Inheritance in Nicotiana Tabacum. Xix. Identification of the Tabacum Chromosome Replaced by One from N. Glutinosa in Mosaic-Resistant Holmes Samsoun Tobacco.

N ORDER to produce better cultivated plants and animab by hydridizaI tion, the breeder usually relies upon an exchange of genic material between varieties or species. This is accomplished by conjugation of the chromosomes in pairs during meiosis and subsequent crossing over. In interspecific hybrids, however, chromosomes frequently do not conjugate. Therefore the utilization of valuable genic material in species closely related to a particular crop is limited by the ability of their chromosomes to pair in the hybrid. Hybrids between wheat and rye are a case in point. Though they have often been produced, only rarely has a stable strain of wheat with one or more rye characters been secured. BLEIER (1933) and LEDINGHAM and THOMPSON (1938), among others, have obtained negative results. These failures presumably are due to the infrequent pairing of chromosomes in the meioses of the hybrids: most investigators found that the modal number of pairs ranged from zero to two (reviewed by BLEIER 1934 and by O’MARA 1940). There are instances, however, in which stable wheat strains with rye traits have been obtained. In those reported, this was accomplished through the introduction of a pair of rye chromosomes into the wheat complement. KATTERMANN (1937, 1938) thus produced strains of wheat with the rye character “hairy neck.” Cytological studies showed his plants to have 2 0 pairs of wheat chromosomes and one pair of rye. Meiosis was regular, with 21 pairs in 94 percent and nonconjunction in only two per cent of more than 900 cells studied. KATTERMANN introduced the term Substitutionsbastard to designate such a type. However, only organisms having some degree of heterozygosis should be called “hybrids.” In the following the term aliert subbstitution race is being used as proposed by PROFESSOR R. E. CLAUSEN. Another case of substitution is reported from Avena, where NISHIYAMA (1939) replaced a pair of chromosomes from A . fatua by a pair from A . barbata. A similar substitution race has been obtained in Nicotiana. HOLMES (1938) derived a mosaic resistant strain’ of N . Tabacum from a N . TabacumXglutinosa amphidiploid (named ‘(Holmes Samsoun”). The author, in an earlier paper in this series (1943), showed that this was accomplished by the substitution of a pair of N . glutinosa chromosomes carrying the resistance factor for a Tabucumpair in the tobacco genome. In the present paper the identity of the replaced Tabacum chromosome will be demonstrated.