Evolution of Drosophila mitochondrial DNAs. Comparison of denaturation maps

Mitochondrial DNA heteroplasmy in Drosophila mauritiana

Mitochondrial DNA extracted from an isofemale strain of Drosophila mauritiana (subgroup melanogaster) appeared to be heterogeneous in size. A short genome [S; 18,500 base pairs (bp)] and a longer one (L; 19,000 bp) coexist in the preparation. The additional 500 bp have been located within the A+T-rich region. Hpa I digest patterns suggest that the S genome may carry a duplication of a 500-bp sequence including an Hpa I site and that the L genome may carry a triplication of the same sequence. At the 30th generation of the isofemale strain, 60 female genotypes were examined individually. Half of the files were pure either for the S or the L DNA. The remaining 50% exhibited various degrees of heteroplasmy for the two DNA types. Among metazoan animals, this D. mauritiana strain offers an exceptional situation with regard to the number of individuals heterogeneous for mtDNA and the relative stability of heteroplasmy through generations.

Contrasting denaturation maps of Xenopus laevis and Xenopus borealis mitochondrial DNAs

Denaturation maps of mitochondrial DNAs of Xenopus laevis and Xenopus borealis are radically different from each other. This is in striking contrast to the invariant denaturation patterns previously recognized among mtDNAs of various Drosophila species, particularly, since the two toads may be even more closely related to each other than the Drosophila species.

Evoluation of Drosophila mitochondrial DNAs. Analysis of heteroduplex molecules

We have mapped the single block of non-homologous sequences and measured the extent and distribution of base-pair substitutions within the homologous sequences in Drosophila melanogaster: Drosophila virilis heteroduplex mitochondrial DNAs (mtDNAs). Of the 4.8 kilobases long, unusually (A + T)-rich region in D. melanogaster mtDNA, only 0.5 kilobases can react with related, but not identical sequences in D. virilis mtDNA, while the rest (4.3 kilobases in the long arm of a heteroduplex loop) is replaced by a shorter, non-homologous region (1.0 kilobases in the short arm of the loop). No additional heterologous regions are evident. Homologous sequences have accumulated on the average 15.5% base-pair changes. Regionally, these substitutions are relatively uniformly distributed (14.5--16.5%) except for a single, more conserved region (10--13%), which presumably represents the ribosomal cistrons. The lack of general sequence stability suggests that the invariant topographic organization of the nucleotide sequence, previously recognized among Drosophila mtDNAs, is under more stringent selection than the sequence per se.

Positioning of the A . T-rich regions in rat mitochondrial DNA by electron microscopy and analysis of the hysteresis of denaturation

Intramolecular heterogeneity in the base composition of rat mitochondrial DNA (mtDNA) was shown by a combination of an improved denaturation mapping technique using electron microscopy and analysis of high-resolution optical melting-renaturation profiles. Circular mtDNA starts to melt in one specific region and then forms loops in four other regions in random order. These five early melting regions are all located in one half of the molecule. The arrangement of the early melting regions in rat mtDNA bears a remarkable resemblance not only to those of mtDNAs from several species of Drosophila but also to those of several species of Drosophila but also to those of several plasmid DNAs and phage DNA.

Complex mitochondrial DNA in Drosophila

The larval mtDNA isolated from D. virilis, D. simulans and D. melanogaster exists in complex molecular forms in addition to the simple monomeric circular form. The frequency of circular dimers and oligomers is highly elevated in apparently normal larval tissues. These complex forms of mtDNA are separable on agarose gels. Hind III restriction endonuclease and electron microscopic analyses used in the present study have revealed that circular dimers are simply the circular concatemers of two monomeric circles which are arranged in a head-to-tail structure with no detectable heterologous regions such as insertions or deletions. The electrophoretic patterns of Hind III digested mtDNAs of D. simulans and D. melanogaster (sibling species) are identical and distinguishable from that of distantly related species, D. virilis.