SIMILARITIES AND RELATIONSHIPS AMONG POPULATIONS OF THE BULB ONION ASESTIMATED BY NUCLEAR RFLPS

Random nuclear restriction fragment length polymorphisms (RFLPs) were used to assess similarities and relationships among open-pollinated (O P) populations of the cultivated bulb onion (Allium cepa). Seventeen O P 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 enzyme s. Sixty (58%) clones detected at least 1 polymorphic fragment scorabl e among the OP populations and were used for analyses. The average num ber of polymorphic fragments per polymorphic probe-enzyme combination was 1.9, reflecting that numerous monomorphic fragments were usually p resent. Similarities were estimated as the proportion of polymorphic f ragments 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 estimate d by parsimony, cluster analysis of similarities using the unweighted- pair-group method (UPGMA), and multivariate analysis using principle c omponents. Parsimony analysis generated a strict consensus tree that g rouped all but 1 LD onion with unresolved relationships to the SD OP p opulations. The UPGMA analysis placed together the LD storage OP popul ations. Principal component analysis grouped all but 2 LD onions; the other OP populations were dispersed. The results suggest that LD and S D onions do not represent distinct germ plasm, but that LD storage oni ons represent a derived group selected for production at higher latitu des. If it is assumed that the sampled populations are representative of all onion OP populations, the lower similarities among SD OP popula tions indicate that their collection and maintenance in germ plasm col lections is important for the preservation of genetic diversity.