EVOLUTION AND STRUCTURAL CONSERVATION OF THE CONTROL REGION OF INSECTMITOCHONDRIAL-DNA

The control regions of mitochondrial DNA of two insects, Schistocerca gregaria and Chorthippus parallelus, have been isolated and sequenced. Their sizes are 752 bp and 1,512 bp, respectively, with the presence of a tandem repeat in C. parallelus. (The sequences of the two repeats are highly conserved, having a homology of 97.5%.) Comparison of thei r nucleotide sequences revealed the presence of several conserved sequ ence blocks dispersed through the whole control region, showing a diff erent evolutionary pattern of this region in these insects as compared to that in Drosophila. A highly conserved secondary structure, locate d in the 3' region near the small rRNA gene, has been identified. Sequ ences immediately flanking this hairpin structure rather than the sequ ences of this structure themselves are conserved between S. gregaria/C . parallelus and Drosophila, having a sequence consensus of ''TATA'' a t 5' and ''GAA(A)T'' at 3'. The motif ''G(A)nT'' is also present in th e 3' flanking sequences of mammalian, amphibian, and fish mitochondria l L-strand replication origins and a potential plant mitochondrial sec ond-strand-replication origin, indicating its universal conservation a nd functional importance related to replication origins. The stem-and- loop structure in S. gregaria/C. parallelus appears to be closely rela ted to that found in Drosophila despite occupying a different position , and may be potentially associated with a second-strand-replication o rigin. This in turn suggests that such a secondary structure might be widely conserved across invertebrates while their location in the cont rol region may be variable. We have looked for such a conserved struct ure in the control regions of two other insects, G. firmus and A. mell ifera, whose DNA sequences have been published, and their possible pre sence is discussed. Mitochondrial control regions characterized to dat e in five different insect taxa (Drosophila, G. firmus, A. mellifera, S. gregaria, and C. parallelus) may be classed into two distinct group s having different evolutionary patterns. It is observed that tandem r epetition of regions containing a probable replication origin occurred in some species from disjunct lineages in bath groups, which would be the result of convergent evolution. We also discuss the possibility o f a mechanism of ''parahomologous recombination by unequal crossing-ov er'' in mitochondria, which can explain the generation of such tandeml y repeated sequences (especially the first critical repetition) in the control region of mtDNA, and also their convergent evolution in disju nct biological lineages during evolution.