THE GENUS COLLINSIA. XVII. PREFERENTIAL PAIRING IN FOUR AMPHIDIPLOIDS AND THREE TRIPLOID INTERSPECIFIC HYBRIDS

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Abstract
Amphidiploids of 3 diploid interspecific hybrids with 1 or 2 interchange complexes of 4 or 8 chromosomes at metaphase I and with 2 dicentric chromatid bridges and fragments at anaphase I, and of 1 diploid interspecific hybrid with bivalents and univalents but no dicentric chromatid bridge were studied cytologically. Each amphidiploid exhibited a different maximum number of quadrivalents and no dicentric chromatid bridge. Except for a higher chiasmata frequency in 1 amphidiploid compared with its diploid interspecific hybrid, the amphidiploids and their corresponding diploid interspecific hybrid had the same chiasmata frequency. Although amphidiploids were fertile, they were not as fertile as their parental species. In general, the amphidiploids had the same floral morphology as their corresponding diploid interspecific hybrid. Triploid interspecific hybrids exhibited univalents, bivalents, and trivalents but not quadrivalents at metaphase I and no dicentric chromatid bridge at anaphase I; the triploids were sterile. The flowers of the triploid hybrids generally resembled those of the species contributing 2 of the 3 genomes. Chromosome associations in the amphidiploids and in the triploid interspecific hybrids were explained on the basis of preferential pairing of those chromosomes which did not have a structurally idential homologue in the diploid interspecific hybrid, that is, chromosomes involved either in an interchange complex or with a paracentric inversion.