A study of gene flow, divergence, hybridization and speciation using chromosomal and molecular genetic markers has been carried out on 2 grasshoppers, the Iberian Chorthippus parallelus erythropus (Cpe) and the North European C.p.parallelus (Cpp) which form a hybrid zone along the Pyrenees. Results have shown that these 2 taxa differ in many characters including song, colour, stridulatory pegs, ovipositors, isoenzymes, nucleoli, heterochromatin content and distribution, levels of methylation in ribosomal deoxyribonucleic acid (rDNA) sequences, particular short non sense DNA sequences.
In a programme of crossing between the 2 taxa, laboratory hybrids show reduced fertility in the male, as well as an asymmetric tendency to homogamy. The C banding technique has shown that introgression of the Cpp sex chromosome has occurred into the range of the other subspecies Cpe. There is a narrow contact region where various recombinant sex chromosomes have been recorded. Silver staining has also shown that the expression of the nucleolar organisers (NOR) decreases through the hybrid zone.
Analysis of repetitive DNA by restriction enzymes showed the same patterns in both subspecies. Therefore such restriction enzyme analysis has been considered not suitable for the analysis of both subspecies. Similarly, Southern blot analysis suggested no difference between subspecies in rDNA copy number although there is evidence of intraspecific variation. The random amplified polymorphic DNA (RAPD) technique was found to be more promising in identifying sequence differences.
Several Lambda clones from the Cpe library have been sequenced. Primers for these sequences have been designed, manufactured and used to amplify Cpp DNA. These amplifications are then sequenced and compared with the Cpe sequence, the different sequences being used as molecular markers. Using this method it was shown that an insertion consisting of 5 base pairs (bp) was found in Cpe but not Cpp. The sequence containing the insertion is being used to analyze the hybrid zone.
The nature and significance of barriers to gene exchange can be particularly well studied in hybrid zones. This is important for an understanding of genetic divergence, speciation and the genetic basis for controlling pests and biological resources.
New techniques of chromosome and molecular genetics will be used to located markers that differentiate two subspecies of Orthoptera that form a hybrid zone in the Pyrenees. This combined with breeding experiments and population studies will open up analysis of gene flow and genetic control of pre and post mating barriers, including acoustical communication and male fertility.
Funding SchemeCSC - Cost-sharing contracts
NR4 7TJ Norwich