| Litchi cv. ‘Guiwei' is famous for its fantastic fruit quality with fragrant crispy sweet flesh and small seed. ‘Guiwei' is a typical partly abortive cultivar bearing both normally developed seeded(large-seed) fruits and aborted-seeded(small-seed) fruits. The seed abortion rate affects not only edible rate but also the taste. Seed abortion rate of ‘Guiwei' is an important determinant of fruit price. The seed abortion rate vary among orchards and years in the same orchard. However, there are few studies about the occurrence and factors affecting seed abortion rate in ‘Guiwei'.In this study, we investigated the occurrence of seed abortion in ‘Guiwei' sampling from different orchards, different production years, inflorescence types and pollen sources. We measured the volume of endosperm and soluble sugar contents in endosperm, and observed the ovule early development status using semi-thin section. We found ‘Guiwei' lines bearing large seed fruit during abortion rate investigation. And SCo T molecular marker technique was applied to test their genetic difference. After analysis the seed abortion pattern and comfirming the impact of pollen source on seed development, we proposed that the seed abortion of ‘Guiwei' might be a epigenetic traits. DNA methyltransferase genes were cloned and full sequences were obtained. The expression patterns of DNA methyltransferase and glycosylase genes were investigated. The main results were as follows:1. The seed development of ‘Guiwei' can be divide into three types: early abortion, late abortion and normal. Seed abortion rate of ‘Guiwei' litchi varied among different orchards, from 40% to 97%. Even the same orchard has different seed abortion rate in different years. The fruits developed from earlier flowers had lower seed abortion rate than those developed from late flowers in the same plant.2. Different pollen sources had a remarkable influence on the seed develoment of ‘Guiwei'. The self-pollination fruits have a high seed abortion rate which are up to 93.3%, while the fruits that pollinated with ‘Xuehuaizi', ‘Huaizhi', ‘Hongmili', ‘Ziniangxi' and so on had much less seed abortion rate(28.6-65.3%) lower than that of ‘Guiwei' self-pollination fruits.3. A retard in the early development of ‘Guiwei' was noticed. Endoperm nucleus were much less in ‘Guiwei' than in ‘Huaizhi'. The ovules development state of ‘Guiwei' at 15 days after flowering was about the state of ‘Huaizhi' at 10 days after flowering. At 15 days after flowering, the embryo of ‘Huaizhi' display active cell division and in the pre-globular stage, while those of ‘Guiwei' were still at pre-embryo stage.4. We found large-seed ‘Guiwei' line, whose biological features are identical to small-seed ‘Guiwei' plants except for the seed development phenotype. The SCo T molecular marker capable to distinguish different litchi cultivars were used to assess their genetic diversity. No polymorphic loci was found, which means the large-seed ‘Guiwei' lines and the apartly abortion ‘Guiwei' had similarity in genetic background.5. Base on the seed abortion patterns, we proposed that the development of ‘Guiwei' seeds are regulated by epigenetic modification. Six DNA methyltransferase genes, named Lc MET1, Lc MET2, Lc CMT1, Lc CMT2, Lc DRM1, Lc DRM2, respectively, and four DNA glycosylase genes, named Lc DML1, Lc DML2, Lc DML3, Lc DML4, respectively, were selected from the litchi genome database. Their expression levels in ovules and pollens were studied using Real-Time quantitative PCR. The DNA methyltransferase are highly expressed during the early ovule development stage and decreased along with the ovule development. Three genes, Lc CMT1?Lc CMT2 and Lc DRM2 displayed expression patterns correlative well with the seed development status were targeted. They full sequence were obtained and bioinformation were analysis. |
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