Lack of Host Specialization on Winter Annual Grasses in the Fungal Seed Bank Pathogen Pyrenophora semeniperda

Generalist plant pathogens may have wide host ranges, but many exhibit varying degrees of host specialization, with multiple pathogen races that have narrower host ranges. These races are often genetically distinct, with each race causing highest disease incidence on its host of origin. We examined host specialization in the seed pathogen Pyrenophora semeniperda by reciprocally inoculating pathogen strains from Bromus tectorum and from four other winter annual grass weeds (Bromus diandrus, Bromus rubens, Bromus arvensis and Taeniatherum caput-medusae) onto dormant seeds of B. tectorum and each alternate host. We found that host species varied in resistance and pathogen strains varied in aggressiveness, but there was no evidence for host specialization. Most variation in aggressiveness was among strains within populations and was expressed similarly on both hosts, resulting in a positive correlation between strain-level disease incidence on B. tectorum and on the alternate host. In spite of this lack of host specialization, we detected weak but significant population genetic structure as a function of host species using two neutral marker systems that yielded similar results. This genetic structure is most likely due to founder effects, as the pathogen is known to be dispersed with host seeds. All host species were highly susceptible to their own pathogen races. Tolerance to infection (i.e., the ability to germinate even when infected and thereby avoid seed mortality) increased as a function of seed germination rate, which in turn increased as dormancy was lost. Pyrenophora semeniperda apparently does not require host specialization to fully exploit these winter annual grass species, which share many life history features that make them ideal hosts for this pathogen.

Hydrothermal time models for conidial germination and mycelial growth of the seed pathogen Pyrenophora semeniperda

Population-based threshold models using hydrothermal time (HTT) have been widely used to model seed germination. We used HTT to model conidial germination and mycelial growth for the seed pathogen Pyrenophora semeniperda in a novel approach to understanding its interactions with host seeds. Germination time courses and mycelial growth rates for P.semeniperda were measured on PDA amended to achieve a series of five water potentials (ca. 0 to -6 MPa) at six constant temperatures (5-30 °C). Conidial germination was described with alternative population-based models using constant or variable base and maximum temperature and water potential parameters. Mycelial growth was modeled as a continuous, linear process with constant base temperature and base water potential. Models based on HTT showed reasonable fit to germination and growth rate data sets. The best-fit conidial germination model (R(2) = 0.859) was based on variable base and maximum temperature as a function of water potential. The good fit of the linear mycelial growth model (R(2) = 0.916) demonstrated the utility of HTT for modeling continuous as well as population-based processes. HTT modeling may be a useful approach to the quantification of germination and growth processes in a wide range of filamentous fungi.

Indirect effects of an invasive annual grass on seed fates of two native perennial grass species

Invasive plants exhibit both direct and indirect negative effects on recruitment of natives following invasion. We examined indirect effects of the invader Bromus tectorum (cheatgrass) on seed fates of two native grass species, Elymus elymoides and Pseudoroegneria spicata, by removing B. tectorum and by adding inoculum of the shared seed pathogen Pyrenophora semeniperda in factorial experiments at xeric and mesic field sites. We also included a supplemental watering treatment to increase emergence and also the potential for pathogen escape. We recorded emergence and survival of native seedlings and also determined the fate of unemerged seeds. At the xeric site, Pyrenophora-caused mortality was high (34%), and effects of other pathogens and failed emergence of germinants were smaller. Cheatgrass removal negatively affected both emergence (35 vs. 25%) and spring survival (69 vs. 42%). Pyrenophora-caused seed mortality increased with inoculum augmentation for both species (22 vs. 47% overall), but emergence was negatively impacted only for P. spicata (20 vs. 34%). At the mesic site, Pyrenophora-caused mortality was low (6%). Cheatgrass removal doubled emergence (26 vs. 14%). Seed mortality increased significantly with inoculum augmentation for P. spicata (12 vs. 5%) but not E. elymoides, while emergence was not significantly affected in either species. A large fraction of seeds produced germinants that failed to emerge (37%), while another large fraction (35%) was killed by other pathogens. We conclude that facilitation by cheatgrass at the xeric site but interference at the mesic site was probably mediated through litter effects that could be ameliorative or suppressive. Apparent competition between cheatgrass and native grasses could occur through Pyrenophora, especially in a xeric environment, but effects were weak or absent at emergence. This was probably because Pyrenophora attacks the same slow-germinating fraction that is subject to pre-emergence mortality from other causes, including attack by other pathogens such as Fusarium.

A Race for survival: can Bromus tectorum seeds escape Pyrenophora semeniperda-caused mortality by germinating quickly?

BACKGROUND AND AIMS

Pathogen-seed interactions may involve a race for seed resources, so that seeds that germinate more quickly, mobilizing reserves, will be more likely to escape seed death than slow-germinating seeds. This race-for-survival hypothesis was tested for the North American seed pathogen Pyrenophora semeniperda on seeds of the annual grass Bromus tectorum, an invasive plant in North America. In this species, the seed germination rate varies as a function of dormancy status; dormant seeds germinate slowly if at all, whereas non-dormant seeds germinate quickly.

