A genetic screen was used to isolate mutants in Arahidopsis thaliana that produce seed without pollination; these should include mutants that are apomictic. The screen is based on the visual difference in length between a seed pod that is seedless (short silique) and one in which seed is formed normally by self-pollination and fertilization (long silique). The rationale of the screen is to mutagenize a stamenless (and therefore pollenless) mutant of Arabidopsis with short siliques into plants that form seed without pollination and therefore have long siliques. Using the shortness of siliques of the Arabidopsis mutant pistillata (pi) compared with wild type as the basis of our genetic screen for apomixis, a number of mutants have been isolated which have fertilization independent partial seed development (fis1, fis2 and fis3) and some mutants which may have a low frequency of fertilization independent complete seed development (fis4). Homozygous pistillata (pi/pi) or heterozygous pi/PI seeds were mutagenized with ethylmethanesulphonate and screened for dominant and recessive mutants, respectively. In 40 000 M_2 pi/pi plants, eight mutants have been identified in which partial development of seeds occurs without fertilization (Fis phenotype). Three mutants, fis1, fis2 and fis3, have been characterized. In each of the three mutants, endosperm develops normally to the point of cellularization. At a lower frequency, embryos are also seen that degenerate without development beyond the globular stage. Following pollination, the maternal embryos formed are arrested at the torpedo stage and the endosperm develops to the cellular breakdown stage. Each of these three mutants has been mapped to a different chromosomal location in the Arabidopsis genome. After screening 15 000 M_1 plants, 22 sectored inflorescences with Fis phenotype were also identified. In particular, the mutant fis4, in which autonomous functional seeds are apparently formed without pollination, is being investigated. All these mutants define genes which promote seed development without fertilization and may be associated with the cases of apomixis seen in many angiosperms.
展开▼
