High level of resistance to Sclerotinia sclerotiorum in introgression lines derived from hybridization between wild crucifers and the crop Brassica species B. napus and B. juncea

Sclerotinia rot caused by the fungus Sclerotinia sclerotiorum is one of the most serious and damaging diseases of oilseed rape and there is keen worldwide interest to identify Brassica genotypes with resistance to this pathogen. Complete resistance against this pathogen has not been reported in the field, with only partial resistance being observed in some Brassica genotypes. Introgression lines were developed following hybridization of three wild crucifers (viz. Erucastrum cardaminoides, Diplotaxis tenuisiliqua and E. abyssinicum) with B. napus or B. juncea. Their resistance responses were characterized by using a stem inoculation test. Seed of 54 lines of B. napus and B. juncea obtained from Australia, India and China through an Australian Centre for International Agricultural Research (ACIAR) collaboration programme were used as susceptible check comparisons. Introgression lines derived from E. cardaminoides, D. tenuisiliqua and E. abyssinicum had much higher levels (P < 0.001) of resistance compared with the ACIAR germplasm. Median values of stem lesion length of introgression lines derived from the wild species were 1.2, 1.7 and 2.0 cm, respectively, as compared with the ACIAR germplasm where the median value for stem lesion length was 8.7 cm. This is the first report of high levels of resistance against S. sclerotiorum in introgression lines derived from E. cardaminoides, D. tenuisiliqua and E. abyssinicum. The novel sources of resistance identified in this study are a highly valuable resource that can be used in oilseed Brassica breeding programmes to enhance resistance in future B. napus and B. juncea cultivars against Sclerotinia stem rot.     

Persistence of an oilseed rape transgene in the environment

Transgenic bromoxynil (BX)-resistant spring oilseed rape (Brassica napus L.) was grown commercially in Canada only for two crop seasons – 2000 and 2001. We investigated the persistence of suspected BX-resistant oilseed rape volunteers in a 64-ha wheat field in Saskatchewan, Canada in 2007, 7 years after the BX-resistant cultivar BX Armor was grown. A small oilseed rape volunteer population, estimated at less than 100 plants, was observed in three main areas or patches in the field. These patches were located in low-lying areas that were too wet to plant or spray with herbicides in 2007. Viable seed was collected from 35 mature volunteers and F₁ progeny screened with BX at 280 g ai/ha in the greenhouse. Progeny of all of the volunteers were visually rated as BX-resistant; the presence of the Oxy 235 transgene in leaf tissue of progeny of all volunteers was confirmed by PCR analysis. This study has documented the longest persistence of oilseed rape volunteers in Canada. Volunteers were not observed in 2008 or 2009, because of drought conditions in spring of both years. Results support the findings from previous studies that persistence of volunteer oilseed rape populations in western Canada is generally ephemeral or transitory in the absence of seed bank immigration. However, this study shows that oilseed rape transgenes can persist in the environment for a number of years even after all cultivars with the conferred trait are removed from the market.     

Both incidence and severity of white rust disease reflect host resistance in Brassica juncea germplasm from Australia,China and India

White rust (Albugo candida) is a highly destructive disease of oilseed Brassicas such as Brassica juncea and B. rapa, and has caused serious yield losses in Australia, China and India on both species. The first commercial B. juncea varieties are now being deployed in Australia, but their response to Australian strains of A. candida is yet to be defined under Australian field conditions. To identify useful sources of host resistance for Australia, China and India, in B. juncea, three field trials were undertaken in Western Australia. Forty-four B. juncea genotypes, viz. 22 from India, 12 from Australia and 10 from China, were tested. Varying levels of host resistance to Australian strains of A. candida (race 2) were identified among the genotypes from the three countries. Genotypes CBJ-001, CBJ-003 and CBJ-004 from China consistently showed high levels of resistance to A. candida on leaves across the three trials. Overall, the genotypes from China showed the best resistance, followed by the genotypes from Australia, with those from India being the most susceptible. The most susceptible genotypes were RL1359, RH30 and Seetha from India. It is noteworthy that both the incidence and severity of disease reflected varying levels of host resistance in the germplasm from the three countries, irrespective of whether screening was undertaken in the field using natural or artificial inoculation. Differentiation of resistance among these genotypes was similar to that we reported previously for artificially-inoculated seedlings or adult plants under glasshouse conditions, indicating that a choice of options is available to plant breeders to reliably differentiate host resistance among genotypes to white rust in B. juncea.     

