Lee Yew Hwa
Email Address
12 results
Publication Search Results
Now showing 1 - 10 of 12
Publication Incidence of cymbidium mosaic and odontoglossum ringspot viruses and their significance in orchid cultivation in Singapore(1994-05) Wong, S.M.; Chng, C.G.; Lee, Y.H.; Tan, K.; Zettler, F.W.; BOTANYA survey of cymbidium mosaic virus (CyMV) and odontoglossum ringspot virus (ORSV) in orchids cultivated in Singapore was conducted during 1988-1991. Of the 12 genera from four major commercial orchid farms, 54.6% of the plants were infected with CyMV, 4.0% with ORSV, and 14.2% with both CyMV and ORSV. Among the 29 genera from the orchid nursery of Botanic Gardens, 34.5% were infected with CyMV, 0.3% with ORSV, and 8.3% with both CyMV and ORSV. From the 13 orchid genera cultivated in tissue-culture flasks in the tissue culture laboratory of the Botanic Gardens, 50.5% were infected with CyMV, none with ORSV alone, and 0.5% with both CyMV and ORSV. These findings emphasize the need to implement a virus-free certification programme that would greatly enhance the management of these two prevalent viruses in the Singapore orchid industry. © 1994.Publication RAPD analysis of some species in the genus Vanda (Orchidaceae)(1999-02) Lim, S.-H.; Teng, P.C.-P.; Lee, Y.-H.; Goh, C.-J.; BIOLOGICAL SCIENCESThe genetic closeness of various species of Vanda was determined using random amplified polymorphic DNA (RAPD). Strap-leaved Vanda species (including Vanda sanderiana) and Ascocentrum miniatum were more closely related to each other than to the terete-leaved Vanda species studied. RAPD analysis supports the suggestion that terete-leaved Vanda teres and Vanda hookeriana be classified in the separate genus Papilionanthe and that Vanda sanderiana should remain in the genus Vanda.Publication Characterization of a Singapore isolate of hibiscus chlorotic ringspot virus(1996) Wong, S.M.; Chng, C.G.; Chong, P.L.; Lee, Y.H.; BOTANYA Singapore isolate of hibiscus chlorotic ringspot virus (HCRSV-S1) was obtained from Hibiscus rosa-sinensis L. exhibiting chlorotic leaf spots. Single lesion transfers were carried out on Chenopodium quinoa Willd. C. quinoa and kenaf were used for virus multiplication. Virus yield of 4 mg/100 g of infected leaves was achieved. Two rabbit antisera were raised against two virus bands obtained after sucrose density gradient centrifugation. Double-stranded RNA profile revealed three distinct bands. Virions were observed in cytoplasm of ultrathin sections from infected leaf tissues. The deduced coat protein amino acid composition was similar to that of HCRSV-F.Publication Breeding vandaceous orchids for commercial cut-flowers in Singapore: An overview(1996) Lee, Y.H.; Wong, S.M.; Tan, W.K.; Goh, C.J.; BOTANYExtensive artificial hybridisation among members of some 86 genera of vandaceous orchids within the subtribe Sarcanthinae in Singapore over the past five decades has created many thousands of exotic hybrids. A small number of these are selected for commercial cut-flower production, while others are cultivated as pot-plants. Two classes of intergeneric hybrids are of special commercial importance as export cut-flowers, the bigeneric Aranda (Arachnis x Vanda) and the trigeneric Mokara (Arachnis xVanda x Ascocentrum) hybrids. Cytogenetic studies of these hybrids in recent years have provided information on the significance of ploidy levels and genomic constitutions on the horticultural performance of cut-flower cultivars, and also on the strategy for effective breeding in vandaceous orchids. © 1996 Kluwer Academic Publishers.Publication Amplified fragment length polymorphism analysis of vandaceous orchids(1999-02-22) Chen, X.; Hoon Lim, S.; Man Wong, S.; Hwa Lee, Y.; Kuo, J.; Wing Yam, T.; Lin, J.-J.; BIOLOGICAL SCIENCESWe investigated the application of the PCR-based fingerprinting technique, amplified fragment length polymorphism (AFLP), in orchids. The optimal AFLP patterns have been determined using primer combinations of EcoRI +4 and MseI +3 selective nucleotides. The same reproducible AFLP patterns were demonstrated in genomic DNAs isolated both from: (1) different orchid tissues, e.g. leaves and flowers; and (2) orchid flowers collected at different times. Genomic variations among different cultivars of vandaceous orchid hybrids were successfully determined by AFLP analysis. More than 10% of the AFLP bands were polymorphic DNA when siblings, derived from the same original crosses (two cultivars of Aranda Christine, five cultivars of Mokara Willie How), were used. Only 0.3-0.7% of the AFLP patterns were shown to be polymorphic when different cultivars, originating from somatic mutations during meristem culture for massive propagation, were used (two cultivars of Ar. Christine, four cultivars of M. Chark Kuan).Publication Genomic and meiotic analysis of Mokara orchids(1994) Lee, Y.H.; BOTANYMokara orchids are the special group of artificially created trigeneric hybrids involving members of Vanda, Arachnis, and Ascocentrum, the genomic symbols of which are often designated as V, A, and As, respectively. Eight Mokara clonal cultivars grown commercially in Singapore were selected for genomic and meiotic