David R. Walker

Theobroma cacao L.: a genetic linkage map and quantitative trait loci analysis.

A genetic linkage map of Theobroma cacao (cocoa) has been constructed from 131 backcross trees derived from a cross between a single tree of the variety Catongo and an F1 tree from the cross of Catongo by Pound 12. The map comprises 138 markers: 104 RAPD loci, 32 RFLP loci and two morphologic loci. Ten linkage groups were found which cover 1068 centimorgans (cM). Only six (4%) molecular-marker loci show a significant deviation from the expected 1∶1 segregation ratio.The average distance between two adjacent markers is 8.3 cM. The final genome-size estimates based on two-point linkage data ranged from 1078 to 1112 cM for the cocoa genome. This backcross progeny segregates for two apparently single gene loci controlling (1) anthocyanidin synthesis (Anth) in seeds, leaves and flowers and (2) self-compatibility (Autoc). The Anth locus was found to be 25 cM from Autoc and two molecular markers co-segregate with Anth. The genetic linkage map was used to localize QTLs for early flowering, trunk diameter, jorquette height and ovule number in the BC1 generation using both single-point ANOVA and interval mapping. A minimum number of 2–4 QTLs (P<0.01) involved in the genetic expression of the traits studied was detected. Coincident map locations of a QTL for jorquette height and trunk diameter suggests the possibility of pleiotropic effects in cocoa for these traits. The combined estimated effects of the different mapped QTLs explained between 11.2% and 25.8% of the phenotypic variance observed in the BC1 population.

Field Evaluation of Soybean Engineered with a Synthetic cry1Ac Transgene for Resistance to Corn Earworm, Soybean Looper, Velvetbean Caterpillar (Lepidoptera: Noctuidae), and Lesser Cornstalk Borer (Lepidoptera: Pyralidae)

Abstract A transgenic line of the soybean ‘Jack’, Glycine max (L.) Merrill, expressing a synthetic cry1Ac gene from Bacillus thuringiensis variety kurstaki (Jack-Bt), was evaluated for resistance to four lepidopteran pests in the field. Jack-Bt and genotypes serving as susceptible and resistant controls were planted in field cages and artificially infested with larvae of corn earworm, Helicoverpa zea (Boddie), and velvetbean caterpillar, Anticarsia gemmatalis (Hübner), in 1996, 1997, and 1998, and also with soybean looper, Pseudoplusia includens (Walker), in 1996. Susceptible controls included Jack (1996–1998), ‘Cobb’ (1996), and Jack-HPH (1996). GatIR 81–296 was used as the resistant control in all 3 yr. Compared with untransformed Jack, Jack-Bt showed three to five times less defoliation from corn earworm and eight to nine times less damage from velvetbean caterpillar. Defoliation of GatIR 81–296 was intermediate between that of Jack and Jack-Bt for corn earworm, and similar to that of Jack for velveltbean caterpillar. Jack-Bt exhibited significant, but lower resistance to soybean looper. Jack-Bt also showed four times greater resistance than Jack to natural infestations of lesser cornstalk borer, Elasmopalpus lignosellus (Zeller), in conventional field plots at two locations in 1998. Data from these experiments suggest that expression of this cry1Ac construct in soybean should provide adequate levels of resistance to several lepidopteran pests under field conditions.

Combining cry1Ac with QTL alleles from PI 229358 to improve soybean resistance to lepidopteran pests

