Assessing Heterodera glycines-Resistant and Susceptible Cultivar Yield Response

The soybean cyst nematode Heterodera glycines (SCN) is of major economic importance and widely distributed throughout soybean production regions of the United States where different maturity groups with the same sources of SCN resistance are grown. The objective of this study was to assess SCN-resistant and -susceptible soybean yield responses in infested soils across the north-central region. In 1994 and 1995, eight SCN-resistant and eight SCN-susceptible public soybean cultivars representing maturity groups (MG) I to IV were planted in 63 fields, either infested or noninfested, in 10 states in the north-central United States. Soil samples were taken to determine initial SCN population density and race, and soil classification. Data were grouped for analysis by adaptation based on MG zones. Soybean yields were 658 to 3,840 kg/ha across the sites. Soybean cyst nematode-resistant cultivars yielded better at SCN-infested sites but lost this superiority to susceptible soybean cultivars at noninfested sites. Interactions were observed among initial SCN population density, cultivar, and location. This study showed that no region-wide predictive equations could be developed for yield loss based on initial nematode populations in the soil and that yield loss due to SCN in our region was greatly confounded by other stress factors, which included temperature and moisture extremes.     

Rhg1 alleles from soybean PI 437654 and PI 88788 respond differentially to isolates of Heterodera glycines in the greenhouse

The production of resistant soybean [Glycine max (L.) Merr.] cultivars is the most effective means for controlling losses from soybean cyst nematode (SCN) (Heterodera glycines Ichinohe). The major resistance gene in most SCN resistance sources is rhg1, which has been mapped as a quantitative trait locus onto linkage group G. Our objective was to determine whether the SCN resistance sources PI 437654 and PI 88788 have different functional alleles at rhg1 based on resistance phenotypes. Populations segregating for resistance alleles at rhg1 from both PI 88788 and PI 437654 and at Rhg4, a second SCN resistance gene from PI 437654, were developed. These populations were screened for resistance to the H. glycines inbred isolates PA3 (HG type 7) and TN14 (HG type 1.2.5.7) in the greenhouse and evaluated with molecular markers linked to both rhg1 and Rhg4. Each isolate test was repeated, and the evaluations were done on a single-plant and a line-mean basis in Test 1, and solely on a single-plant basis in Test 2. Across two tests with the TN14 isolate, plants with the PI 437654 allele for a marker linked to rhg1 had significantly (P<0.0001) less SCN reproduction than plants carrying the PI 88788 allele. A marker linked to Rhg4, however, was not significantly associated with resistance to TN14. Across two tests with the PA3 isolate, alleles of rhg1 from both sources gave a resistant reaction, although plants homozygous for the PI 88788 allele had significantly (P<0.05) greater resistance than plants with the PI 437654 allele. The marker allele from PI 437654 linked to Rhg4 was significantly (P<0.0005) associated with greater resistance than the PI 88788 allele in both PA3 tests, and resistance was dominant. There was a significant interaction between alleles at rhg1 and Rhg4 in both PA3 tests. These results suggest that PI 437654 and PI 88788 each have a different functional SCN resistance allele at or close to rhg1. These allelic differences have implications that breeders should consider before incorporation into cultivars.     

Selection of Heterodera glycines chorismate mutase-1 alleles on nematode-resistant soybean

The soybean cyst nematode Heterodera glycines is the most destructive pathogen of soybean in the Unites States. Diversity in the parasitic ability of the nematode allows it to reproduce on nematode-resistant soybean. H. glycines chorismate mutase-1 (Hg-CM-1) is a nematode enzyme with the potential to suppress host plant defense compounds; therefore, it has the potential to enhance the parasitic ability of nematodes expressing the gene. Hg-cm-1 is a member of a gene family where two alleles, Hg-cm-1A and Hg-cm-1B, have been identified. Analysis of the Hg-cm-1 gene copy number revealed that there are multiple copies of Hg-cm-1 alleles in the H. glycines genome. H. glycines inbred lines were crossed to ultimately generate three F2 populations of second-stage juveniles (J2s) segregating for Hg-cm-1A and Hg-cm-1B. Segregation of Hg-cm-1A and 1B approximated a 1:2:1 ratio, which suggested that Hg-cm-1 is organized in a cluster of genes that segregate roughly as a single locus. The F2 H. glycines J2 populations were used to infect nematode-resistant (Hartwig, PI88788, and PI90763) and susceptible (Lee 74) soybean plants. H. glycines grown on Hartwig, Lee 74, and PI90763 showed allelic frequencies similar to Hg-cm-1A/B, but nematodes grown on PI88788 contained predominately Hg-cm-1A allele as a result of a statistically significant drop of Hg-cm-1B in the population. This result suggests that specific Hg-cm-1 alleles, or a closely linked gene, may aid H. glycines in adapting to particular soybean hosts.     