Perennial O. sativa × O. rufipogon interspecific hybrids: I. Photosynthetic characteristics and their inheritance

A survey of 24 wild Oryza accessions identified Oryza australiensis and Oryza rufipogon as potential sources of enhanced photosynthetic rate for introgression into cultivated rice. Photosynthetic capacity per unit leaf area (CER) was associated with leaf N content but not with leaf chlorophyll concentration, flag leaf area, or specific leaf area. Eight fertile, perennial F₁ hybrids between O. sativa and O. rufipogon were grown in non-flooded soil, and CER was measured at flowering under saturating light. Two F₁ hybrids had greater CER than the average of 26.1 μmol m² s⁻¹. The F₂ progeny from these hybrids were screened for CER in the field, and segregants with even greater rates of photosynthesis were selected. The basis of high photosynthetic rate in the F₂ populations was not leaf thickness or leaf chlorophyll content. One F₂ line had exceptionally high CER and stomatal conductance. Broad-sense heritability on an individual plant basis for CER in two F₂ populations was 0.44 and 0.37. A highly significant offspring-parent regression of 0.89 for CER was observed in a replicated field evaluation (four blocks, five plants per plot) of 20 vegetatively propagated F₂ selections and their F₃ seedling progeny. Broad-sense heritability for CER on a plot-mean basis was estimated as 0.74 for both selected F_2:3 families and for the selected F₂ clones. Genetic resources in the genus Oryza may represent a source of alleles to increase leaf photosynthetic rate in the cultivated species, which we have demonstrated to be a heritable, though environmentally variable, trait in an O. sativa/O. rufipogon population.     

Segregation analysis indicates that Puroindoline b-2 variants 2 and 3 are allelic in Triticum aestivum and that a revision to Puroindoline b-2 gene symbolization is indicated

Genetic and kernel texture relationships between Puroindoline b-2 variants 2 and 3 have not been fully established in wheat (Triticum aestivum L.). Here, 480 F₂ plants, derived from three hard spring wheat populations were used to test the segregation of Puroindoline b-2 (Pinb-2) variants 2 and 3. Chi-square analysis indicated that Pinb-2 variants 2 and 3 in all three F₂ populations segregated as a single bi-allelic locus, with segregation ratios fitting a 1:2:1 ratio. Using 448 of the 480 plants derived from these three F₂ populations, the average SKCS hardness index of plants homozygous for Pinb-2 variant 2 vs. those homozygous for variant 3 was not significantly different (67.5 vs. 67.9). Results indicated that plants with Pina-D1b/Pinb-D1a were on average 10.0 Single Kernel Characterization System (SKCS) hardness index units harder than those carrying the Pina-D1a/Pinb-D1b haplotype. In conclusion, Pinb-2 variants 2 and 3 are allelic and exert little effect on kernel texture in hard-kernel T. aestivum germplasm. Further, the designation of Pinb-2v2 and Pinb-2v3 should be changed to Pinb-B2a and Pinb-B2b, respectively. We propose that Pinb-2 variants 1 and 4 of Chinese Spring be designated Pinb-D2a and Pinb-A2a, respectively.     

