Cycles Of Selection For Resistance Research Articles

Genetic Variances and Heritabilities of Traits of an Early Yellow Maize Population after Cycles of Improvement for Striga Resistance and Drought Tolerance.

Drought and Striga are principal constraints to maize (Zea mays L.) production in sub-Saharan Africa. An early yellow maize population, TZE-Y Pop DT STR, which had undergone five cycles of selection for resistance to Striga, followed by three cycles of improvement for drought tolerance, was investigated for yield gains, changes in genetic variance, and interrelationships among traits under drought stress and optimum environments. Two hundred and forty S1 lines comprising 60 each from the base population and subsequent populations from three selection cycles improved for grain yield and drought tolerance were assessed under drought and optimal environments in Nigeria from 2010 to 2012. Genetic improvements in grain yield of 423 and 518 kg ha–1 cycle–1 were achieved under drought stress and optimal environments. Predicted improvements in selection for yield were 348 and 377 kg ha–1 cycle–1 under drought stress and optimum environments, respectively. The highest yield observed in C3 was accompanied by reduced days to silking and anthesis–silking interval, improved plant aspect and ear aspect, and increased plant height and ears per plant across research environments, as well as improved stay-green characteristic under drought. The level of genetic variability for yield and a few other traits were maintained under drought and optimal environments in the population. The presence of residual genetic variability for yield and other assayed traits in C3 indicated that progress could be made from future selection in the population depending on the ability of breeders to identify outstanding genotypes and the precision level of experimentation. Substantial improvement has been made in yield and drought tolerance in C3 of the population.

Phytochemical and Nutraceutical Changes during Recurrent Selection for Storage Pest Resistance in Tropical Maize

ABSTRACTMaize (Zea mays L.) is a staple crop used for subsistence by smallholder farmers around the world, who usually store grain for gradual use. During the course of storage, losses due to storage pests often reach 20%. Over three cycles of recurrent selection, population improvement to develop sources of resistance to the larger grain borer (Prostephanus truncates) and the maize weevil (Sitophilus zeamais) resulted in a new population called Population 84 (P84). The objective of this study was to evaluate the biophysical, phytochemical, and nutraceutical profile changes in P84 derived from three cycles of selection for resistance. Insect resistance parameters were evaluated using bioassay and biophysical, phytochemical, and nutraceutical parameters. Resistance to storage pests increased by two‐ to threefold after three cycles of selection for maize weevil and larger grain borer, respectively. The major factors related to enhanced resistance are kernel hardness (10% increase) and proportion of pericarp (68% increase) in the kernel. Comparing the phytochemical composition of the pericarp cell wall before and after selection revealed an elevated level of cell wall–bound components (42% increase), including ferulic and diferulic acids (14% and twofold increase, respectively). Increases of 18% in free phenolic acid were found in the endosperm. A significant association between insect resistance with antioxidant activity (r > 0.75) establishes the possibility of seeking increased nutrients beneficial for human health in the same maize germplasm selected for increased insect resistance and stemming from the same compounds. Population 84 is a good source for resistance to postharvest pests with enhanced nutraceutical properties.

Registration of ‘Ezra’ Alfalfa

‘Ezra’ (Reg. No. CV‐207, PI 664904) alfalfa (Medicago sativa L.) was developed by Cornell University for higher forage yield and multiple disease resistance. After evaluating it as experimental designation NY 0240, Ezra was released by the Cornell University Agricultural Experiment Station. This cultivar is the result of a cross between two populations. The first population was derived by completing one cycle of selection for resistance to Phytophthora root rot (caused by Phytophthora megasperma Drechs. f. sp. medicaginis Kuan & Erwin) in ‘Seedway 9558’. The second population traces back to a cross between ‘Magnum III’, which had been selected for resistance to anthracnose (Race 1) (caused by Colletotrichum trifolii Bain & Essary) and to Verticillium wilt (caused by Verticillium albo‐atrum Reinke & Berth.), and plants from multiple Cornell breeding populations related to ‘Oneida VR’, the resulting population which had been further selected for resistance to Verticillium wilt, anthracnose (Race 1), and Phytophthora root rot. Full‐sib crosses between the first and second populations were made by hand (125 plants per population) by crossing an individual plant from one population with one plant from the other population. Equal amounts of seed from each full‐sib cross were bulked to form Syn. 1 seed. Ezra is adapted to the North Central and Northeast regions of the United States and has fall dormancy similar to the FD3 check. Flower color is 78% purple and 22% variegated, with traces of yellow, cream, and white. Pod shape is 76% tightly coiled, 21% loosely coiled, and 3% crescent. Ezra has high resistance to anthracnose (Race 1) and Fusarium wilt [caused by Fusarium oxysporum Schlecht. f. sp. medicaginis (Weimer) Snyd. & Hans.] and resistance to Verticillium wilt, bacterial wilt, and Phytophthora root rot. It is susceptible to Aphanomyces root rot (Race 1) (caused by Aphanomyces euteiches Drechs.).

