Submitted to: HortScience
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/8/2004
Publication Date: 4/3/2004
Citation: Reed, S. 2004. Cytological Analysis of a Clethra Alnifolia 'Hokie Pink' x C. Pringlei Hybrid. HortScience. 40:339-342. Interpretive Summary: Clethra alnifolia, which is commomly known as summersweet, is a native deciduous shrub that is cultivated as an ornamental. This species was recently hybridized with C. pringlei, which is native to Mexico and has glossy evergreen foliage. New foliage of C. pringlei is a deep bronze-red and is extremely attractive; however, the species is not hardy in most areas of the U.S. The purpose of this hybridization was to combine the cold hardiness and adaptability of C. alnifolia with the ornamental foliage of C. pringlei. While most of the C. alnifolia × C. pringlei hybrids more closely resembled C. alnifolia than the paternal species, a 'Hokie Pink' × C. pringlei hybrid (NA71586) that was intermediate in appearance between its parents was recovered. This study involved examining chromosome numbers and chromosome pairing in the hybrid and the parents and developing a second generation of progeny from the hybrid. Chromosome number of NA71586 was found to be 32. Since 'Hokie Pink' has 32 chromosomes and C. pringlei was found to have 16 chromosomes, it appears that NA71586 resulted from the fertilization of an egg cell of 'Hokie Pink' with a male gamete from C. pringlei that had twice the number of chromosomes as expected. Chromosome pairing indicated that C. alnifolia may be a tetraploid, having four sets of chromosomes. Self-pollination of NA71586 resulted in over 600 progeny, which varied in height and foliage characteristics. We will continue to evaluate this population for the purpose of selecting cold-hardy, well-adapted plants with ornamental foliage.
Technical Abstract: Clethra alnifolia L., which is a native deciduous shrub cultivated as an ornamental, was recently hybridized with C. pringlei S. Wats. The purpose of this hybridization was to combine the cold hardiness and adaptability of C. alnifolia with the ornamental foliage of C. pringlei. While most of the C. alnifolia × C. pringlei hybrids more closely resembled C. alnifolia than the paternal species, a 'Hokie Pink' × C. pringlei hybrid (NA71586) with foliage that flushes red like C. pringlei was recovered. The objectives of this study were to cytologically analyze and produce a F2 population from NA71586. Chromosome counts from root tips cells indicated that NA71586 has 32 chromosomes. Since the chromosome number of C. alnifolia is 2n = 32 and that of C. pringlei was found to be 2n = 16, NA71586 appears to have developed following fertilization of a C. alnifolia egg with an unreduced male gamete from C. pringlei. Both 'Hokie Pink' and C. pringlei exhibited primarily bivalent pairing in pollen mother cells (PMCs). Over half of the PMCs from NA71586 contained 16 bivalents, indicating substantial homology within the C. alnifolia genome. It was theorized that C. alnifolia is either an autotetraploid that exhibits bivalent pairing or a segmental allotetraploid produced from hybridization of species with similar genomes. Over 700 F2 progeny were obtained from self-pollination of NA71586. Although many of the F2 progeny resembled NA71586, variation in foliage color, size and shape was apparent in the population.