Interspecific hybrids between cultivated rice (Oryza sativa, 2n = 24, AA) and two wild species (O. minuta, 2n = 48, BBCC and O. officinalis, 2n = 24, CC) have been produced by cross pollination. The embryo development in these interspecific hybrids was observed to study the deterioration features in detail. All hybrids showed either abnormal embryo development with degenerating endosperm or embryo retardation itself. Abnormally developed embryos were observed by the end of 7 days after pollination and the percentage of abnormally developed embryos increased from 7 to 14 DAP. The embryos were excised at 7 to 14 DAP and cultured on MS medium supplemented with 1, 3 and 5 mg/l benzylaminopurine, 1 g/l casein hydrolysate, 0.8 % agar and 3 % sucrose. The optimal time for rescue turned out to be 11-14 DAP. There was a significant difference (P = 0.05) in the regeneration efficiency rate with rescue time but no significant difference with of culture medium in these cross combinations. The F 1 hybrids were morphologically intermediate between their parents. All F 1 hybrid plants thus obtained were completely sterile. Chromosome doubling was attempted by treating F 1 hybrid tillers with colchicine to overcome their sterility. The morphology of the colchicine-treated F 1 plants closely resembled the female parent and fertility.
In Thailand, Dendrobium is the most popular crop grown for cut flowers and as a pot plant for local use and export. Some native species are used for hybrid production while some are specific to particular parts of the country and need to be conserved. The sequences of rDNA ITS (ITS1-5.8S-ITS2) and the matK gene were used to reconstruct the phylogenetic relationships of 27 native Dendrobium spp. in Thailand, which are composed of 9 sections: Breviflores, Callista, Dendrobium, Distichophyllum, Formosae, Pedilonum, Rhopalanthe, Stachyobium, and Strongyle. The average levels of nucleotide divergence (p-distances) of matK and rDNA ITS sequences were 1.3% and 11.2%, respectively. The topologies of the phylogenetic trees reconstructed based on matK and rDNA ITS sequences were slightly different. Use of the rDNA ITS sequence was shown to be a more authoritative systematic approach than use of the matK gene sequence in the genus Dendrobium. The result of the combined data (matK-rDNA ITS) was highly similar to the result from rDNA ITS. From rDNA ITS and combined data, the twenty-seven Dendrobium spp. were clearly grouped into two clades with some differences from the morphological classification. The present study provides the first molecular phylogeny of 27 native Dendrobium species in Thailand.
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