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Near-amphihaploid hybrid plants of Nicotiana tabacwn x N. alata disomic for some N. alata chromosomes were nearly completely self- and cross-sterile but under open pollination conditions occasionally set poorly filled seed capsules containing very small numbers of seeds. Six plants from those seeds which survived to maturity resembled N. alata. Upon cytological examinations five plants were found to have the diploid chromosome complement of N. alata (2n=18), which suggested their androgenetic origin following outcrossing with paternal N. alata. The sixth plant had 9 bivalents and 11 univalents, which was indicative of its hybrid origin. Unlike the self-incompatible N. alata parent, N. alata-like hybrid derivatives were self-compatible. Surprisingly, the selfed progeny of the 18-chromosome plants showed sensitivity to tentoxin, a chloroplast trait characteristic of paternal N. alata but hardly compatible with its cytoplasmic lineage since the tentoxin-insensitive N. tabacum was the cytoplasmic parent of the hybrid. The progeny of the 29-chromosome plant was tentoxin-insensitive, which agreed with the parentage of the hybrid. Populations derived from the 18-chromosome plants were highly uniform but differed sharply from one another. The offspring of the 29-plant was highly heterogeneous mainly because of univalent segregation in that progeny.
A vigorous hybrid (N. tabacum cv. TB-566 tetra x N. alata) x N. alata was obtained by backcrossing a partly viable sesquidiploid hybrid N. tabacum cv. TB-566 tetra x N. alata to N. alata. The hybrid was a 35-chromosome near-amphihaploid with a pair of N. alata chromosomes in disomic condition. It was completely self- and cross-sterile and formed from 7 to 8 bivalents in pollen mother cells. By using stem pith culture, polyploidized regenerants were obtained from the 35-chromosome hybrid with somatic chromosome numbers from 65 to 70. These regenerants showed fairly regular meiosis with the number of bivalents in pollen mother cells ranging from 27.3 to 30.4. Irregularities in meiosis included a high number of univalents, aberrant tetrads, and a high frequency of micronuclei. The percentage of acetocarmine-stainable pollen ranged from 22.1 to 78.4. A 66-chromosome regenerant showed fairly regular meiosis and was self-fertile but could not be backcrossed to N. tabacum. This barrier seems to be caused by genic imbalance rather than irregularities of meiotic divisions. Hence transfer programs based on the introgression of entire linkage groups (sexual and somatic hybridization) seem to be of little use in the case of that species.
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