Variability and uniformity of mitochondrial DNA in populations of putative diploid ancestors of common wheat.

By using restriction endonuclease digestion patterns, the degree of intraspecific polymorphism of mitochondrial DNA in four diploid species of wheat and Aegilops, Ae. speltoides, Ae. longissima, Ae. squarrosa, and Triticum monococcum, was assessed. The outbreeding Ae. speltoides was found to possess the highest degree of variability, the mean number of nucleotide substitutions among conspecific individuals being 0.027 substitutions per nucleotide site. A very low degree of mtDNA variation was detected among Ae. longissima accessions, with most of the enzyme-probe combinations exhibiting uniform hybridization patterns. The mean number of substitutions among Ae. longissima individuals was 0.001 substitutions per nucleotide site. The domesticated diploid wheat T. monococcum var. monococcum and its conspecific variant T. monococcum var. boeoticum seem to lack mitochondrial DNA variability altogether. Thus, the restriction fragment pattern can be used as a characteristic identifier of the T. monococcum cytoplasmic genome. Similarly, Ae. squarrosa accessions were found to be genetically uniform. A higher degree of variation among accessions is observed when noncoding sequences are used as probes then when adjacent coding regions are used. Thus, while noncoding regions may contain regulatory functions, they are subject to less stringent functional constraints than protein-coding regions. Intraspecific variation in mitochondrial DNA correlates perfectly with the nuclear variability detected by using protein electrophoretic characters. This correlation indicates that both types of variation are selectively neutral and are affected only by the effective population size.