Improving Birdsfoot Trefoil for Resistance to Fusarium Wilt

ABSTRACTFusarium wilt (caused by Fusarium oxysporum f. sp. loti) is a serious disease of birdsfoot trefoil, Lotus corniculatus L., reducing yield of forage and seed. In the early 1990s a breeding program was begun to breed a Fusarium wilt‐resistant birdsfoot trefoil. Our objectives were to (i) determine progress from recurrent phenotypic selection for resistance to Fusarium wilt in six birdsfoot trefoil populations and (ii) compare forage yield and persistence of Fusarium wilt‐resistant populations to unselected populations in field trials. Recurrent phenotypic selection was effective in improving resistance in the greenhouse, with mean improvement of 35% across all populations after three cycles of selection. In a field with high natural disease pressure, a population developed after two cycles of selection for resistance had lower levels of disease incidence and yielded 1.33 Mg ha−1 more than the mean of nine cultivars in the seeding year. The Fusarium wilt‐resistant population had 52% higher plant stand than the mean of the other cultivars at the end of the first production year. In a trial inoculated with the Fusarium wilt organism, two cycle 3 populations averaged 28% more yield by June of the fourth production year and 22 more plants per m2 than the mean of the unselected populations. Birdsfoot trefoil with resistance to Fusarium wilt yielded more and persisted longer than other cultivars under Fusarium wilt‐infested field conditions.

Single-gene resistance to powdery mildew in zucchini squash (Cucurbita pepo)

Cucurbita pepo (pumpkin, squash,gourd) is an economically important species that is susceptible to the cucurbit powdery mildew fungus, Podosphaera xanthii(syn. Sphaerotheca fuliginea). ‘True French’, an open-pollinated cultivar of the Zucchini Group of C. pepo, was crossed with an unnamed powdery-mildew resistant straight neck-type accession, the resistance of which was apparently derived from an interspecific cross with a resistant wild species of Cucurbita,and resistant plants were selected in the F2 generation. This was followed by six cycles of backcross-pedigree selection for resistance, and resulted in the development of an accession true-breeding for resistance to powdery mildew and nearly isogenic to ‘True French’. The resistant and susceptible near-isogenics were crossed and seeds of the filial and backcross generations were produced. Plants of the parental accessions and their progenies were grown together in a controlled-environment chamber, exposed to the pathogenic fungus, and scored as resistant, partially resistant, or susceptible 27–33 days after sowing. The results indicated that resistance is conferred by a single incompletely dominant gene, designated Pm-0.

Inheritance of Resistance to Aspergillus Ear Rot and Aflatoxin Production of Corn from Tex6

ABSTRACT The inheritance of resistance to Aspergillus ear rot and aflatoxin production in corn (Zea mays L.) caused by Aspergillus flavus was studied in progeny derived from crosses between the resistant corn inbred cv. Tex6 and susceptible inbred cvs. B73 and Mo17. From 1994 to 1996, plant generations included were the P(1) (susceptible B73 or Mo17), P(2) (resistant Tex6), F(1), F(2), F(3), BCP(1), BCP(1)-selfed, and BCP(2). The BCP(2)-selfed generation was added in 1995 and 1996 for the B73 x Tex6 cross. Primary ears were pinboard inoculated and evaluated for Aspergillus ear rot severity. F(1) means deviated from the midparent value toward resistance for aflatoxin production and toward susceptibility for ear rot in both crosses. Analyses of generation means indicate that additive gene action was most important in the resistance to both ear rot and aflatoxin production in the B73 x Tex6 cross. Mo17 was somewhat resistant to both traits, so resistance from Tex6 was not well defined in this cross. Broad-sense heritabilities for ear rot and aflatoxin production were 58 and 63% for Mo17 x Tex6, and 66 and 65% for B73 x Tex6. Narrow-sense heritabilities for ear rot and aflatoxin production were 39 and 45% for B73 x Tex6. It is estimated that one cycle of selection for resistance within B73 x Tex6 F(3) families would reduce the percentage of ear rot severity by 8.5% and aflatoxin concentration by 19 ng/g.

Selection for resistance to root rot caused by Fusarium spp. in red clover (Trifolium pratense L.)