Highly efficient in vitro regeneration of the industrial oilseed crop Crambe abyssinica

A highly efficient regeneration protocol for oilseed crop Crambe abyssinica has been developed using hypocotyls as explants in this study. Crambe is a potential engineering oilseed crop for industrial purposes as it contains 55-60% erucic acid in its oil and, more importantly, it does not outcross with any food oil seed crops. However, the low regeneration frequency with the currently available protocols is still a limiting factor for genetic modification of Crambe. In this study, we investigated the effects of N-source, C-source, AgNO₃, cultural conditions as well as the concentration and combination of plant growth regulators (PGR) on the regeneration frequency of C. abyssinica. The results showed that all these factors, especially the N-source and PGR concentrations and combinations, played an important role in shoot regeneration. Among all the factors tested, the combination of using hypocotyls from C. abyssinica cv. galactica, the Lepiovre basal medium supplemented with 16 g l⁻¹ glucose, 0.5 g l⁻¹ AgNO₃, 2.2 mg l⁻¹ thidiazuron (TDZ), 0.5 mg l⁻¹ α-naphthaleneacetic acid (NAA), 2.5 g l⁻¹ Gelrite, seeds germinated in dark for 3 days and explants cultured in light, gave the best regeneration frequency (over 95%). The results also suggest that reducing the content of NH₄⁺ or keeping a suitable NO₃⁻/NH₄⁺ ratio in the regeneration medium would be crucial to Crambe shoot regeneration.     

Susceptibility of Helicoverpa armigera from different host plants in northern China to Bacillus thuringiensis toxin Cry1Ac

Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) is a serious pest of cotton and many other crops in northern China. To evaluate the contribution of alternative hosts as an effective refuge for transgenic cotton expressing the Bacillus thuringiensis (Bt) Cry1Ac toxin, the susceptibility to this toxin was measured in progeny derived from field-collected H. armigera larvae and pupae from different hosts in the Xiajin’s region of the Shandong Province in northern China. During 2008-2010, progeny from a total of 258,56,184 and 160 single-pair crosses derived from wheat (first-generation), Bt cotton (second-generation), Bt cotton (third-generation), and corn (third-generation) were screened on Cry1Ac diets, respectively. Based on relative average development rates (RADR) of H. armigera larvae in these F₁ tests, the second and third-generation moths emerging from Bt cotton fields were more tolerant to the Bt toxin than the first and third-generation moths emerging from wheat and corn each year. These results suggest that there is significant variation in susceptibility to Bt toxins among H. armigera populations derived from different host crops. Alternate crops, such as corn, that maintain Bt susceptible populations of H. armigera could be used as refugia to minimize the evolution of resistance to Bt cotton.     

Frequency of Bt resistance alleles in Helicoverpa armigera in 2007-2009 in the Henan cotton growing region of China

Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) is a key insect pest of cotton in the Henan cotton growing region of China. In this region, cotton is grown on small acreages in rich agricultural landscapes, contrary to cropping systems in the United States or Australia. Under such cropping regimes, naturally occurring refuges (with non-Bt plants) may be sufficient to prevent H. armigera resistance development to Bt toxins. In order to gain a timely understanding of the evolution of resistance of H. armigera to Bt toxin after continuous cultivation of Bt cotton for c. 10 years, we assessed the frequency of alleles conferring resistance to Cry1Ac toxin in field populations of H. armigera sampled from Xinxiang County in Henan province during 2007-2009. Screening F₁ and F₂ generations from isofemale lines, derived from female moths trapped in the field, were used with a discriminating dose of Cry1Ac diet to estimate the frequency of resistance alleles. Totals of 625, 516 and 488 isofemale lines were screened for the F₁ generation in 2007, 2008 and 2009, respectively. Resistance gene frequency in Xinxiang fluctuated between 0.0000 and 0.0005, and it did not increase significantly from 2007 to 2009. Based on the relative average development rates (RADR) of H. armigera larvae in F₁ tests, no substantial increase in Cry1Ac tolerance was found in the Xinxiang region over the 3-yr period.     