A QTL conditioning corn earworm resistance in soybean PI 229358 and asynthetic Bacillus thuringiensis cry1Ac transgene from therecurrent parent ‘Jack-Bt’ were pyramided intoBC2F3 plants by marker-assisted selection. Segregatingindividuals were genotyped at SSR markers linked to an anitbiosis/antixenosisQTL on linkage group M, and were tested for the presence ofcry1Ac. Marker-assisted selection was used during andafter the two backcrosses to develop a series of BC2F3plants with or without the crylAc transgene and the QTLconditioning for resistance BC2F3 plants that werehomozygous for parental alleles at markers on LG M, and whicheither had or lacked cry1Ac, were assigned to one of fourpossible genotype classes. These plants were used in no-choice, detached leaffeeding bioassays with corn earworm and soybean looper larvae (Lepidoptera:Noctuidae) to evaluate the relative antibiosis in the different genotypeclasses. Resistance was measured as larval weight gain and degree of foliageconsumption. Few larvae of either species survived on leaves expressing theCry1Ac protein. Though not as great as the effect of Cry1Ac, the PI229358-derived LG M QTL also had a detrimental effect on larval weights of bothpest species, and on defoliation by corn earworm, but did not reduce defoliation bysoybean looper. Weights of soybean looper larvae fed foliage from transgenicplants with the PI-derived QTL were lower than those of larvae fed transgenictissue with the corresponding Jack chromosomal segment. This work demonstratesthe usefulness of SSRs for marker-assisted selection in soybean, and shows thatcombining transgene-and QTL-mediated resistance to lepidopteran pests may be aviable strategy for insect control.

A QTL that enhances and broadens Bt insect resistance in soybean.

Effective strategies are needed to manage insect resistance to Bacillus thuringiensis (Bt) proteins expressed in transgenic crops. To evaluate a multiple resistance gene pyramiding strategy, eight soybean (Glycine max) lines possessing factorial combinations of two quantitative trait loci (QTLs) from plant introduction (PI) 229358 and a synthetic Bt cry1Ac gene were developed using marker-assisted selection with simple sequence repeat markers. Field studies were conducted in 2000 and 2001 to evaluate resistance to corn earworm (Helicoverpa zea) and soybean looper (Pseudoplusia includens), and detached leaf bioassays were used to test antibiosis resistance to Bt-resistant and Bt-susceptible strains of tobacco budworm (TBW; Heliothis virescens). Based on defoliation in the field and larval weight gain on detached leaves, lines carrying a combination of cry1Ac and the PI 229358 allele at a QTL on linkage group M were significantly more resistant to the lepidopteran pests, including the Bt-resistant TBW strain, than were the other lines. This is the first report of a complementary additive effect between a Bt transgene and a plant insect resistance QTL with an uncharacterized mode of action that was introgressed using marker-assisted selection.

Pathogenic diversity of Phytophthora sojae and breeding strategies to develop Phytophthora-resistant soybeans

Phytophthora stem and root rot, caused by Phytophthora sojae, is one of the most destructive diseases of soybean [Glycine max (L.) Merr.], and the incidence of this disease has been increasing in several soybean-producing areas around the world. This presents serious limitations for soybean production, with yield losses from 4 to 100%. The most effective method to reduce damage would be to grow Phytophthora-resistant soybean cultivars, and two types of host resistance have been described. Race-specific resistance conditioned by single dominant Rps (“resistance to Phytophthora sojae”) genes and quantitatively inherited partial resistance conferred by multiple genes could both provide protection from the pathogen. Molecular markers linked to Rps genes or quantitative trait loci (QTLs) underlying partial resistance have been identified on several molecular linkage groups corresponding to chromosomes. These markers can be used to screen for Phytophthora-resistant plants rapidly and efficiently, and to combine multiple resistance genes in the same background. This paper reviews what is currently known about pathogenic races of P. sojae in the USA and Japan, selection of sources of Rps genes or minor genes providing partial resistance, and the current state and future scope of breeding Phytophthora-resistant soybean cultivars.

EMBRYOGENIC RESPONSE OF MULTIPLE SOYBEAN (GLYCINE MAX (L.) MERR.) CULTIVARS ACROSS THREE LOCATIONS

SummaryNine soybean [Glycine max (L.) Merr.] cultivars representing midwestern, mid-south, and southern US growing regions were evaluated at each of three locations (Athens, GA; Lexington, KY; and Wooster, OH) using uniform embryogenic induction and proliferation protocols in order to evaluate the portability of soybean somatic embryogenic protocols to different locations. The experimental design minimized variation between locations by having all cultivars present at all locations on all days. A quantitative weighted score for primary embryo induction was developed on average embryo number per explant and was used to describe non-embryogenic, poorly embryogenic, moderately embryogenic, and highly embryogenic responses. Ranking of cultivars remained similar across all locations, indicating a uniform transportability of the protocol, at least as far as embryo induction is concerned. Continued proliferation of embryogenic cultures was also measured using a repetitive growth measure but few meaningful conclusions could be made due to the high level of variability including inconsistent growth of cultures between each subculture. Overall, several cultivars were identified as being uniformly embryogenic or non-embryogenic at the primary induction phase at all locations, and we predict that those embryogenic cultivars could be used by any laboratory for high-efficiency induction of embryogenesis. The best of these cultivars, ‘Jack’, was uniformly responsive across all locations and should be selected as the genotype most likely to yield positive results when attempting to culture and genetically engineer soybeans via embryogenic protocols.