Root and crown rots caused by Fusarium spp. are a major cause of stand decline of red clover (Trifolium pratense L.) in North America. The goal of this study was to evaluate the effectiveness of one cycle of selection for resistance to Fusarium root rot. Plants of the red clover cultivars Florex and Arlington were screened for their resistance to infection by two isolates of Fusarium acuminatum and one isolate of F. avenaceum. Plants of various infection categories were intercrossed, and progenies were screened to estimate the heritability of resistance to these pathogens. Synthetics produced by intercrossing either highly resistant (R) or highly susceptible plants were evaluated under sward conditions in the field for dry matter yield, root rot and adventitious root development. There were significant (P = 0.05) differences in aggressiveness among the three Fusarium isolates used, with F. avenaceum 814 being the most aggressive. Plants classed as R for all three isolates were found in both cultivars; however, disease reaction was not always correlated among the isolates. Parent-progeny regressions or correlations of disease reaction were negative in the cultivar Arlington, while for Florex, a relatively low regression coefficient of 0.26 was found. In the field evaluation in the fall of the third year after seeding, synthetics of plants selected for resistance were either not different from, or higher in infection score than the parental cultivars, indicating that selection using one or two isolates/species was ineffective in improving field root rot resistance. Plants with more severely rotted roots showed a tendency to produce more adventitious roots from the crown. It was concluded that progeny testing would be necessary for selection to be effective for resistance to Fusarium root rot in red clover and that more that one isolate or species of the fungus should be used in screening programs. In addition, not all populations of red clover contain sufficient genetic variation for resistance to justify a selection program. Key words: Red clover, disease resistance, selection, root rot, adventitious roots, Trifolium pratense L., Fusarium spp.

Glycoalkaloid contents in tubers fromSolanum tuberosum populations selected for potato leafhopper resistance

Solanum tuberosum L. Gp. Tuberosum populations that had been improved for resistance to potato leafhopper,Empoasca fabae (Harris), by recurrent selection, were analyzed for tuber contents of the glycoalkaloids α-solanine and α-chaconine. The mean content of both glycoalkaloids was significantly higher in the more resistant populations compared with the unselected population. In a population generated from seven cycles of selection for resistance, the mean content of solanine + chaconine was 9.5 mg/100 g fresh weight compared with 4.0 mg/100 g in the original, unselected population; a 137% increase. Kennebec, in the same experiment, averaged 10.9 mg/100 g; Katahdin averaged 4.6 mg/100 g.

Selection for Resistance to Aphanomyces euteiches in Red Clover

Red clover (Trifolium pratense L.) is known to be a moderate host for Aphanomyces euteiches Drechs. Recently, an isolate of A. euteiches (Ae‐572) was observed to be highly virulent on red clover. This research was conducted to determine inheritance of, and select for, resistance to A. euteiches in red clover using an inoculated seedling assay. Resistant and susceptible red clover plants were intercrossed to study the inheritance of resistance. Frequency distributions of resistant and susceptible F1 plants indicated that resistance to A. euteiches is inherited as a quantitative trait. Three cycles of selection for resistance to A. euteiches in red clover resulted in significant improvement in resistance from the original selected population (Cycle 0 = 3.64 mean disease severity index (DSI); Cycle 3 = 2.43 mean DSI). Narrow‐sense heritability estimates were high enough ((h2n = 51% for Cycle 3) to assure that a recurrent selection program utilizing family selection could be used to improve red clover populations for resistance to A. euteiches.

Genetic variation and selection response to iron deficiency chlorosis in arrowleaf clover1

Abstract All cultivars of arrowleaf clover (Trifolium vesiculosum Savi) exhibit symptoms of iron (Fe)‐deficiency chlorosis when grown on calcareous soils. Previous research has shown that arrowleaf clover is adapted to the climatic areas where calcareous soils do occur. Since arrowleaf clover is highly cross‐pollinated, cultivars are made up of heterogeneous populations of plants. When individual seedlings of ‘Yuchi’ arrowleaf clover were grown in the greenhouse in saturated calcareous soils, most seedling expressed severe chlorosis. Resistant (green) plants were retained and intermated through three cycles of recurrent selection for resistance to Fe‐deficiency chlorosis. The frequency of occurrence of resistant plants has been increased by this selection process. Cycle 3 seedlings averaged 44% resistant vs 17% for unselected Yuchi. Highly susceptible seedlings accounted for 16% and 48% of Cycle 3 and Yuchi arrowleaf clover, respectively.

