We discuss a series of studies in our units employing molecular genetic markers in collection management, primarily for identity and diversity assessment and partitioning of genetic variation. Isozymes, random amplified polymorphic DNAs (RAPDs), and simple sequence repeat DNAs (SSRs) have been used for these purposes. We analyzed a range of Brassica oleracea accessions at six isozyme loci. Unique isozyme profiles (or fingerprints) were found for 40% of the individual genotypes within accessions. While isozymes were extremely valuable for partitioning genetic variability between and among subspecies, they failed to identify accessions and subspecies. Furthermore, relationships found did not correspond to those predicted by taxonomy. In a study of three species of Chinese vegetable brassicas using 112 RAPD markers, we were able to unambiguously distinguish all 52 accessions studied, despite some intra-accession variability. In addition, cluster analysis correctly grouped all individuals of the same species, but below that rank, taxonomic groupings occasionally broke down. RAPD profiles were found that unambiguously distinguished the three Brassica species from one another, but, for subspecies, no such profiles were found. In another RAPD study of B. oleracea subsp. capitata (cabbage), a closely related set of cultivars were not distinguishable, although more distantly related cultivars were. We had disappointing results with a RAPD study of Vitis accessions. DNA was extracted from the leaves of 23 greenhouse-grown and 52 field-grown vines. Twelve of the 23 greenhouse vines were rooted cuttings collected from 12 of the field-grown vines. Unfortunately, the RAPD profiles of all vines grown in the same location (whether greenhouse or vineyard) were more similar to one another than were profiles from the same clone grown in the two different locations. We are studying whether this result is due to physiological differences in plants growing under different conditions, to differences between PCR reagent lots, to pathogen infestation, or to DNA sample contamination. In a study of 23 accessions representing 15 Vitis species and three species hybrids, we used six different SSR markers to identify individual genotypes. We were able to unambiguously distinguish all genotypes, except two that were identical at all six loci. Review of planting records revealed that the two genotypes were probably the same grape clone. SSR results were not congruent with known taxonomic relationships or geographic origin of genotypes. The SSR polymorphisms found in even this small subset of the Vitis collection in principle make possible the identification of more than 130 trillion different genotypes. This high level of polymorphism, however, makes our particular SSR loci of limited use for identification of species and for the determination of genetic relationships. Molecular genetic markers offer a powerful, efficient approach to assessing questions of identity, relationship, and diversity in germplasm collections, but markers need to be selected based on their suitability for the particular task.
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