THE HETEROCHROMATIN OF GRASSHOPPERS FROM THE CALEDIA-CAPTIVA SPECIES COMPLEX .2. CYTOLOGICAL ORGANIZATION OF TANDEMLY REPEATED DNA-SEQUENCES

C-band variation between the Caledia taxa is extensive with numerous large interstitial and telomeric blocks of heterochromatin being present in the South-east Australian and Moreton taxa while the Torresian types possess small centromeric or telomeric C-bands. In situ hybridization using 3H-cRNA from a 168 bp (base pairs) highly repeated sequence, originally isolated from the South-east Australian taxon, defined further variation between the C. captiva taxa. This sequence family is present in reach of the interstitial and telomeric constitutive heterochromatic blocks in the South-east Australian and Moreton taxa. However, it is represented in only a fraction of the heterochromatic regions, defined by C-banding, within the 3 Torresian types. A second, unrelated 144 bp sequence family, originally isolated from the Daintree taxon, is restricted to the procentric blocks of heterochromatin of chromosomes 2-7, 9 and 10 in the Daintree taxon. This sequence is A-T rich and possesses a region of dyad symmetry. Quantitative measurements for the 2 sequence families revealed a wide range of copy numbers between the C. captiva taxa. The 168 bp family has .apprx. 150,000, 35,000 and 4,000 copies, respectively, in the South-east Australian/Moreton, Torresian and Daintree genomes. There are 2,000,000 and 100,000 copies of the 144 bp sequence in the Daintree and Papuan Torresian taxa, respectively. The distributional, quantitative and sequence characteristics of these repeat families imply that past amplification or introgression has played a major role in the evolution of these sequences. There is an overall negative correlation between the quantity of the 168 bp sequence and the levels of reproductive isolation and genic divergence between the various taxa. It is possible that some of the reduction in the viability of the hybrid individuals is due to quantitative changes in these sequences. Moreover, the quantitative and qualitative characteristics of highly repeated DNA families may play a role in the modulation of such essential cellular functions as cell cycle duration, nuclear organization and gene expression.