HOW WIDE CAN A WIDE CROSS BE

Wide crosses in wheat are reviewed in relation to various factors that facilitate wide crossing to show that wide crosses can be as wide as one can make them. Included in this review is a particular reference t o wheat-wheatgrass (Agropyron complex) crosses and an update on wide c rosses of wheat with various genera of Agropyron complex. Hybrid seed set is too variable to predict whether a wide hybrid, where no seed wa s obtained in one attempt, will not be possible. High crossability gen es seem to facilitate not only fertilization but also seed development , enabling embryo rescue. Variability for crossability occurs not only in wheat but also in alien species. Contrary to conventional thinking , several wide hybrids with wheat can be produced when species with lo wer chromosome numbers are used as female parents. Pre- and post-ferti lization barriers to wide crosses do not appear to be equally strong, and can be overcome by the development and application of various tech nologies. Considering these aspects of wide hybridization, and based o n recent successes in producing previously unsuccessful and very wide hybrids, it is concluded that how wide cross between plant species can be made is an open question and that many new and wider hybrids can b e produced in future.