We report mutants in Arabidopsis thaliana (fertilization-independent seed: fis) in which certain processes of seed development are uncoupled from the double fertilization event that occurs after pollination. These mutants were isolated as ethyl methanesulfonate-induced pseudo-revertants of the pistillata phenotype. Although the pistillata (pi) mutant has short siliques devoid of seed, the fis mutants in the pi background have long siliques containing developing seeds, even though the f lowers remain free of pollen. The three fis mutations map to loci on three different chromosomes. In fis1 and fis2 seeds, the autonomous endosperm nuclei are diploid and the endosperm develops to the point of cellularization; the partially developed seeds then atrophy. In these two mutants, proembryos are formed in a low proportion of seeds and do not develop beyond the globular stage. When FIS͞fis plants are pollinated by pollen from FIS͞FIS plants, Ϸ50% of the resulting seeds contain fully developed embryos; these seeds germinate and form viable seedlings (FIS͞FIS). The other 50% of seeds shrivel and do not germinate; they contain embryos arrested at the torpedo stage (FIS͞fis). In normal sexual reproduction, the products of the FIS genes are likely to play important regulatory roles in the development of seed after fertilization.Arabidopsis seed, like the seed of other angiosperms, is a product of double fertilization, in which one of the two sperm cells fertilizes the haploid egg cell, giving rise to a diploid embryo, and the other sperm cell fertilizes the polar nuclei in the central cell, giving rise to the triploid endosperm (1). As the embryo and the endosperm develop, the ovule enlarges into a seed; the maternal tissues of the inner and outer integuments surrounding the embryo sac form the seed coat.We have proposed a strategy for identifying genes that uncouple components of seed development from the fertilization process (2). Inactivation of genes that normally repress seed development may lead to precocious seed development without fertilization. In many apomictic plants, seed development does occur without fertilization or with only partial fertilization (3). In autonomous apomixis, seed development occurs without pollination and thus without fertilization of either the egg cell or the polar cell. In pseudogamous apomixis, pollination is required; in some cases, the pollination event results in fertilization of the polar cell but not of the egg cell. Apomixis has been described in a close relative of Arabidopsis, Arabis holboellii (3), and some genetic data support a one or two gene control of apomixis (3), so we reasoned that a mutational approach in Arabidopsis might detect mutants displaying some components of apomixis.For the isolation of these mutants, we used stamenless pistillata (pi) (4). If pi plants are not pollinated, the siliques remain short; they only elongate when seed is formed. We identified mutants in which the siliques elongated without pollination (5), and recently Ohad et al (6) described ...