Differential reproductive maturity between geographically separated populations of Homalodisca vitripennis (Germar) in California

The glassy-winged sharpshooter, Homalodisca vitripennis (Germar), is native to the southeastern United States and northeastern Mexico. It was detected in southern California in the late 1980s and in the San Joaquin Valley in 1999, where it transmits the bacterium Xylella fastidiosa to grapevines and other crops. The reproductive success of hybrid and pure line H. vitripennis from two geographically separated populations in California (Riverside (RIV) and Bakersfield (BAK)) was evaluated under identical conditions. The RIV and BAK populations had different preoviposition periods that persisted through the second generation of each lineage. From adult molt, the preoviposition period in both female generations was significantly shorter for RIV (F₀ = 28.2 days and F₁ = 62.3 days) than BAK females (F₀ = 46.1 days and F₁ = 170.4 days). After a 21-day mating period, F₀ and F₁ females deposited on average 391 (range, 21–967) and 196 (range, 0–755) eggs, respectively, without significant differences in fecundity among the F₀ and F₁ mating pair treatments. Egg accumulation rates among F₁ treatments showed that females in the RIV groups rapidly deposited their eggs within the first 120 days after adult molt while BAK females maintained a steady accumulation rate during their life. The performance of both hybrid lines was intermediate between the pure lineages. The F₀ mating pairs: ♀RIV × ♂RIV, ♀RIV × ♂BAK, ♀BAK × ♂RIV, and ♀BAK × ♂BAK produced on average 185, 94, 79, and 0 viable eggs, respectively, which suggested a delayed sexual maturity of BAK males and females. The proportion of viable eggs deposited decreased gradually, which suggests that females completely exhausted sperm reserves. From a management perspective, delayed reproductive maturity and polyandry are weak links in H. vitripennis’ biology that may be exploited through mating disruption or insect sterilization strategies to reduce population growth and augment pressure by natural enemies.     

利用花青素基因标记研究正反交对玉米杂交种主要性状的影响

以转花青素合成转录因子Bi和Cl基因玉米的稳定表达株系与自交系B73-329(CK)进行杂交,研究花青素基因在玉米杂种F_1植株组织、器官中表达情况。结果表明,用转基因株系作为母本的杂交种F_1植株部分存在表型分离,用自交系B73-329作为母本的杂交种F_1植株不存在表型分离。转基因杂交种F_1植株,颖紫色,苞叶、茎鞘紫色,气生根紫色,与CK差异显著;转基因杂交种F_1植株叶片为绿色,转基因株系自交分离群体株系的叶片为紫色;转基因杂交种F_1植株绝大部分花丝为黄色,个别花丝红色,子粒绝大部分为黄色。抽丝、散粉期,转基因杂交种株高、穗位高等农艺性状与对照差异不显著,但是大部分产量显著低于对照,推测其主要原因在于花青素合成途径的引入导致植株能量代谢损耗所致。研究表明,转基因玉米杂交制种应用应以转基因株系作为父本。     

Comparative susceptibility of Ostrinia furnacalis, Ostrinia nubilalis and Diatraea saccharalis (Lepidoptera: Crambidae) to Bacillus thuringiensis Cry1 toxins

Transgenic corn hybrids that express toxins from Bacillus thuringiensis (Bt) are highly effective against the European corn borer, Ostrinia nubilalis (Hübner), and the closely related Asian corn borer, Ostrinia furnacalis (Guenée). Since the registration of Bt corn hybrids in the U.S. in 1996, there has been a great deal of information generated on O. nubilalis. However, relatively little information exists for O. furnacalis. To help determine whether the information generated for O. nubilalis can be leveraged for decisions regarding the use of transgenic Bt corn against O. furnacalis, experiments were designed to determine whether the pattern of sensitivity to various Bt Cry1 toxins is similar between the two species. Test insects included laboratory-reared O. furnacalis originating from Malaysia, a Bt-susceptible laboratory colony of O. nubilalis maintained at the University of Nebraska-Lincoln (UNL) and an out-group consisting of the sugarcane borer, Diatraea saccharalis (F.), from Louisiana which represents a different genus from the same family. O. furnacalis and O. nubilalis exhibited a similar pattern of susceptibility to all the Cry1 toxins and were highly susceptible to the range of Bt toxins tested including Cry1Aa, Cry1Ab, Cry1Ac and Cry1F. Both of the Ostrinia species were more tolerant to Cry1Ba compared with D. saccharalis, although sensitivity of O. furnacalis was intermediate and did not differ significantly from that of O. nubilalis and D. saccharalis. D. saccharalis was also susceptible to the range of toxins tested but unlike the two Ostrinia species, was more tolerant to Cry1F and more susceptible to Cry1Ba. These results indicate that both of the Ostrinia corn borer species are similar in sensitivity to the Cry1Aa, Cry1Ab, Cry1Ac, Cry1Ba and Cry1F toxins, thus suggesting shared toxin receptors and mechanisms of toxicity for the two species.     

