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Embryo Sac Morpho-structural Analysis of Developmental Pathways of Fertilization Failure, Seed Abortion and Poor Germination in Cassava (Manihot esculenta Crantz).

1995; International Society of Cytology; Volume: 60; Issue: 1 Linguagem: Inglês

10.1508/cytologia.60.75

1348-7019

M. N. Ogburia, Tetsuya Adachi,

Agricultural Science and Fertilization

Embryo sac developmental examination in four cultivated Asian cassava clones: 'Rayong 3' (MMex 57×Mven 307), 'Rayong 60' (MCol 1684×Rayong 1), 'Rayong 90' (CMC 76×V 43, Sarakarn 1993), and 'CMR 25-105-112' (27-77-10×Rayong 3, Kawano et al. 1990) was conducted to elucidate the reproductive and conventional breeding barriers (fertilization failure, seed abortion and poor germination) in the genus Manihot. Late pre- and post-pollination embryology revealed absence of functional megaspore mother cell and megagametophytes respectively. Malformed megagametophytes were characterized with reduction in embryo sac volume (8-12.5% of normal), devoid of nucleate cells especially, polar and the antipodal cells, massively vacuolated with no recognizable zygotes or embryos. Regular embryo sacs commenced degeneration 8 days after anthesis in unfertilized ovules of all zero pollinated pistils. Degeneration signal initiated from its thin cell wall and transducted radially inwards and outwards to the central cell and the surrounding nucellus respectively. Both integuments (inner and outer) as well as the nucellus of functional (possessing nucleate cells) and non-functional (deprived of nucleate cells) megagametophytes appeared healthy and developed normally. Physical seed analysis showed that phenotypically normal but functionally deprived 'pseudo-seed' (without endosperm and embryo) and 'true seed' are produced at a ratio of approximately 1:2. Two distinct but complementary developmental pathways of fertilization failure, seed abortion as well as poor germination in cassava have been identified. They are: 1. Ovules devoid of functional megaspore due to lack of megasporocyte during megasporogenesis and 2. Embryo sacs devoid of nucleate cells due to mitotic inactivity at megagametogenesis resulting in structurally and functionally deprived megagametophyte but morphologically normal ovules. These pathways, either separately or collectively are implicated as largely responsible for the reproductive and conventional breeding barriers in the genus Manihot.