In addition to their application in the production of F-1 hybrids in rape (Brassica napus L.), cytoplasmic male-sterility (CMS) systems may be used to produce synthetic varieties with much higher levels of heterozygosity than those expected in conventional rape synthetics. CMS synthetics are produced by compositing a CMS A-line with several male-fertile (MF) B-lines lacking nuclear alleles for fertility restoration, and increasing the resulting mixture by natural pollination. Over generations of increase, pollination of the CMS component by the MF component of the synthetic results in the progressive loss of A-line nuclear genes from the population. The initial proportions of CMS and MF plants are expected to be preserved over several generations of natural pollination if CMS and MF plants are equal in yield. Methods for estimating the heterozygosity level of CMS synthetics, taking into account the proportion of CMS plants, number of MF parents in Syn 0, and selfing rate (s) of MF plants, are presented. If completely inbred Syn 0 parents and s of 0 and 0.75 for CMS and MF plants respectively, are assumed, the heterozygosity level (1-F) of a synthetic derived from four inbred MF parents each comprising 6.25% of Syn 0 and one inbred CMS parent comprising 75% of Syn 0 is predicted to be 0.66 in Syn 5, compared to 0.30 in synthetics derived from four MF parents only. CMS synthetics offer a novel, low-cost approach to the exploitation of heterosis in rape and other species with mixed mating systems in which self-pollination predominates.