Discovery and utilization of QTLs for insect resistance in soybean

Insect resistance in soybean has been an objective in numerous breeding programs, but efforts to develop high yielding cultivars with insect resistance have been unsuccessful. Three Japanese plant introductions, PIs 171451, 227687 and 229358, have been the primary sources of insect resistance alleles, but a combination of quantitative inheritance of resistance and poor agronomic performance has hindered progress. Linkage drag caused by co-introgression of undesirable agronomic trait alleles linked to the resistance quantitative trait loci (QTLs) is a persistent problem. Molecular marker studies have helped to elucidate the numbers, effects and interactions of insect resistance QTLs in the Japanese PIs, and markers are now being used in breeding programs to facilitate transfer of resistance alleles while minimizing linkage drag. Molecular markers also make it possible to evaluate QTLs independently and together in different genetic backgrounds, and in combination with transgenes from Bacillus thuringiensis.

Weather-based crop and disease advisories for peanuts in Virginia

orld peanut production averages 23 million metric tons from harvest of approximately 20 million hectares on six continents (2). India, China, and the United States produced 70% of the world’s peanuts in 1990 and 1991. Production in the United States is concentrated in the southeast (Georgia, Florida, and Alabama), southwest (Texas and Oklahoma), and the mid-Atlantic region (Virginia, North Carolina, and South Carolina). Peanuts are grown in sandy-textured soils in warm, moist climates having a growing season of 140 or more days. Southeastern Virginia provides an excellent environment for commercial production, as evidenced by high yield and quality dating back to the Civil War (8). Today, the industries of producing, shelling, and processing peanuts are major pillars of the economy in southeastern Virginia. These industries are concentrated in an eight-county area, where the value of the crop to growers often ranges from

From select agent to an established pathogen: The response to Phakopsora pachyrhizi (soybean rust) in North America

80 to

Effects of defoliating insect resistance QTLs and a cry1Ac transgene in soybean near-isogenic lines

102 million, as in 1994 and 1990, respectively. Unfortunately, the warm and moist climate necessary for peanut production makes fungal diseases a chronic threat to profits. The rapid spread and the unforgiving nature of early leaf spot, caused by Cercospora arachidicola S. Hori, have earned the respect of growers in Virginia and around the world. Peanut growers are also keenly aware of soilborne diseases caused by fungi such as Sclerotinia minor Jagger (Sclerotinia blight), Sclerotium rolfsii Sacc. (southern stem rot), and Cylindrocladium parasiticum Crous, Wingfield & Alfenas (Cylindrocladium black rot). In 1990 and 1991, a systematic survey of peanut growers in southeastern Virginia was conducted to evaluate their perception of losses of yield to specific diseases and the use of pesticides (11; P. M. Phipps, unpublished). Sclerotinia blight and southern stem rot were named most frequently as responsible for yield losses exceeding 10% (Table 1). Sclerotinia blight was considered by the highest percentage of growers to be the most damaging disease. Clinical records, on-farm tests, and reports by extension agents and specialists supported these estimates, except for the claims of yield losses to southern stem rot. In this case, it was thought that the growers’ estimate of losses may have been excessive and possibly reflected some confusion with Sclerotinia blight. Peanuts require intensive management because of the crop’s vulnerability to yield losses caused by diseases, insects, and weeds. Chemicals play a major role in pest management strategies for peanut production due to the limited availability of acceptable alternatives. Based on the 1990 survey of peanut growers in Virginia, the input of pesticide active ingredients (a.i.) totaled 747 t and averaged 19 kg/ha (Table 2). The cost of these materials averaged