Polymorphisms in the ALS gene of weedy rice (Oryza sativa L.) accessions with differential tolerance to imazethapyr

Herbicide-resistant Clearfield™ rice technology allows the use of ALS inhibitors to control weedy rice. Weedy rice plants have differential tolerance to imazethapyr, perhaps due to ALS polymorphisms. We aimed to assess ALS polymorphisms in weedy rice accessions from Arkansas, USA, with differential tolerance to imazethapyr in seedling growth bioassays. Six base changes were identified in the ALS of 14 weedy rice accessions. Three of these nucleotide changes resulted in amino acid substitutions — Pro⁹³Thr, Glu⁶³⁰Asp, and Val⁶⁶⁹Met — in four accessions: Ark-4, Ark-9, Poi-1 and Poi-6. The GR₅₀ values and inhibition of root and shoot growth (%) of these accessions differed. The Glu⁶³⁰Asp substitution occurs in the herbicide binding domain B and Val⁶⁶⁹Met occurs at the C-terminal tail where the co-factor binds. Variability in weedy rice ALS exists, but polymorphism patterns did not relate to tolerance levels. The observed mutations presented the possibility that sustained selection pressure will eventually lead to selection of a herbicide-tolerant individual that will be the progenitor of a resistant population. Concomitantly, pollen-mediated gene flow from Clearfield™ rice to weedy rice will lead to the evolution of ALS-resistant weedy rice populations.     

Genetic structure of natural Tunisian Hypericum humifusum L. (Hypericacae) populations as assessed by allozymes and RAPDs

The genetic diversity within and among seven Tunisian natural populations of Hypericum humifusum L., from different geographic regions and bioclimates, was assessed using 11 isozymic polymorphic loci, and 166 RAPD markers amplified by 8 primers. The genetic diversity within populations based on allozymes was higher (P = 72.46%, Ap = 2.01 and H_e = 0.29), than that revealed by RAPDs (29.52 < P < 39.16% and 0.150 < H < 0.200). Both markers yielded high estimates of genetic differentiation and low gene flow (Nm = 0.257 and 0.508 for RAPD and allozymes, respectively) among populations at all space scales. However, the level of differentiation revealed by RAPDs (Φ_ST = 0.494; G_ST = 0.561) was higher than that based on allozymes (F_ST = 0.117). No correlation (Mantel test) among genetic (F_ST and Φ_ST matrices) and geographical distance matrices was observed indicating no isolation by distance. Cluster analyses from allozyme and RAPD loci did not completely agree. The dendrogram based on allozymes yielded higher separation among most populations, while that from RAPDs separated populations into three distinct subclusters. Groupings of populations, in both dendrograms, did not reflect spatial geographic or bioclimatic patterns, indicating specific adaptation of populations to local environments. The correlation between matrices of allozyme and RAPD band frequencies was not significant (Mantel test). The dendrogram obtained from combined data yielded similar population groupings to that probed by RAPDs suggesting higher accuracy of these markers. Given the high differentiation among all populations even at a low geographic distance, ex situ conservation should involve extensive seed collection from all populations from all bioclimatic zones. The populations from the upper semi-arid bioclimate exhibiting relatively high level of genetic diversity should be first protected.     

