Embryological studies of cross compatibility of species within the genus trifolium 3. development of the embryo and endosperm in crossing trifolium repens with trifolium hybridum and trifolium fragiferum
Genetica Polonica 21(1): 37-62
Fertilization and development of the embryo and endosperm in T. repens, T. hybridum and T. fragiferum, as well as fertilization and development of the hybrid embryo and endosperm, were studied. The percentage of fertilized ovules within the species ranges 92.4%-98.2%, whereas the percentage while intercrossing the species ranges 64.1%-73.7%. As a rule, ovules with a dying embryo had endosperm, which excludes the assumption that the cause of the hybrid system disintegration is the underdevelopment of endosperm. This is due to the incompatibility of complete genomes or of their large part in the species crossed. This incompatibility initiates disharmony in the developing embryo and endosperm, as a result of which no self-regulating systems arise and the hybrid organism dies. Such phenomena as polyembryony, aposporic embryony, early transformation of nuclear endosperm into cellular, differentiation in the size and shape of endosperm nuclei and rapid development of integumental tapetum may be considered as additional signs of genome incompatibility of the crossed species. The affiliation to a section within the subgenus had no influence on the frequency of fertilization and development of the embryo and endosperm in the intercrossed species.