Cytology of a pentaploid hybrid and genome analysis in Solanum nigrum L.

Cytology of a pentaploid hybrid (2n=60) produced by crossing an autotetraploid (2n=48) of a diploid race of Solanum nigrum L. as seed parent with the naturally occurring hexaploid S. nigrum (2n=72) was studied which showed approximately 24_II+12_I. Chromosome pairing in the gametic complement of the hexaploid was deduced to be 12_II+12_I and on this basis it is concluded that the natural hexaploid is an autoallohexaploid, which has three sets (genomes) of 12 chromosomes each, two of which are to a large extent homologous to each other. Theoretically the genomic formula can be tentatively written as AAAABB. Since the natural hexaploid does not show any quadrivalents between the four homologous A genomes, chromosome pairing appears to be restricted to bivalent formation by a special genetic mechanism. The probability that S. nigrum could be an autohexaploid, parallelling Pheleum pratense, has been discussed.