 Hawaii Cooperative Extension Service
Seed-propagated dendrobium cultivars developed by the University of Hawaii are widely used by local growers. The success of many of these cultivars relies on utilizing amphidiploid parents for seed production. A major advantage of seed-propagated cultivars is that the seedlings are free of cymbidium mosaic virus (CyMV). Unfortunately, the amphidiploid parents used in our breeding program are heavily infected with the virus. Plant vigor has weakened over the years, thereby affecting the production of seed pods for distribution to grower organizations. In our breeding program we have frequently encountered some androgenetic offspring from crosses involving amphidiploids as the pollen parent (Amore and Kamemoto, 1991). Androgenesis is the development of a haploid individual with only the paternal chromosome complement. Androgenetic individuals therefore resemble the male parent, and have half the chromosome number of the male parent. When we crossed Dendrobium Louis Bleriot, a triploid, with amphidiploid (tetraploid) D. Jaquelyn Thomas clones 'Y166-1', 'O580', and 'D168-12' or amphidiploid D. Neo Hawaii 'Y972', we were able to recover some individuals that resembled the male parent. Chromosome counts obtained from root tip squashes revealed their diploid nature. Leaf samples from these plants were assayed for virus using the ELISA technique and were found to be negative for both CyMV and odontoglossum ringspot virus (ORSV). Chromosome doubling of the androgenetic diploids with colchicine should enable the recovery of virus-free amphidiploids similar or identical to the original parental amphidiploid. Thus, androgenesis via D. Louis Bleriot as the female parent presented an opportunity to rejuvenate the existing virus-infected amphidiploid stocks and recover virus-free amphidiploids for the production of seed-propagated cultivars. Crosses were made in 1985 between D. Louis Bleriot and the following D. Jaquelyn Thomas amphidiploids and D. Neo Hawaii:
Table 1. Diploids selected for cloning. Selection Amphidiploid parent Parent of UH Cultivars ________________________________________________________________ K1056-8 Jaquelyn Thomas 'Y166-1' UH44 K1059-51 Jaquelyn Thomas 'D168-12' UH503, UH507 K1064-64 Jaquelyn Thomas 'O580' UH232, UH503 K1062-18 Neo Hawaii 'Y972' UH306, UH1041, UH1233When sufficient numbers of protocorm-like bodies (PLBs) were produced, these were then treated with colchicine for 4-6 days to induce chromosome doubling. Plantlets were transferred to finished flasks in April 1990, to compots in fall 1990, and to 2-inch pots in May 1991. Plants were then transferred to 6-inch pots in winter 1991, where they were observed and classified as diploid or tetraploid as they flowered. Ploidy level was determined by visual observations on flower size, spray length, and floriferousness. The conversion from diploid to tetraploid ranged from 51.0 percent for 'D168-12' to 70.7 percent for 'O580' (Table 2).
Table 2. Tetraploid conversion rates for androgenetic selections.
Number of Number of Percent
diploid tetraploid Total conversion
Selection individuals individuals observed to tetraploid
________________________________________________________________
K1056-8 17 39 58 67.2
K1059-51 25 26 51 51.0
K1062-18 22 27 49 52.9
K1064-64 13 41 58 70.7
No maternal bands were amplified in the RAPD analyses, and only paternal
bands were present, indicating that these newly induced amphidiploids are
virtually identical to the male parent, and should behave no differently
from the original amphidiploid.The converted amphidiploids were re-tested for virus in 1994, and were found to be still negative for CyMV and ORSV. Selected virus-free amphidiploids have been utilized for the production of seed pods of UH cut flower cultivars for distribution to the industry. We made additional crosses in 1994 using D. Louis Bleriot with D. Jaquelyn Thomas 'D192', as well as with D. Jaquelyn Thomas 'K159-21' to rejuvenate the parent plants of UH800. Androgenetic seedlings of 'D192' and 'K159-2' are now in 2-inch pots and will be flowered before going through the process to convert them into virus-free amphidiploids. We wish to acknowledge the contribution of the Dendrobium Orchid Growers Association of Hawaii for partial support of this project.
Literature cited:
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