Efficiency of mesosulfuron-methyl and clodinafop-propargyl dose for the control of Lolium perenne in wheat

Increasing environmental and economic concerns have resulted in considering the potential for the successful use of herbicides at lower doses within an integrated approach to weed management. Therefore, field experiments were carried out to evaluate the efficiency of mesosulfuron-methyl and clodinafop-propargyl dose to control Lolium perenne in pure stand and in mixture with wheat. The herbicides at 5 different doses were assigned in a factorial arrangement (subplots) within planting systems (mainplots). L. perenne biomass was totally different in pure stand and in mixed planting system, irrespective of the herbicide effect. More than 3-fold decrease was caused in the L. perenne biomass by the presence of wheat. Mesosulfuron-methyl was more potent than clodinafop-propargyl in the L. perenne control. Mixed planting enhanced the herbicide performance, increasing the relative potency value of mesosulfuron-methyl. The wheat grain yield received no additional benefit from applying mesosulfuron-methyl at the full recommended dose rather than at half dose. This study demonstrates that L. perenne suppression involving sub-lethal herbicide dose is associated with the wheat competitiveness. Combining competitive cropping systems with reduced herbicide could be an approach to reducing weed populations over time with lower crop production costs.     

Plant protection potential and ultrastructure of Bacillus subtilis strain 3A25

Strain 3A25 was isolated from Indian soil during a screening study for new plant protecting candidates. The present study further characterized this strain regarding its species identity, cellular ultrastructure and potential for protecting Brassica juncea against exposure to ozone. The strain was identified as Bacillus subtilis by 16S rDNA sequencing. Transmission electron microscopy revealed that strain 3A25 had intact bacterial flagella, endospores were found only occasionally. The growth-promoting capacity of strain 3A25 on B. juncea seedlings was investigated in a series of treatments with or without ozone stress in open top chambers. Chlorophyll content, shoot length, root length and leaf area were significantly increased by the treatment with B. subtilis 3A25. The ozone treatment (150 pbb) had a significant inhibiting effect on the development of B. juncea, however, the number of secondary roots was increased. Application of B. subtilis 3A25 either as foliar spray or directly on the soil significantly reduced the negative ozone effect, indicating that this strain has capacities in plant protection against environmental stress.     

Influences of Cry1Ac Broccoli on Larval Survival and Oviposition of Diamondback Moth

Larval survival and oviposition behavior of three genotypes of diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), (homozygous Cry1Ac-susceptibile, Cry1Ac-resistant, and their F₁ hybrids), on transgenic Bacillus thuringiensis (Bt) broccoli expressing different levels of Cry1Ac protein were evaluated in laboratory. These Bt broccoli lines were designated as relative low, medium, and high, respectively, according to the Cry1Ac content. Untransformed brocccoli plants were used as control. Larval survival of diamondback moth on non-Bt leaves was not significantly different among the three genotypes. The Cry1Ac-resistant larvae could survive on the low level of Bt broccoli plants, while Cry1Ac-susceptible and F₁ larvae could not survive on them. The three genotypes of P. xylostella larvae could not survive on medium and high levels of Bt broccoli. In oviposition choice tests, there was no significant difference in the number of eggs laid by the three P. xylostella genotypes among different Bt broccoli plants. The development of Cry1Ac-susceptible and Cry1Ac-resistant P. xylostella on intact Bt plants was also tested in greenhouse. All susceptible P. xylostella larvae died on all Bt plants, while resistant larvae could survive on broccoli, which expresses low Cry1Ac protein under greenhouse conditions. The results of the greenhouse trials were similar to that of laboratory tests. This study indicated that high dose of Bt toxins in broccoli cultivars or germplasm lines is required for effective resistance management.     

Damage and survivorship of fall armyworm (Lepidoptera: Noctuidae) on transgenic field corn expressing Bacillus thuringiensis Cry proteins