European Corn Borer Resistance and Cell Wall Composition of Three Maize Populations

Feeding activities of herbivorous insects are influenced by host plant nutritional quality. Improved insect resistance resulting from either natural or artificial selection may be due, in part, to changes in nutritive constituents of plants. The first objective of this study was to measure changes in detergent fiber, lignin, ash, and N concentrations in whorls, leaf‐sheaths, and stalks of the BS9 maize (Zea mays L.) population across five cycles of selection for resistance to the European corn borer (ECB) [Ostrinia nubilalis (Hübner)]. The second objective was to evaluate ECB resistance in the WFISIHI and WFISILO maize populations, which were developed for high and low concentrations, respectively, of indigestible plant constituents (acid detergent fiber, lignin, and silica) in the leaf sheath. Leafsheath composition for all five cycles of BS9 was measured in three environments in Iowa. Whorl, leaf‐sheath and stalk composition, as well as first‐ and second‐generation ECB resistance of populations WFISIHI, WFISILO and Cycles 0, 2, 4, and 5 of BS9 were evaluated in two environments in Wisconsin. Whorl composition was not related to changes in ECB resistance in anypopulation. In BS9, leaf‐sheath and stalk concentrations of neutral and acid detergent fiber, cellulose, and lignin increased linearly over selection cycles. In contrast, WFISIHI was as susceptible to second‐generation ECB as WFISILO, suggesting that the responses in BS9 may be due to linkage or unintentional selection. Populations BS9, WFISIHI, and WFISILO, however, were derived from diverse sources, and it is likely that mechanisms for resistance differ for the three populations.

Recurrent selection for resistance to Stemphylium vesicarium within the lucerne cultivars Trifecta and Sequel

Two cycles of recurrent selection for resistance to Stemphylium leaf spot (caused by Stemphylium vesicarium) based on half-sib family performance were made within the lucerne cultivars Trifecta and Sequel. Within Trifecta, 1 generation of selection was sufficient to increase the level of resistance to that of UC 1249 (resistant check), while 2 generations were necessary to obtain equivalent levels of resistance in Sequel. The Stemphylium resistant lines maintained adequate levels of resistance to anthracnose (caused by Colletotrichum trifolii) and Phytophthora root rot (caused by Phytophthora megasperma f. sp. medicaginis) on the basis of glasshouse tests.

COMPARISON OF FOUR METHODS OF SELECTION FOR RESISTANCE TO LEPTOSPHAERULINA BRIOSIANA IN ALFALFA

Three cycles of selection for resistance to Leptosphaerulina briosiana (Poll.) Graham &Luttrell were conducted in two alfalfa (Medicago sativa, L.) germplasm pools, MSA and MSB. Each germplasm pool was used to compare four methods of selection: phenotypic recurrent, half-sib family, full-sib family, and alternating generations of selfed family and half-sib family. Response to selection for resistance to L. briosiana was greater in MSA than in MSB. Differences between selection methods were not significant. Selection for resistance to L. briosiana generally increased resistance to Stemphylium botryosum Wallr., but the magnitude of the correlated response varied with germplasm pool and selection method. The initial selfed families in both germplasm pools were significantly less resistant to Colletotrichum trifolii Bain than the other family types. Resistance to C. trifolii increased with selfed family selection for resistance to L. briosiana in MSA but not in MSB.

GENETICS OF RESISTANCE AND SUSCEPTIBILITY IN ALFALFA TO ALFALFA SICK SOIL

Alfalfa sickness occurs in central Alberta. It is characterized by yellowed, stunted, and poorly nodulated seedlings of alfalfa (Medicago media Pers.). Alfalfa seedlings were grown on "sick soil" in a growth room and evaluated for height, disease rating, and yield of dry matter, during three cycles of selection for resistance and susceptibility. Results showed that recurrent phenotypic selection was an effective breeding method to increase resistance to alfalfa sickness. High broad sense heritabilities for height and disease rating suggested that genotypic variability exists within locally adapted cultivars. Low narrow sense heritabilities for height and disease rating in resistant genotypes indicated that additive genetic variation was small and successive cycles of recurrent selection would be necessary to develop resistance. High specific combining ability variance in the resistant and susceptible diallel crosses demonstrated large nonadditive genetic variation.

Associated Effects of Mass Selection for Soil-Insect Resistances in Sweet Potato1

Abstract Character associations in a sweet potato [Ipomoea batatas (L.) Lam.] population after 4 cycles of selection for resistance to soil insects and in a control population with no selection were studied by use of contingency tables of pairs of traits. Possible common factors were indicated for resistances to the sweet potato flea beetle and the wireworm-Diabrotica-Systena (WDS) complex. Selection changed the means and distributions of 6 of 13 unselected root and vine traits. None of these changes were directly associated with insect resistances, but that in root cracking was caused by the grading techniques. Two traits appeared genetically associated (cortex thickness and leaf-whorl color), and 2 appeared to be expressions of the same character change (flesh-color changes were also expressed as skin-color changes). The selected population had shorter internodes than the unselected. These changes in unselected traits were probably due to drift caused by small population sizes in the selected generations. No barriers to development of insect-resistant cultivars were detected.

Effects of Three Cycles of Selection for Resistance to Common Leafspot in Alfalfa1

Effects of Three Cycles of Selection for Resistance to Common Leafspot in Alfalfa¹