AbstractsBiology & Animal Science

Post-fertilization breakdown in the reproductive processes is frequently the cause of incompatibility in interspecific and intergeneric crosses in plants. Cooper and Brink (1940) report that sterility in the crosses Nicotiana rustica X N. glutinosa, N. rustica X Petunia violacea and N. rustica X Lycopersicon esculentum is due to a type of seed failure which they call somatoplastic sterility. Fertilization, although delayed, takes place, and the hybrid embryos grow normally until the collapse of the young seeds. The endosperm however is retarded in development; it loses ascendancy over the maternal tissues of the ovule, which proliferate and finally occlude the endosperm. Associated with these events is the non-differentiation of the vascular bundle to the ovule. Starvation of the endosperm results. Final seed collapse is initiated by the breakdown of the endosperm, which begins in the chalazal region and progresses towards the embryo. Ledingham (1940) has investigated the development of the embryo and endosperm in the cross Medicago falcata X M. sativa and the reciprocal. He finds that there is some evidence that if the development of the endosperm and embryo is too slow some part of the ovule or ovary develops at their expense until abortion occurs. Autogenous endosperm breakdown is reported by several investigators. Kihara and Nishiyama (1932) found that in the cross Avena strigosa X A. fatua the endosperm grows very rapidly but abnormally. Mitotic divisions are disturbed, so that giant masses of chromatin are formed. Large irregular nuclei, unhealthy cytoplasm, and weak cell wall formation are also found. Seeds are formed but they do not germinate. In the reciprocal cross, A. fatua X A. Strigosa endosperm development is very slow and soon ceases. In some ovules it is regenerated later from a group of cells in the antipodal region, and these ovules may form fertile seeds. Boyes and Thompson (1937) describe endosperm and embryo development in reciprocal crosses in Triticum and Secale. The embryos develop normally. In the endosperm all conditions are found from a slight retardation in growth to extreme abnormalities and early abortion, depending on the width of the cross and the direction in which it is made. Kostoff (1930) suggests that abortion of endosperm and embryo in Nicotiana hybrids may be due to an immunity reaction. Antibodies are produced in the maternal tissues which attack the hybrid embryo and endosperm. Laibach’s theory (1929) which explains the non­germination of hybrid embryos in Linum by unfavorable influences of the mother plant, is similar. When the embryos are dissected out it is found possible to raise them to maturity. Preliminary work (Thompson, 1939) indicates that fertilization takes place in the cross Hordeum vulgare x Secale cereale, and that seed failure is due to a characteristic type of endosperm abortion. The development of the hybrid embryo and endosperm is traced in the present study.