Field corn, Zea mays L., plants expressing Cry1Ab and Cry1F insecticidal crystal (Cry) proteins of Bacillus thuringiensis (Bt) Berliner are planted on considerable acreage across the Southern region of the United States. The fall armyworm, Spodoptera frugiperda (J.E. Smith), is an economically important pest during the mid-to-late season on non-Bt and some commercial Bt corn hybrids. The objective of this study was to quantify foliar injury and survivorship of fall armyworm on transgenic corn lines expressing Cry1Ab or Cry1F Bt proteins. Corn lines/hybrids expressing Cry1Ab, Cry1F, and a conventional non-Bt cultivar were evaluated against artificial infestations of fall armyworm in field trials. Larvae (second instars) of fall armyworm were placed on corn plants (V8-V10 stages). Leaf injury ratings were recorded 14 d after infestation. Hybrids expressing Cry1F had significantly lower feeding injury ratings than non-Bt corn plants. Development and survivorship of fall armyworm on Bt corn lines/hybrids were also evaluated in no-choice laboratory assays by offering freshly harvested corn leaf tissue to third instars. Transgenic corn hybrids expressing Cry1Ab or Cry1F significantly reduced growth, development, and survivorship of fall armyworm compared to those offered non-Bt corn tissue. However, 25-76% of third instars offered Bt corn leaf tissues successfully pupated and emerged as adults. These results suggest Cry1Ab has limited effects on fall armyworm; whereas Cry1F demonstrated significant reductions in foliar injury and lower survivorship compared to that on non-Bt corn tissues. Although fall armyworm is not considered a primary target for insect resistance management by the U.S. Environmental Protection Agency, these levels of survivorship could impact selection pressures across the farmscape, especially when considering that transgenic Bt cotton cultivars express similar Cry (Cry1Ac or Cry1F) proteins.     

Genetic diversity, population structure and relationships of Tunisian Thymus algeriensis Boiss. et Reut. and Thymus capitatus Hoffm. et Link. assessed by isozymes

The genetic diversity and population structure of two Tunisian Thymus species (Thymus algeriensis and Thymus capitatus), from 46 natural populations growing in six bioclimates, were analysed by starch gel electrophoresis using eight isozymes. The genetic diversity within populations varied at species level. Variation in T. algeriensis was higher than that observed for T. capitatus, and exclusive alleles were detected for the two studied species. A high differentiation among populations, for each species, estimated by Wright's F-statistics was revealed. In each species, a high level of inbreeding within populations induced by Wahlund effect was observed. A relatively high level of differentiation associated with a restricted gene flow among species was revealed. The PCA and UPGMA analysis, performed on all populations, showed two distinct groups with respect of specific differentiation level. The high genetic divergence between the two species corroborates their taxonomic status, as previously reported using morphological traits. The strategy for the management and conservation of populations should be made for each taxa according to its level of diversity and bioclimate.     

Heritability of,and genotypic correlations between,aflatoxin traits and physiological traits for drought tolerance under end of season drought in peanut (Arachis hypogaea L.)

More rapid progress in breeding peanut for reduced aflatoxin contamination should be achievable with a better understanding of the inheritance of, aflatoxin trait and physiological traits that are associated with reduced contamination. The objectives of this study were to estimate the heritability of aflatoxin traits and genotypic (r_G) and phenotypic (r_P) correlations between drought resistance traits and aflatoxin traits in peanut. One hundred-forty peanut lines in the F_4:6 and F_4:7 generations were generated from four crosses, and tested under well-watered and terminal drought conditions. Field experiments were conducted under the dry seasons 2006/2007 and 2007/2008. Data were recorded for biomass (BIO), pod yield (PY), drought tolerance traits [harvest index (HI), drought tolerance index (DTI) of BIO and PY, specific leaf area (SLA), and SPAD chlorophyll meter reading (SCMR)], and aflatoxin traits [seed infection and aflatoxin contamination]. Heritabilities of A. flavus infection and aflatoxin contamination in this study were low to moderate. The heritabilities for seed infection and aflatoxin contamination ranged from 0.48 to 0.58 and 0.24 to 0.68, respectively. Significant correlations between aflatoxin traits and DTI (PY), DTI (BIO), HI, biomass and pod yield under terminal drought conditions were found (r_P = −0.25** to 0.32**, r_G = −0.57** to 0.53**). Strong correlations between SLA and SCMR with A. flavus infection and aflatoxin contamination were also found. Positive correlations between SLA at 80, 90, and 100 DAP and aflatoxin traits were significant (r_P = 0.13** to 0.46**, r_G = 0.26** to 0.81**). SCMR was negatively correlated with aflatoxin traits (r_P = −0.10** to −0.40**, r_G = −0.11** to −0.66**). These results indicated that physiological-based selection approaches using SLA and SCMR might be effective for improving aflatoxin resistance in peanut.