Parent-Specific Gene Expression and the Triploid Endosperm

The endosperm of flowering plants is a tissue that acquires resources from the maternal sporophyte and is in turn digested by the developing embryo in its own seed. In this respect, the endosperm takes the role filled by the female gametophyte in gymnosperms. Endosperm is a tissue unique to angiosperms. At fertilization, a male gametophyte (pollen tube) releases two sperm into the female gametophyte. One sperm fuses with the egg to form a diploid zygote, and the other sperm fuses with two female-gametophyte nuclei (known as polar nuclei) to form the triploid primary-endosperm nucleus. Therefore, the endosperm, which develops from this triploid nucleus, has an unusual genetic composition. Each nucleus contains one copy of the paternal contribution to the associated embryo and two copies of the maternal contribution. A minority of flowering plant species have endosperms of different genetic composition, but almost all share the characteristic that more of the endosperm genome is derived from the mother than from the father. Double fertilization and the formation of endosperm are often considered the major features that distinguish flowering plants from other seed plants. Because of this importance, many authors have suggested adaptive explanations for the unusual genetic constitution of endosperm. Heterozygote vigor (Brink and Cooper 1940) can explain the presence of a paternal contribution but not why the maternal contribution should be doubled. Polyploid vigor (Stebbins 1974) can explain the advantage of triploid over diploid endosperm but not the weighting toward the maternal contribution. In other words, the polyploid-vigor explanation does not explain why genome doubling should occur in the female gametophyte before fertilization, particularly given that endosperm cells often increase their ploidy by other means after fertilization (D'Amato 1984). More-recent hypotheses have treated the endosperm as a participant in conflicts of interest among mother, father, and offspring. Conflict arises because sibling offspring of one mother tend to compete with each other. Each offspring's prospects of survival and eventual reproduction are sensitive to the quantity of resources it obtains from the mother, and when some offspring obtain more resources, others tend to obtain less. Because the mother is equally related to (has an equal genetic investment in) each of her offspring, her inclusive fitness is greatest when all nonabortive offspring are provisioned equally (Smith and Fretwell 1974; Trivers 1974). However, each offspring is more closely related to itself than to its siblings. For this reason, individual offspring would benefit from some redistribution of the mother's resources toward themselves and away from their