Similarities and relationships among populations of the bulb onion as estimated by nuclear RFLPs

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Abstract
Random nuclear restriction fragment length polymorphisms (RFLPs) were used to assess similarities and relationships among open-pollinated (OP) populations of the cultivated bulb onion (Allium cepa). Seventeen OP populations and 2 inbreds of contrasting daylength response [termed by convention as long (LD) and short (SD) day], 1 shallot (A. cepa var. ascalonicum), and one cultivar of bunching onion (Allium fistulosum) were examined with 104 cDNA clones and two to four restriction enzymes. Sixty (58%) clones detected at least 1 polymorphic fragment scorable among the OP populations and were used for analyses. The average number of polymorphic fragments per polymorphic probe-enzyme combination was 1.9, reflecting that numerous monomorphic fragments were usually present. Similarities were estimated as the proportion of polymorphic fragments shared by 2 populations. Average similarity values among LD, among SD, and between LD and SD OP populations were 0.79, 0.67, and 0.68, respectively. Relationships among the OP populations were estimated by parsimony, cluster analysis of similarities using the unweighted-pair-group method (UPGMA), and multivariate analysis using principle components. Parsimony analysis generated a strict consensus tree that grouped all but 1 LD onion with unresolved relationships to the SD OP populations. The UPGMA analysis placed together the LD storage OP populations. Principal component analysis grouped all but 2 LD onions; the other OP populations were dispersed. The results suggest that LD and SD onions do not represent distinct germ plasm, but that LD storage onions represent a derived group selected for production at higher latitudes. If it is assumed that the sampled populations are representative of all onion OP populations, the lower similarities among SD OP populations indicate that their collection and maintenance in germ plasm collections is important for the preservation of genetic diversity.