or female parent had 2 B chromosomes the B chromosomes did not segregate perfectly, nor did they segregate in a random fashion. The closest resemblance to the behavior or normal bivalents occurred when the 2 B chromosomes were of the same, rather than different, G-banding variants, and when they were present in the male rather than the female parent. B chromosomes occurred at 8 localities scattered throughout New South Wales. No locality was found where they were not present. The average frequency of B chromosomes was 14.0% for 1-B individuals and 0.8% for 2-B individuals. There were no significant differences in B-chromosome frequency between males and females. Of the 8 localities, only 1 had a B-chromosome frequency significantly different from any other locality. This relatively uniformity of B-chromosome distribution is interpreted as a consequence of the migratory nature of C. terminifera. A model was constructed to simulate the dynamics of B chromosomes in locust populations. Three main dynamic patterns were recognized, and these were related to differences in the fitness of 1-B and 2-B individuals. The B chromosome of C. terminifera is probably parasitic, although the simulation model revealed some difficulties which suggest that caution should be applied to the description of any B chromosome as purely parasitic.