Dormancy of Heterodera glycines in Missouri

A 2-year study was conducted in field microplots to determine the relative importance of soybean phenology and soil temperature on induction of dormancy in Heterodera glycines in Missouri. Four near-isogenic soybean lines differing for maturity date were planted in microplots infested with a race 5 isolate of H. glycines. Soil temperature was monitored at a depth of 15 cm. Eggs of H. glycines, extracted from cysts collected monthly from each microplot, were used in hatching tests and bioassays to determine dormancy. Egg hatching and second-stage juvenile (J2) infectivity rates decreased sharply from their highest levels in midsummer (July-August) to a low level by October of each year and remained low (< 10% hatching and < 0.2 J2/cm root) until May or June of the following year. The patterns of numbers of females and eggs in the bioassays were similar. The decreases were not related to soil temperature and did not differ consistently among soybean isolines. The monophasic changes in all nematode responses with peak midsummer rates suggest that H. glycines produces one primary generation per year in central Missouri. Changes in hatching rates and the timing of minimum and maximum rates suggested that H. glycines eggs exhibit more than one type of dormancy.     

Giant Starship Elements Mobilize Accessory Genes in Fungal Genomes

Accessory genes are variably present among members of a species and are a reservoir of adaptive functions. In bacteria, differences in gene distributions among individuals largely result from mobile elements that acquire and disperse accessory genes as cargo. In contrast, the impact of cargo-carrying elements on eukaryotic evolution remains largely unknown. Here, we show that variation in genome content within multiple fungal species is facilitated by Starships, a newly discovered group of massive mobile elements that are 110 kb long on average, share conserved components, and carry diverse arrays of accessory genes. We identified hundreds of Starship-like regions across every major class of filamentous Ascomycetes, including 28 distinct Starships that range from 27 to 393 kb and last shared a common ancestor ca. 400 Ma. Using new long-read assemblies of the plant pathogen Macrophomina phaseolina, we characterize four additional Starships whose activities contribute to standing variation in genome structure and content. One of these elements, Voyager, inserts into 5S rDNA and contains a candidate virulence factor whose increasing copy number has contrasting associations with pathogenic and saprophytic growth, suggesting Voyager’s activity underlies an ecological trade-off. We propose that Starships are eukaryotic analogs of bacterial integrative and conjugative elements based on parallels between their conserved components and may therefore represent the first dedicated agents of active gene transfer in eukaryotes. Our results suggest that Starships have shaped the content and structure of fungal genomes for millions of years and reveal a new concerted route for evolution throughout an entire eukaryotic phylum.     

HLA frequencies, diabetes mellitus and autoimmunity in Turner's patients and their relatives.

In a preliminary study of twenty-three patients with gonadal dysgenesis (Turner's syndrome) and their families, correlation was sought between their serum defined HLA allele frequencies and their known tendencies toward abnormal immune responses and diabetes mellitus, since individuals with the latter disorders have been shown to have an increased frequency of certain HLA types. We were unable to demonstrate an association between these major serum-defined histocompatibility antigens, immune homeostasis disturbances and sex chromosome aneuploidy in this group. It is felt, however, that testing involving the patterns of HLA-D and "HLA-D related" antigen frequencies should be obtained to further evaluate the possibility of such an association.     

