Leaf and stem resistance to gummy stem blight [Didymella bryoniae (Auersw.) Rehm.] in five resistant by susceptible crosses of cucumber (Cucumis sativus L.) was investigated using generation means analysis. No single gene of major effect controls either leaf or stem resistance to gummy stem blight in these five crosses. The mean number of effective factors controlling leaf resistance in the cross `Slice' × `Wis. SMR 18' was estimated to be at least five. Estimates of broad- and narrow-sense heritabilities indicated that environmental effects were larger than genetic effects. In general, additive variance was the larger component of genetic variance. Epistasis was significant in most crosses, and dominance was present in several crosses. Additive gene effects contributed more to resistance than to susceptibility in contrast with dominance gene effects. Reciprocal differences for leaf rating were detected in the crosses M 17 × `Wis. SMR 18' and `Slice' × `Wis. SMR 18'. Phenotypic correlations between leaf and stem ratings were moderate (r = 0.52 to 0.72). Estimates of genetic gain for resistance to gummy stem blight ranged from low to moderate. Breeding methods that make best use of additive variance should be used because much of the variance for resistance is additive, and dominance effects, at least in these crosses, tended to contribute to susceptibility.
Paul C. St. Amand and Todd C. Wehner
Timothy J Ng
Antonia Y. Tetteh, Todd C. Wehner, and Angela R. Davis
and digenic epistasis. Inheritance of powdery mildew resistance in watermelon was investigated by generation means analysis by Tetteh et al. (2013) . Studies have demonstrated a close correspondence between generation mean analysis and QTL mapping
Kenneth R. Schroeder and Dennis P. Stimart
Genetics of Antirrhinum majus L. (snapdragon) cut flower postharvest longevity (PHL) was investigated by generation means analysis using a white short-lived inbred (WS) and white long-lived inbred (WL) to determine mode of inheritance and heritability. Broad and narrow sense PHL heritability was estimated at 78% and 30%, respectively. Scaling tests for adequacy of an additive-dominance model in explaining PHL inheritance suggested absence of epistasis. However, joint scaling indicated digenic or higher order epistatic interactions. Fitting of a digenic epistatic model revealed significant additive effects and nonsignificant dominance and epistatic interactions. Additionally, based on sequential model fittings all six parameters [mean, additive (a), dominance (d), a×a, d×d, and a×d] proved necessary to explain observed PHL variation. Continuous variation for PHL observed in the F2 and backcross generations suggests PHL is quantitative. Assessment of associated traits revealed a positive relationship between number of flowers opening postharvest on a cut flower and PHL. In addition, floret wilting led to short PHL while floret browning was associated with long PHL.
Katy M. Rainey and Phillip D. Griffiths
The genetic basis for heat tolerance during reproductive development in snap bean was investigated in a heat-tolerant × heat-sensitive common bean cross. Parental, F1, F2, and backcross generations of a cross between the heat-tolerant snap bean breeding line `Cornell 503' and the heat-sensitive wax bean cultivar Majestic were grown in a high-temperature controlled environment (32 °C day/28 °C night), initiated prior to anthesis and continued through plant senescence. During flowering, individual plants of all generations were visually rated and scored for extent of abscission of reproductive organs. The distribution of abscission scores in segregating generations (F2 and backcrosses) indicated that a high rate of abscission in response to heat stress was controlled by a single recessive gene from `Majestic'. Abscission of reproductive organs is the primary determinant of yield under heat stress in many annual grain legumes; this is the first known report of single gene control of this reaction in common bean or similar legumes. Generation means analysis indicated that genetic variation among generations for pod number under heat stress was best explained by a six-parameter model that includes nonallelic interaction terms, perhaps the result of the hypothetical abscission gene interacting with other genes for pod number in the populations. A simple additive/dominance model accounted for genetic variance for seeds per pod. Dominance [h] and epistatic dominance × dominance [l] genetic parameters for yield components under high temperatures were the largest in magnitude. Results suggest `Cornell 503' can improve heat tolerance in sensitive cultivars, and heat tolerance in common bean may be influenced by major genes.
