| dc.description.abstract | Tea, camellias and oil camellia have brought huge profits to people worldwide. These plants belong to the genus Camellia. The variable morphological characters of the group make the classification of Camellia very difficult and controversial. After the work of Sealy who laid the foundation of modern classification of Camellia, monographers, especially Zhang and Min, held very different opinions on the species boundaries and hotly debated the infrageneric classification. However, all
three taxonomists shaped their views largely based on the collections from China. Similarly, using the samples mainly collected in China, subsequent researchers investigated the molecular phylogeny of Camellia with DNA markers but without yielding a formal classification. Here, species from Indochina, including Cambodia, Laos, Myanmar, Thailand and Vietnam, were added to fill the data gaps in previous studies, to yield a new taxonomic treatment of the group and to infer the origin, evolution and dispersal of Camellia in this area. The species were also reviewed, described and assessed to clarify their nomenclatural and conservation status, geographic
distributions and diagnostic character states.
The single copy nuclear DNA RPB2 11?16 and 23 intron regions were found to be very
informative for the phylogenetic reconstruction in Camellia, especially when used in combination. The phylogenetic accuracy and resolution of RPB2 gene trees were effectively improved with the addition of Indochinese species of Camellia compared to previous studies lacking these taxa. A new classification of Camellia is proposed here based on the RPB2 and other gene trees, and the morphological and other data to generally categorise Indochinese species into ten amended sections: Archecamellia, Camelliopsis, Corallina, Cylindriea, Furfuracea, Heterogenea, Paracamellia, Piquetia, Stereocarpus and Thea. Among them, sect. Thea is enlarged to include C. longissima, which has been barely investigated and may supply new genetic characters for tea
breeding. Gene trees reconstructed using chloroplast DNA markers in Camellia are usually incongruent with those of nuclear DNA and analyses using morphological, chemical, cross tests and other data. These incongruences may weaken its value in the phylogenetic analyses. Multicopy nuclear DNA markers, such as the LEAFY region and ITS, are affected by gene duplications in Camellia and need to be carefully interpreted in phylogenetic investigations.
Not only the topologies of gene trees but also the fossil calibration points selected can influence the results of molecular dating. To minimize the potential problems of calibration, six different combinations of calibration points using between one and four fossils were analysed with the concatenated RPB2 (intron 11?16 and intron 23) data. The results suggested that the crown age of Camellia was 48.5 Ma with a 95% highest posterior density interval of 39.25?58.92 Ma. This places the evolution of Camellia earlier than that estimated previously by the LEAFY region analysis in which the fossil C. japonoxyla was applied to constrain the stem age of Camellia. However, the ancestral areas of Camellia cannot be simply inferred by the molecular dating analyses using the present distribution data because of a lack of fossil records and a changeable climatic and geographic history. Most of the recognised sections diversified in the Early and Middle Miocene probably due to the rapid uplift of Qinghai-Tibetan Plateau and a series of mountain building events introduced by the India-Eurasia collision which created great ecological opportunities to drive speciation in Indochina. The varying types of seed dispersal mechanisms of Camellia would allow long-distance terrestrial dispersal over millions of years. It is proposed that when there was a land bridge between Indochina and Indonesian Archipelago at 20?30 Ma, some species of Camellia colonized the islands of the latter and subsequently became isolated by the South China Sea, leading to the allopatric speciation of C. lanceolata.
Species of Camellia in Indochina were examined, described and discussed with illustrations, distribution maps, conservation assessments, nomenclatural notes and a key to them. In total, 11 sections, including the new sect. Gilbertia, 43 species and four varieties of Camellia are recognised in Indochina. Two new combinations, C. drupifera var. kissi and C. drupifera var. stenophylla are proposed here. Among them, 27 species are endemic to Indochina. There are 17 synonyms of Camellia identified, including one new synonym, Bembiciopsis, and three of them are lectotypified here. In terms of sections, 25 synonyms are recognised, including ten new ones, and two of them
are lectotypified. Under the 47 accepted specific and infraspecific taxa, 112 synonyms are recognised, including 30 names that are lectotypified or neotypified here and 42 new synonyms.
Many species of Camellia in Indochina, such as yellow camellias, have been exposed to critical over-exploitation for medicinal or ornamental purposes in their natural habitats. The natural resources of Camellia are of fundamental importance for the breeding of tea, oil camellia and horticultural species and thus require a strict protection for their sustainable development. This thesis provides the taxonomic framework in the region for such conservation. | en |
