| Summary: | 碩士 === 國立屏東科技大學 === 熱帶農業暨國際合作系所 === 95 === Development of new varieties in Doritaenopsis orchids may be hindered by different factors including low seed set in hybrids. However, through cytological studies it may be feasible to develop new hybrids by selecting varieties with good chromosome pairing and high pollen viability, good pairing is an approximate indicator of genome homology without serious chromosomal rearrangements such as paracentric inversions and translocations. Inversions and translocations are liabilities since they may reduce the ability of pollen to develop and fertilize ovules.
Cross pollinations, pollen viability, and chromosomal behavior during meiosis were analyzed to reveal the correlation between seed fertility and capsule set in Doritaenopsis hybrids. The number of mature capsules harvested and their relative seed content were used as indexes of crossing availability.
The results of meiosis were evaluated according to pollen viability detected by fluorescein diacetate, which indicates membrane metabolic activity, and by quantification of sporad types with acid fuchsin, which stains the cytoplasm. Chromosome number and pairing at meiosis were observed by using root tips, usually sampled from 11 am to 12 pm, or pollen mother cell samples from flower buds at 3/4 of their final size. A positive correlation was found among high seed set, high frequency of viable tetrads, high degree of chromosome pairing, and low frequency of chromosomal aberrations such as inversions and translocations.
Three types of hybrids could be distinguished on the basis of these factors. In type one hybrids, chromosomes paired as bivalents, pollen mother cells divided into tetrads and capsule setting occurred after pollination of pollen acceptor. Type one hybrids included Dtps. Jiuhbao Red Rose (4x), Dtps. Chian-Huey Red Rose ‘F904’ (4x), Dtps. Ruey Lih Beauty (4x) and to some extent Dtps. Purple Gem (2x). In type two hybrids, meiotic chromosomes showed low or partial homology, with most chromosomes configured as univalents and in some cases tetravalents, while the prevalence of micronucleated pollen cells was high. Seed harvest was low after pollination. Type two hybrids included Dtps. Minho Diamond ‘F729’ (2x), Dtps. Sogo Gem ‘F859’ (2x) and Dtps. Bin You Danseuse ‘F574’ (4x). In type three hybrids, chromosomes showed aberrant configurations such as tetravalents and multivalents (chains) during meiosis, inversion loops and translocation junctions at pachytene, and chromosomal bridges with fragments at anaphase. Pollen mother cell division showed massive failure and bare fertility occurred. Type three hybrids include Dtps. Sogo Gem ‘F1089’ (3x), Dtps. Sogo Gem ‘F752’ (3x), Dtps. Fuchsia Princess ‘KHM 648’ (2x) and Dtps. I-Hsin Purple Jewel (3x).
High pollen viability and high fertility seems linked to a high frequency of normal tetrads, while low seed set in cross pollinations could be predicted if micro-nuclei exist in end-products of meiosis. For breeders the existence of chromosomal rearrangements such as inversions and translocations in parental genomes implies that plant improvement through chromosome doubling might not restore fertility, thus careful screening of cultivars prior to breeding is necessary. Thus a deeper understanding of the biology of chromosomes in Phalaenopsis and Doritaenopsis orchids is essential to predict how chromosomal-led speciation may arise.
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