| Summary: | 博士 === 國立臺灣大學 === 農藝學研究所 === 85 === Napier grass (Pennisetum purpureum) is one of the most important forage grasses in Taiwan. Breeding for higher forage quality was in progress through inter-specific crossing with pearl millet (P. americanum). However, the cost for establishment and renewing is prohibitory high due to its vegetative propagation nature. The objective of this research is to transmit apomixis from P. squamulatum, an obligate apomictic wild relative ofnapier grass, into the inter-specific hybrid of pearl millet and napier grass.
Apomixis is an asexual mode of reproduction through the seed, which bypasses female meiosis and undergoes reproduction without the union of egg and sperm. The apomixis process leads to the production of embryos genetically identical to the maternal plant. Incorporating apomixis into plant breeding programs can facilitate the process of fixing heterosis and producing true-bred seed. Using apomixis to develop true hybrid through seed in napier grass can substantially reduce the costs in planting and concomitantly eliminate or reduce propagule-borne diseases.
Embryo sac examination of the three species showed that the most obvious characters of apomictic reproduction are multiple embryo sacs and the lack of antipodals. The apomixis mode of P. squamulatum is highly inheritable and stable across different locations. Results of reproduction mode testing in tri-specific hybrid [(pearl millet X napier grass) X P. squamulatum}, indicated that apomixis is dominant expression. An open-pollinated population was derived from trispecific hybrids, from which an obligate apomictic individual with high seed set and pollen production rates was selected (No. 9124-1). This plant was used as the male parent of a backcross with the interspecific hybrid of pearl millet and napier grass.
Morphological characteristics of the backcross population including plant height, leaf width, stem diameter, flowering period and panicle type all showed large genetic variation which forms the basis for further improvement. By excluding no heading and aborted embryo sac individuals, the backcross population can be grouped into three categories including sexual, facultative apomixis and obligate apomixis with the ratio of 2:1:1, based on observation on embryo sac development. This result indicated that apomixis is controlled by simple inheritance. The sexual and obligate apomictic sub-population showed significant differences in stem diameter, somatic chromosome number, and aborted embryo sac proportion. Obligate apomictic plants have narrow stem diameter, more somatic chromosome number, and lower proportion of aborted embryo sac, which demonstrated that apomixis can escape from sterility.
Screening 520 random primers for pearl millet, napier grass, and P. squamulatun and the backcross population obtained one polymorphic RAPD (random amplified polymorphic DNA) marker which is co- segregated with apomixis. Specific primers were designed and synthesized based on the sequence of this marker and successfully converted it to a SCAR (sequence characterized amplified region) marker for apomixis, which could provide fast and accurate screening for apomictic plants in further improvement programs.
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