Diploid endosperm formation in Tulipa spp. and identification of a 1:1 maternal-to-paternal genome ratio in endosperms of T. gesneriana L. |
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Authors: | Hitoshi Mizuochi Hironori Matsuzaki Takehiko Moue Keiichi Okazaki |
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Institution: | (1) Faculty of Agriculture, Niigata University, Ikarashi Niigata, 950-2181, Japan |
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Abstract: | Most Liliaceae plants have the tetrasporic Fritillaria-type embryo sac and normally form diploid embryos and pentaploid endosperms derived
from a 4:1 maternal-to-paternal genome ratio (4m:1p) after double fertilization. Here we characterize embryo sac and endosperm
formation in Tulipa spp. of Liliaceae. Chromosome analysis using seeds derived from 2x × 2x crosses of Tulipa
gesneriana (2n = 2x = 24) identified diploid chromosome number in the endosperm. Similarly, flow cytometric analysis confirmed diploid endosperm
formation in T. gesneriana, T. fosteriana (2n = 2x = 24) and T. greigii (2n = 2x = 24). To further study the possible mechanism of diploid endosperm formation, we made interploidy crosses of triploid (2n = 3x = 36) × diploid in which aneuploid seeds with various chromosome numbers (2n = 25–36) were produced. Again, flow cytometric analysis confirmed the same ploidy level in both embryos and endosperms at
all aneuploidy levels, suggesting that only a single haploid polar nucleus contributes to endosperm formation at fertilization.
Histological observation further confirmed the physical separation of two polar nuclei by a large vacuole in the Fritillaria-type
embryo sac of T.
gesneriana that appeared to prevent the fusion of the two polar nuclei that originated at the micropylar and chalazal ends before fertilization.
Taken together, these results indicate that diploid endosperms (1m:1p) are normally formed in Tulipa spp. by fusion of the micropylar polar nucleus (n) and a spermatid (n) but not by normal triple fusion. We also show that tulip endosperm partially overcomes the triploid block mechanism that
occurs in interploidy crosses. Based on these observations, the possible role of triple nuclear fusion in double fertilization
is discussed. |
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Keywords: | Embryo sac Endosperm Evolution Triploidy Tulips |
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