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Seed Development Following Matings of Trifolium nigrescens x T. occidentale at Different Ploidy Levels¹

Fertilization ensued from all matings obtained by pollinating diploid and autotetraploid Trifolium nigrescens Viv. with pollen from diploid and autotetraploid T. occidentale Coombe. However, only fertilized ovules from the cross of T. nigrescens (2x) x T occidentale (4x) appeared to be developing successfully toward viable seed; fertilized ovules from other crosses aborted at early stages of development. Seed failure in the different crosses followed a similar pattern. The first evidence of abnormality was slow development of the endosperm. Sequentially, the endosperm failed to develop signs of specialization, such as formation of a haustorium, and signs of breakdown followed in 1 or 2 days. The embryo began to degenerate when the endosperm was in a highly collapsed state. Concomitant with failure of the embryo and endosperm, abnormalities occurred in the adjacent maternal tissues. The endothelial cells stained darkly, whereas the integumentary cells showed little breakdown from digestion which characteristically has occurred at this time in normal seed development. These observations indicate that lack of a functional endosperm blocks the process of food transfer from the maternal tissues to the young embryo, thus causing nutrients to accumulate in the endothelium. Consequently, the embryo starves. However, the observation of a welldeveloped endosperm when either the egg was not fertilized or the zygote did not divide in two exceptional seed from the cross T. nigrescens (2x) x T. occidentale (2x) suggests the possibility that the hybrid embryo was involved in endosperm failure.

Key Words: Interspecific incompatibility • Seed failure • Abnormal endosperm • Embryogenesis

² Former Research Geneticist (now Geneticist, Taiwan Agriculutral Research Institute, Taipei, Taiwan) and Research Agronomist, respectively, ARS, USDA. Clemson, SC 29631.

Received for publication June 30, 1973.

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