Plasmid preparations were made from 110 isolates of Enterobacter cloacae taken from the guts of members of the Caledia captiva complex of grasshoppers to ascertain whether a relationship exists between these extrachromosomal elements and taxonomic variation in the grasshoppers themselves. Fifty-two plasmids, distinguishable by mobility or restriction fragment pattern differences, were identified. Thirty-seven of these were similar in size. Five plasmids were nick translated and used to probe Southern blots. Only three instances of cross homology with another plasmid were found, implying a very high level of sequence diversity in the samples. No explanation of the size uniformity and sequence diversity of the plasmids is entirely satisfactory but it appears most likely that the variation is maintained to serve a variety of adaptive functions. No plasmid was found in grasshoppers of more than one taxon of C. captiva. This may be due to geographical limitations on the distribution of plasmids. If this is so, it remains possible that there is an association of one or more plasmids with taxonomic divergence in this grasshopper complex. Plasmid preparations were also made from 68 bacterial isolates (predominantly E. aerogenes) from laboratory-reared Locusta migratoria and from 72 isolates from other acridid grasshoppers. Plasmids of the size general in C. captiva were discovered in most of these isolates. Some smaller plasmids were also found. As judged by restriction endonuclease digests and Southern blotting, plasmid diversity is much less in this sample of L. migratoria bacteria than in the field-collected C. captiva. The plasmids reported in this paper may be considered as possible vectors for use in the genetic control of locusts. Key words: host–parasite, plasmids, grasshoppers, Enterobacter.
Spatial ordering of charge and spin in quasi-one-dimensional Wigner molecules