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In plants, the location, size, shape, and number of secondary constrictions and related structures sometimes differ among species or among populations or individuals of the same species. The number of secondary constrictions or nucleolar organizer regions (NORs) varies in number in many species. The size of the secondary constriction also fluctuates in organisms such as Ornithogalum montanum, as revealed by conventional staining (Maggini et al. 1980), and Pseudotsuga menziesii, shown by chromomycin A3 banding (Hizume and Akiyama 1992). The size differences are explained by variation in the number of 45S rDNA genes as a result of unequal recombination. Variation in the number of rDNA repeats among individuals has been demonstrated using molecular techniques in flax (Timmis and Ingle 1973), Zea mays (Givens and Phillips 1976), Pisum sativum (Cullis and Davies 1975), Allium fistulosum (Yampol and Pospelov 1978), 0. montanum (Maggini et al. 1980), and P menziesii (Strauss and Tsai 1988). The addition of a secondary constriction with the ability to behave as a mobile genetic element was reported in Allium hybrid plants (Schubert 1984) and Oryza species (Shishido et al. 2000). Despite widespread reports of variation in the number of secondary constrictions, the origin of these differences has not been investigated. The Scilla scilloides complex is comprised of diploids and polyploids of genomes A and B. In the amphidiploid plants (AABB), except for plants growing in the Okinawa islands, and polyploids (ABB, ABBB) derived from the amphidiploid, secondary constrictions of genome B remain, but the secondary constriction of genome A is lost (Araki 1972, Araki et al. 1979, Haga and Noda 1976). In situ hybridization (ISH) using a 45S rDNA probe revealed that loss of the secondary constriction was a consequence of a large deletion of 45S rDNA (Hizume and Araki 1994), not of differential amphiplasity, as in interspecific hybrids of Crepis (Nawashin 1934), Hordeum (Subrahamanyam and Azad 1978), triticale (Lacadena et al. 1984), wheat (Mukai et al. 1991), and Aegilopus (Mukai 1996). The secondary constriction of the longest chromosome of genome B (b1 chromosome) moved as a result of a translocation of the chromosome arm carrying the secondary constriction in

A New 45S rDNA Locus and Its Genomic Origin in the Amphidiploid of Scilla scilloides Druce.