8. Molecular biology and genetic conservation programmes

The techniques of molecular biology will become a standard part of germplasm conservation and exploitation. They are being used to gather information very rapidly about chromosome structure and genetic variation within the major crop species. Genetic maps with hundreds of DNA sequence markers covering the whole genome have already been created for some crops, such as maize, soybean, wheat and potato. Genetic variation is being revealed by the combined use of restriction endonucleases, fractionation of DNA fragments by electrophoresis and investigation of the size of specific allelic fragments. This kind of approach offers new opportunities to assess the extent of genetic variation among accessions in germplasm collections, thereby helping to decide which accessions are essentially duplicates and which should be maintained in a core collection. I recommend that germplasm banks will in the future also contain diagnostic DNA markers for characterizing and screening germplasm.
When material from germplasm banks is used in crop plant breeding programmes to transfer specific traits into the crop, the availability of a complete set of molecular markers covering the entire genome makes it straightforward to discover which segments have been transferred and which are essential to maintain, so as to preserve the introduced trait.
Germplasm banks are obviously a source of new genetic variation for the molecular geneticist as well as the plant breeder. The isolation of specific alleles determining self-incompatibility from Brassica oleracea accessions for subsequent introduction into oil seed rape is described as an example.     

Mitochondrial Complementation and Grain Yield in Hybrid Wheat

ADP/O ratios, cytochrome c oxidase and adenosine triphosphatasehave been measured in mitochondria and mixtures of mitochondriaisolated from two day-old shoots of wheat of known F₁ hybridgrain yield performance. Mixtures of mitochondria from two varieties,Peko and Cappelle-Desprez, which have considerable F₁ hybridyield heterosis, showed a significantly increased ADP/O ratioover the mean value for mitochondria from the varieties assayedindividually, i.e. these varieties showed ‘mitochondrialcomplementation’. No mitochondrial complementation wasdetected for cytochrome c oxidase or adenosine triphosphatase.In other mitochondrial mixtures no complementation in ADP/Oratios were found even when the varieties showed F₁ hybrid yieldheterosis. Mitochondrial ADP/O ratios were studied in six varietiesindividually and in mixtures. In only one mixture was any significantcomplementation detected. However, when all the results wereconsidered together, mitochondrial complementation was significantlycorrelated with F₁ hybrid grain yield heterosis when the plantswere grown at a low seed density but not at a high seed density.New hypotheses are offered to account for mitochondrial complementationand its statistical relationship with yield heterosis.     

Mitochondrial Efficiency and Grain Yield in Wheat

Mitochondrial ADP/O and RC ratios have been determined in two-day-oldshoots of nine wheat (Triticum aestivum) varieties of knownaverage grain yield. Both ratios were highly correlated withaverage grain yield. ADP/O and RC ratios of germinating shootswere also correlated with yield within a variety when yieldwas varied by irrigation and different nitrogen fertilizer treatments.It is proposed that the observed correlations are the resultof mitochondrial ADP/O and RC ratios being partly controlledduring germination directly or indirectly by seed-based metabolismwhich reflects the metabolic activity and total carbohydratedeposited in seed during seed formation.