Samuel F. Hutton, Jay W. Scott, and Jeffrey B. Jones
%, 4 = 6% to 12%, 5 = 12% to 25%, 6 = 25% to 50%, 7 = 50% to 75%, 8 = 75% to 87%, 9 = 87% to 94%, 10 = 94% to 97%, 11 = 97% to 100%, and 12 = 100% diseased tissue. Data were subjected to generation means analysis, and the estimated parameters were used
David W. Wolff, Wanda W. Collins, and Thomas J. Monaco
Several inheritance experiments with bentazon herbicide-tolerant Capsicum annuum `Bohemian Chili' (BCH P1) and susceptible `Keystone Resistant Giant' (KRG, P2) and `Sweet Banana' (SB, P2) were conducted. Populations of plants at the three- to five-leaf stage were treated with a bentazon rate of 4.5 kg·ha-1. Tolerance expression was affected by environment and varied across experiments. F2 and BCP2 generations from both susceptible parent crosses fit the expected ratios for a single, dominant gene conferring tolerance. Reciprocal F1s showed a maternal effect on tolerance intensity not consistently observed in reciprocal BCP2s or at all in reciprocal F2s. Segregation ratios of reciprocal crosses, however, were not heterogeneous, based on x2 tests of observed ratios in seven of eight cases. Variable tolerance expression in expected homogeneous populations (P1, P2, and F1) and lower tolerance in BC3 families suggested that modifying factors affected tolerance. Analysis of genetic components of shoot height and fresh weight generation means showed significant digenic epistasis, primarily additive × dominance. Modifying genes that affect the major gene controlling tolerance in BCH are, therefore, present. The simple inheritance of bentazon tolerance, even though modifying factors were present, facilitated transfer of bentazon tolerance into KRG via backcrossing. Chemical name used: 3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide (bentazon).
Kimberly H. Krahl and William M. Randle
Ornamental plant breeding has focused almost exclusively on floral prolificacy rather than floral longevity. The importance of floral longevity is obvious in any ornamental crop grown primarily for floral display. This study was undertaken to determine the genetic basis of floral longevity in an important ornamental crop, petunia.
Four parental lines representing the extremes for floral longevity were used to generate 6 F1 families which were selfed to generate F2 families and backcrossed to create 12 backcross families. Generation means analysis was utilizied to determine estimates of additive. dominance, and epistatic genetic variance for floral longevity in petunia.
Susan M. Stieve and Dennis P. Stimart
Selecting for increased postharvest longevity through use of natural variation is being investigated in Antirrhinum majus (snapdragon) in order to decrease postharvest chemical treatments for cut flowers. The postharvest longevity of eighteen white commercial inbreds was evaluated. Twelve stems of each inbred were cut to 40 cm and placed in distilled water. Stems were discarded when 50% of spike florets wilted or browned. Postharvest longevity ranged from 3.0 (Inbred 1) to 16.3 (Inbred 18) days. Crossing Inbred 18 × Inbred 1 yields commercially used Hybrid 1 (6.6 days postharvest). The F2 population averaged 9.1 days postharvest (range 1 to 21 days). F3 plants indicate short life postharvest may be conferred by a recessive gene in this germplasm. Populations for generation means analysis as well as hybrids between short, medium and long-lived inbreds were generated and evaluated for postharvest longevity.
Karl J. Sauter, David W. Davis, Paul H. Li, and I.S. Wallerstein
Yield in common bean, Phaseolus vulgaris L., can be significantly reduced by high temperature (I-IT) during bloom. Ethylene production from plant tissue increases as a consequence of various stresses, including heat stress. The inheritance of leaf ethylene evolution rate (EER) of HT-stressed (35/30C day/night) progenies from crosses among bean genotypes previously categorized as HT sensitive or tolerant, based on cell electrolyte leakage, was investigated. Evidence from generation means analysis of Fl, F2, and backcross progenies shows EER to be genetically controlled, with additive, dominance, and epistatic effects indicated for low EER. The range (0.62 to 2.52 μg-1·hr-1) of EER from field-grown lines and cultivars suggests the existence of considerable genetic variability. EER was associated (r = –0.70) with heat tolerance, as estimated by cell electrolyte; leakage.