METHODS

Three experimental approaches were utilized: (a) artificial inoculations of mature seeds that varied in primary dormancy status and wounding treatment; (b) naturally inoculated undispersed seeds that varied in primary dormancy status; and (c) naturally inoculated seeds from the carry-over seed bank that varied in degree of secondary dormancy, habitat of origin and seed age.

KEY RESULTS

In all three approaches, seeds that germinated slowly were usually killed by the pathogen, whereas seeds that germinated quickly frequently escaped. Pyrenophora semeniperda reduced B. tectorum seed banks. Populations in drier habitats sustained 50 times more seed mortality than a population in a mesic habitat. Older carry-over seeds experienced 30 % more mortality than younger seeds.

CONCLUSIONS

Given the dramatic levels of seed death and the ability of this pathogen to reduce seed carry-over, it is intriguing to consider whether P. semeniperda could be used to control B. tectorum through direct reduction of its seed bank.

Use of antibodies to RCC and CD10 in the differential diagnosis of renal neoplasms

The majority of renal neoplasms can be distinguished on the basis of histologic examination alone; however, there are morphologic similarities between clear cell renal carcinoma and chromophobe cell carcinoma, as well as between the granular/eosinophilic variants of these tumors and renal oncocytoma. Only a limited number of histochemical markers are available to aid in the differential diagnosis of these neoplasms. Hale's colloidal iron usually yields strong, diffuse cytoplasmic staining of chromophobe cell carcinomas whereas clear cell carcinomas are generally negative; however, interpretation of this stain is not always straightforward. By immunohistochemistry, vimentin is detectable in most clear cell carcinomas and is absent from most chromophobe cell tumors and oncocytomas, but reliance on a single antibody can be misleading. In this report we examine the use of commercially available monoclonal antibodies to RCC and CD10 in the differential diagnosis of common renal tumors. Eighty-five percent of clear cell carcinomas (53 of 62) had detectable surface membrane staining for RCC, and 94% (58 of 62) were positive for CD10. Papillary carcinomas were likewise strongly positive for RCC and CD10 in nearly all cases (13 of 14 each). In contrast, all 19 chromophobe cell carcinomas examined were completely negative for surface membrane staining with both of these markers. Oncocytomas were also negative for RCC (0 of 9), but CD10 was detectable in some cases (3 of 9). These results suggest that the presence of surface membrane staining for RCC and CD10 may be used to confirm a diagnosis of suspected clear cell or papillary renal carcinoma. Chromophobe cell carcinomas should be negative for both markers. The absence of RCC staining may also be helpful in the diagnosis of renal oncocytoma.

CD99, keratin, and vimentin staining of sex cord-stromal tumors, normal ovary, and testis

CD99, a marker for MIC-2, reacts with normal Sertoli cells and granulosa cells. We investigated CD99 expression in the development of normal ovary and testis as well as in 25 sex cord-stromal tumors (SCSTs), 7 epithelial neoplasms, and 6 germ cell tumors. Normal Sertoli cells and mature granulosa cells showed 3+ staining with CD99. Pregranulosa cells of primordial follicles were negative. All of the eight Sertoli-Leydig cell tumors were positive with antibody to CD99, with the well-differentiated tumors showing the greatest degree of staining intensity. Reactivity of 2+ to 3+ with CD99 was observed in all of the 11 granulosa cell tumors and in yolk sac components of the germ cell tumors investigated. All of the poorly differentiated carcinomas were negative with CD99. We concluded that CD99 might be a useful marker for SCSTs and that its degree of reactivity correlates with the degree of differentiation in Sertoli-Leydig cell tumors. Additionally, CD99 might aid in distinguishing granulosa cell tumors of the ovary from poorly differentiated carcinomas.

Angiomatoid melanoma: a case of metastatic melanoma mimicking a vascular malignancy

We present a case of melanoma metastatic to forehead skin and sacral vertebra, without known primary, which, in the cutaneous metastasis, displayed striking histologic features suggesting vascular differentiation. Histopathologic examination of the lesion revealed a large, deeply extending spindle cell malignancy with numerous, cavernous, erythrocyte-filled spaces throughout and only scant melanin pigmentation, making initial diagnosis challenging. The neoplastic cells demonstrated positive staining with antibodies to S-100 protein, HMB 45, and vimentin. We suggest the term angiomatoid melanoma for this histopathologic variant.