investigation. They were classified into three categories based on their diverse origins. Three cultivars derived from 2x-Arachnis x 2x-(Vanda x Ascocentrum) were found to be diploid (2n = 38), and their genomic constitutions were likely to be A + 1/2 (As + V). Their microsporocytes exhibited low frequencies of bivalents, which could have involved A-As and/or A-V chromosome pairs. All four cultivars derived from 2x-(Arachnis x Vanda) x 2x-(Ascocentrum x Vanda) were triploid with an inferred genomic content of A + V + 1/2 (As + V). Up to 19 bivalents were detected in the microsporocytes. These were most likely formed between related pairs of V-As and V-V chromosomes and possibly A-As. The only cultivar obtained from 2x-Mokara x 2x-(Ascocentrum x Vanda) was also triploid, but its genomic makeup was likely to be A + 1/2 (As + V) + 1/2 (As + V). Large numbers of bivalents occurred, probably representing mainly As-As, As-V, and V-V chromosome pairs. However, the integrity of individual chromosomes could not be assumed because prior intergenomic recombination could have occurred in parents. Triploid cultivars of both genomic types were more vigorous and generally more horticulturally desirable for cut-flower production than their diploid counterparts.Publication Cytology and fertility of an intergeneric orchid hybrid(1987-09) Lee, Y.H.; BOTANYThe orchid, Aranda Lucy Laycock, is an intergeneric hybrid between Arachnis hookerana and Vanda tricolor. It is well known among breeders because of its unusually high fertility among the over 200 existing Aranda hybrids, but only when used as a maternal parent in crosses. Melotic and sporad analysis of one fertile and one infertile clones of this hybrid were made. The fertite clone was found to produce 47.1 percent dyads of unreduced spores. Cytological studies revealed that 66.4 percent of its pollen mother cells (PMCs) showed noncongressed chromosomes at Ml. Failure of disoriented chromosomes in many of these PMCs to move apart at AI gave rise to first division restitution nuclei. On the other hand, most PMCs of the Infertile clone showed some degree of alignment of chromosomes at the equatorial region at Ml even though most chromosomes remained as unpaired unlvalents. It appeared that the formation of three or more blvalents were necessary to Induce some degree of chromosome alignment at Ml. © 1987, American Genetic Association.Publication Genomic constitutions and flower characteristics of selected Aranda orchid cultivars(1991-05) Lee, Y.H.; BOTANYTen Aranda cultivars commercially grown in Singapore were selected to study their genomic constitutions and flower characteristics. Cytological evidence and breeding records of these cultivars showed that they are of three genomic classes. Four of them are diploid with AV genomes (one Arachnis and one Vanda genome), another four are triploid with AVV genomes and the remaining two are tetraploid with AVVV genomes. Sizes of flowers as well as of sepals and petals generally show significant increases from diploid to tetraploid. This trend reflects the increasing influence of Vanda resulting from additional one and two Vanda genomes in triploid and tetraploid respectively as compared to diploid cultivars. Among the three genomic classes, diploid cultivars generally bear less flowers per spray than those of triploid and tetraploid although exceptions may occur. There is no clear trend in the length of inflorescences although diploid cultivars tend to have less compact spray with flowers more distantly spaced out. © 1991 Kluwer Academic Publishers.Publication An appraisal of the banded and paraerystailine cytoplasmic inclusions induced in cymbidium mosaic potexvirus- and odontoglossum ringspot tobamovirus infected orchid cells using confocal laser scanning microscopy(1996) Wong, S.-M.; Chng, C.G.; Lee, Y.H.; Lim, T.M.; BOTANY; ZOOLOGYThe cymbidium mosaic potexvirus (CyMV) banded inclusions and the odontoglossum ringspot tobamovirus (ORSV) paracrystalline inclusions were studied in flowers and leaves of nine orchid cultivars using the confocal laser scanning microscope. The inclusions varied in length and width and were mostly located adjacent to the cell walls. Some ORSV inclusions fully extended across entire infected cells. We propose that the CyMV banded inclusion was formed from virus aggregates which aligned as periodical stacked layers, appearing as cross bands. The virus aggregates were folded into flexible inclusions, giving rise to various shapes and forms. The ORSV paracrystalline inclusions were observed as needle-like crystals. The confocal laser scanning microscope is an effective tool for the study of the three-dimensional structures of plant virus induced inclusions. © Springer-Verlag 1996.Publication A simple and efficient method of DNA isolation from orchid species and hybrids(1998) Lim, S.H.; Liew, C.F.; Lim, C.N.; Lee, Y.H.; Goh, C.J.; BIOLOGICAL SCIENCESA simple and reliable method for extracting DNA has been developed for orchid species and hybrids. The high quality of DNA obtained is suitable for amplification via the polymerase chain reaction (PCR) for producing random amplified polymorphic DNA (RAPD) markers.