Reverse breeding (RB) is a novel plant breeding technique designed to directly produce parental lines for any heterozygous plant, one of the most sought after goals in plant breeding. RB generates perfectly complementing homozygous parental lines through engineered meiosis. The method is based on reducing genetic recombination in the selected heterozygote by eliminating meiotic crossing over. Male or female spores obtained from such plants contain combinations of non-recombinant parental chromosomes which can be cultured in vitro to generate homozygous doubled haploid plants (DHs). From these DHs, complementary parents can be selected and used to reconstitute the heterozygote in perpetuity. Since the fixation of unknown heterozygous genotypes is impossible in traditional plant breeding, RB could fundamentally change future plant breeding. In this review, we discuss various other applications of RB, including breeding per chromosome.

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A total of 1789 accessions of several lettuce collections was screened to find new major gene resistance to the downy mildew fungus Bremia lactucae Regel . The accessions belonged to the species Lactuca sativa (N = 1288), L . serriola (N = 399), L . saligna (N = 52) and L . virosa (N = 50) . A total of 20 races of B . lactucae were used, 14 of which were NL-races, isolated from cultivated lettuce in the Netherlands . The other six races were isolated from wild L . serriola in Czechoslovakia . The accessions were initially screened with two races : NL1 and NL3 . Accessions with resistance to one or both of these races were tested with the other races . Phenotypes with new resistance were found in accessions of all four Lactuca species . Of L . sativa, four accessions were found with resistance phenotypes that could not be explained by combinations of known major genes . Many accessions of L . serriola had resistance phenotypes that indicated the presence of unknown resistance genes . All interactions between accessions of L . saligna and races of B . lactucae were incompatible in leaf disc tests, except for four accessions, which showed some sporulation with race NL6 . Several accessions of L . virosa were resistant to all races used . Other accessions of L . virosa gave a race-specific interaction with B . lactucae .

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The early stages of development of Bremia lactucae (lettuce downy mildew) were examined on lettuce cultivars possessing high (Iceberg and Regina di Maggio) and low (Great Lakes and Plenos) levels of field resistance. Germ tubes, appressoria, penetration, primary and secondary vesicles, intercellular hyphae and haustoria were observed 3. 6 and 24 h after inoculation of cotyledons and of leaf discs from adult plants. Differences were observed between cv. Iceberg and susceptible genotypes in the percentage of spores germinating and the incidence and speed of development of infection structures. Secondary vesicles were first observed 24 h and 6 h after inoculation in Iceberg and susceptible genotypes, respectively. The lowest incidence of secondary vesicle formation 24 h after inoculation (48 and/or 43%) was recorded in Iceberg and Regina di Maggio, and the highest incidence (68%) occurred in Plenos. The formation of intercellular hyphae and haustoria was not observed in cv. Iceberg some 24 h after inoculation. There were significant differences in the lengths of germ tubes formed on different cultivars. Those on cv. Iceberg were longer than those formed on susceptible cultivars. The results indicate that the field resistance of B. lactucae may result from mechanisms which are effective in the early stages of infection.