Bromus tectorum seed germination: between-population and between-year variation

Cheatgrass (Bromus tectorum L., Poaceae), an introduced winter annual, has invaded a variety of habitats in western North America. This study examines variation in cheatgrass germination response and after-ripening patterns that are related to differences in habitat and to yearly differences in weather conditions during seed maturation. Seeds collected from five contrasting populations in 1992 and 1993 were subjected to controlled dry storage and then incubated across a range of temperatures. Recently harvested seeds were dormant and germinated slowly, while fully after-ripened seeds were nondormant and germinated rapidly. The optimal incubation temperature for mean germination time shifted from 5:15 to 20:30 °C as a result of after-ripening. Between-population differences in germination response appear to be related to the potential risk of precocious summer germination. The results from this 2-year study suggest that the more extreme yet predictable environments select for seed germination and after-ripening patterns that are genetically fixed, while populations from more favorable environments tended to show more between-year variations, suggesting more phenotypic plasticity. Germination percentage showed greater between-year variation than mean germination time. Between-year differences could not be explained simply by differences in maximum temperature or total precipitation during maturation. Adaptive germination responses in cheatgrass populations from contrasting habitats may have both genetic and environmental components, thus explaining why this species can become established in such a variety of habitats. Keywords: after-ripening, invading species, dormancy, mean germination time, cheatgrass, downy brome.

Basic fibroblast growth factor and epidermal growth factor downmodulate the growth of hematopoietic cells in long-term stromal cultures

The bone marrow microenvironment consists of stromal cells and extracellular matrix components which act in concert to regulate the growth and differentiation of hematopoietic stem cells. There is little understanding of the mechanisms which modulate the regulatory role of stromal cells. This study examined the hypothesis that mesenchymal growth factors such as basic fibroblast growth factor (bFGF) and epidermal growth factor (EGF) modulate stromal cell activities and thereby influence the course of hematopoiesis. Both bFGF and EGF were potent mitogens for marrow stroma. However, both factors proved to be inhibitory to hematopoiesis in primary long-term marrow cultures. Inhibition was also observed when hematopoietic cells and bFGF or EGF were added to subconfluent irradiated stromal layers, demonstrating that the decline of hematopoiesis was not due to overgrowth of the stromal layer. Loss of hematopoietic support in bFGF and EGF was dose-dependent. Removal of bFGF and EGF permitted stromal layers to regain their normal capacity to support hematopoiesis. In stroma-free long-term cultures, neither factor affected CFU-GM expansion. Basic FGF slightly enhanced granulocyte-macrophage colony forming unit (CFU-GM) cloning efficiency in short-term agarose culture. Basic FGF did not reduce the levels of interleukin-6 (IL-6), GM-CSF, or G-CSF released by steady state or IL-1-stimulated stroma. Similarly, the constitutive levels of steel factor (SF) mRNA and protein were not affected by bFGF. Basic FGF did not alter the level of TGF-beta 1 in stromal cultures. We conclude that bFGF and EGF can act as indirect negative modulators of hematopoietic growth in stromal cultures. The actual mediators of regulation, whether bound or soluble, remain to be identified.

Kaposi's sarcoma in the bone marrow of a patient with AIDS

The authors report a case of Kaposi's sarcoma (KS) involving the bone marrow of a patient with acquired immune deficiency syndrome (AIDS). The morphologic features, differential diagnosis, immunostaining pattern, and unusually low frequency of marrow involvement by AIDS-related KS are discussed.

Histiocytic disorders of the orbit

Four cases of different types of orbital, benign and malignant histiocytosis syndromes are presented to demonstrate the diagnostic presentation, medical evaluation, and therapy of these diseases with orbital involvement. Newer concepts have altered our understanding of the pathophysiology of histiocytic syndromes. An abnormality of Langerhans' cells and the T-cell suppressor population appears to be important in the development of histiocytosis. It is important for the ophthalmologist to be aware that histiocytic lesions of the orbit may be an indication of widespread and at times life-threatening disease.

Woringer-Kolopp disease (pagetoid reticulosis). Four cases with histopathologic, ultrastructural, and immunohistologic observations

Four patients had localized lesions of Woringer-Kolopp disease (pagetoid reticulosis). The clinical appearances and histopathologic features with light microscopy of the four cases were similar. However, electron microscopy in all four cases and enzyme histochemistry studies and immunophenotyping with monoclonal antibodies in three cases disclosed two subgroups of patients. The features of the predominant cells in the intraepidermal infiltrate of three cases were most consistent with those of helper T lymphocytes with cerebriform nuclei, as found in many cases of mycosis fungoides. In one case, however, the epidermis contained a predominant population of small, medium, and large lymphoid cells of cytotoxic/suppressor T-lymphocyte cell phenotype. It remains to be determined whether Woringer-Kolopp disease is a variant of mycosis fungoides (ie, within the spectrum of the cutaneous T-cell lymphomas), a benign reactive process, or a syndrome with various causes.

Outbreak of Burkitt's-like lymphoma in homosexual men

Four cases of Burkitt's-like lymphoma (undifferentiated, monoclonal, B-cell tumours) in homosexual men were seen in a 9-month period in San Francisco. One tumour contained both Epstein-Barr-virus nuclear antigen (EBNA) and cytomegalovirus (CMV) antigen. Another tumour contained EBNA, and a third contained no viral antigen, but EBNA and CMV antigens were detected in the overlying epithelium. This outbreak widens the array of neoplasms affecting immunosuppressed homosexual men and provides further evidence of an oncogenic role for EBV and CMV.