Ovarian development in 46,XY gonadal dysgenesis

Summary In human the XY ovary is degenerative, there being scant evidence of persistence of that organ beyond the perinatal period. Here we describe indications of functional ovarian tissue in a 17-year-old female with male karyotype, H-Y⁺ cellular phenotype, and some signs of the Turner syndrome. Her gonads were removed after the onset of secondary amenorrhea. Histological examination revealed a degenerative right ovary devoid of germ cells and follicles, and a left streak gonad. There was no trace of testicular development in either side.     

Seasonal Biochemical Changes in Eggs of Heterodera glycines in Missouri

Changes in the carbohydrate (glucose, trehalose, and glycogen) and total protein contents of eggs retained within Heterodera glycines cysts were monitored monthly in a field microplot experiment conducted from March 1993 to March 1995. Treatments included two near-isogenic lines of soybean cv. Clark differing for date of maturity, and one corn hybrid. The soybean lines were planted in microplots infested with H. glycines at a high average initial population density (Pi) (23,810 eggs/100 cm³ soil), and the corn was planted in microplots infested at high (24,640) and low (5,485) Pi. Soil temperatures at 15 cm depth and rainfall were monitored. Carbohydrate contents varied in the same pattern, with the highest levels measured before planting (May) and after harvest (October) in both years. Neither Pi nor soybean isoline had an effect on any measured response, but the carbohydrate contents of eggs from corn and soybean microplots differed during the overwinter (October-May) periods (P < 0.0001). Trehalose accumulation was negatively correlated with soil temperature (r = -0.78 and r = -0.84, P = 0.0001, July through November 1993 and 1994, respectively), which reflects its role as a cryoprotectant. In contrast to the pattern for carbohydrates, total protein was lowest before planting and after harvest, and highest (>20 μg/1,000 eggs) June through October. Protein content was unaffected by plant cultivar or species. Protein and carbohydrate levels in H. glycines eggs showed seasonal changes that appeared to be primarily temperature-dependent.     

Plant-Parasitic Nematode Communities in Dogwood,Maple, and Peach Nurseries in Tennessee

Nursery blocks (48 dogwood, 27 red maple, and 17 peach) distributed among 20 Tennessee nurseries were sampled for nematodes in March, July, and October 1981. Plant-parasitic nematodes were extracted from soil, counted by genera, and identified to species after fixation. A total of 57 species in 24 genera were found, with 1-16 species occurring in a site. The species most commonly detected were Paratylenchus projectus and Xiphinema americanum, which were found in 88% and 78% of the sites, respectively. Relationships existed between distribution and densities of some species present in more than 10% of the sites and certain soil factors (pH, bulk density, texture, and organic matter content). Plant-parasitic nematode community diversity was related to tree age, percentage of weed ground cover, and number of weed species. Site similarities in community ordinations were dependent on the individual nurseries sampled, tree age, and soil type, but clusters of sites of similar tree ages and soil types were not exclusive.     

A genetic linkage map of the soybean cyst nematode <Emphasis Type="Italic">Heterodera glycines</Emphasis>

A genetic linkage map of the soybean cyst nematode (SCN) Heterodera glycines was constructed using a population of F2 individuals obtained from matings between two highly inbred SCN lines, TN16 and TN20. The AFLP fingerprinting technique was used to genotype 63 F2 progeny with two restriction enzyme combinations (EcoRI/MseI and PstI/TaqI) and 38 primer combinations. The same F2 population was also genotyped for Hg-cm-1 (H. glycines chorismate mutase-1), a putative virulence gene, using real-time quantitative PCR. Some of the markers were found to be distributed non-randomly. Even so, of the 230 markers analyzed, 131 could be mapped onto ten linkage groups at a minimum LOD of 3.0, for a total map distance of 539 cM. The Hg-cm-1 locus mapped to linkage group III together with 16 other markers. The size of the H. glycines genome was estimated to be in the range of 630-743 cM, indicating that the current map represents 73-86% of the genome, with a marker density of one per 4.5 cM, and a physical/genetic distance ratio of between 124 kb/cM and 147 kb/cM. This genetic map will be of great assistance in mapping H. glycines markers to genes of interest, such as nematode virulence genes and genes that control aspects of nematode parasitism.