Segregation data from crosses between necrotic and mottled lettuce (Lactuca sativa L.) parents showed that a single gene controls the difference in type of reaction to lettuce mosaic virus: necrosis is dominant to mottled. Segregation data from crosses between resistant and necrotic parents differed, depending on the necrotic parent. In crosses with the necrotic cultivar Prizehead, there were two independent genes, one controlling necrotic vs. mottled and the other resistant vs. susceptible. In a cross with the necrotic cultivar Maikonig, resistance was epistatic to necrotic, suggesting a second necrotic allele. Crosses among necrotic cultivars indicated a single gene for the necrotic reaction, with the possibility of more than one necrotic allele. Necrotic alleles identified are named Necrotic-1m and Necrotic-1p.
In crosses of lettuce (Lactuca sativa L.) between parents producing a mild or susceptible reaction to lettuce mosaic virus, a single gene segregated. The heterozygote reacted in an intermediate manner. In crosses between mild-reacting and resistant parents, the mild reaction gene and the resistant gene segregated independently. The resistant and mild alleles together produced a new phenotype that is usually symptomless. The gene symbol proposed is Mi'Mi, where Mi' gives the mild phenotype. Breeding is in progress to combine the mild and resistant traits in new lettuce cultivars.
Two new lettuce (Luctuca sativa L.) genes are described and named truncated leaf (tn), and sickly (si). A gene for reflexed involucre is identical to that previously described in wild lettuce (L. serriola L.). Mosaic reaction (me) and light green (lg) are linked, with P = 0.448. Six gene pairs tested for linkage are independently inherited. Sickly is epistatic to light green.
Mosaic-susceptible and -resistant lettuce cultivars and breeding lines were evaluated for seed transmission of the virus. Two methods of indexing were used: the seedling method, in which seedlings from infected plants were observed; and the Chenopodium method, in which a local lesion host, C. quinoa, was inoculated with seed samples ground in buffer solution. Susceptible lines transmitted at a rate averaging about 2% per plant. Over 90% of resistant lines failed to transmit the virus; the rest transmitted at extremely low rates, averaging 0.49% in one experiment. The parent plant’s environment appears to affect the transmission rate. Low day temperature results in a higher rate than high.
Each of 3 albino characters is governed by a single recessive gene. Virescent chlorophyll deficiency, pale yellow flower, golden flower, abaxial leaf hairs, and plump involucre shape are also regulated by single recessive genes. Male-sterility is inherited as a single dominant.
Two sources of resistance to lettuce mosaic are described. Resistance is inherited as a single recessive gene. The same gene occurs in both sources. Symptoms on resistant plants are small chlorotic areas distributed sparsely on leaf surface. Later leaves may have more severe symptoms.
In 5 spring trials, the percentage of lettuce plants (Lactuca sativa L.) showing big vein symptoms was consistently low for the resistant line 72-136 and consistently high for the susceptible ‘Great Lakes 65.’ Results were less consistent for the moderately resistant ‘Merit’ and for ‘Calmar.’ During periods of low temperatures a lower percentage of big vein infected plants than healthy plants was harvested. Results were variable at high temperatures but, in general, differences were less.
A partially dominant allele for early flowering in lettuce (Lactuca sativa L.) reduces flowering time by about one-half. This property is useful in backcross breeding procedure to accelerate the transfer of useful alleles to a desired recurrent parent. Application of the technique to transfer resistance to lettuce mosaic is described through 4 backcrosses. Generalization of the technique for other species is discussed.
Two new lettuce genes are described and named: chlorophyll deficient-2 (cd-2) and Early flowering (Ef). Brown pericarp is either a pleiotropic effect of plump involucre (pl) or is caused by a closely linked gene. White pericarp (w) is epistatic to brown. Endive-like leaf (en) and white pericarp are linked, with P = 0.47. Virescent (vi) and fringe (fr) are linked, with a recombination value of p = 0.25. Genes for spininess and anthocyanin color are linked. Endive, white, virescent, fringe, and male sterile-6 (ms-6) form a linkage group. Three other double recessives have interactive relationships. Endive and fringe together are phenotypically extreme endive; virescent and golden (go) together are golden; golden and chlorophyll deficient-2 are golden and partially lethal.
Achene color and leaf-type genes are linked, with p = 0.34. Achene color and male-sterility genes are linked, with p = 0.39. Leaf-type and male-sterility genes are independently inherited. Achene color and virescent chlorophyll-deficiency genes are linked, with p = 0.03. Thirty-four other linkage comparisons showed independent inheritance.
Endive-type leaf and chlorophyll deficiency are each regulated by recessive alleles. Three separate recessive alleles control albinism.