Species specificity of mitochondrial DNA flanking the 18S rRNA gene in two species of cucurbitaceae

Mitochondrial DNA is evolving slowly among species of Brassica and Lycopersicon. In contrast we have observed in cucurbits that at least 101 kilo bases (kb) out of 330 kb of watermelon mitochondrial DNA sequences are not found in muskmelon. The organization of the DNA flanking the mitochondrial 18S rRNA genes was compared in watermelon ( Citrullus vulgaris Schrad., cv. Dixie Queen) and muskmelon ( Cucumis melo L., cv. Iroquois), which differ 7-fold in mitochondrial genome size. Seventeen clones containing the 18S rRNA gene, averaging 14.9 ± 2.0 kb in size, were isolated from a lambda 1059 library of muskmelon mitochondrial DNA. The data are consistent with a single 18S rRNA gene in muskmelon mitochondrial DNA. Electron microscopy of heteroduplexes between watermelon and muskmelon clones containing the 18S rRNA gene show that the regions flanking the 18S gene in these two species are unrelated in nucleotide sequence. Southern blot analysis showed that the flanking sequences are unique to the respective species. At a reduced hybridization criterion these flanking sequences showed additional inter and intra species hybridization to both mitochondrial and chloroplast DNA indicating that the flanking regions contain either short reiterated sequences or longer sequences that have diverged greatly. Thus, in a region spanning 5% of the entire watermelon mitochondrial genome only a single highly conserved sequence, the 18S rRNA gene, is found.