Spiroplasma Kunkelii Research Articles

Exploring Natural Infection in Alternative Hosts of Corn Stunt Spiroplasma and Tracking the Temporal Dynamics of Corn Stunt Disease in the Brazilian Cerrado Biome

The corn stunt disease complex, caused by the mollicutes Spiroplasma kunkelii (Css) and the maize bushy stunt (MBS) phytoplasma, is a major phytosanitary issue for corn production in the neotropical region of Brazil. In this study, we investigated the presence of alternative hosts for S. kunkelii (Css) in the Brazilian Cerrado biome and explored the potential for asymptomatic Css infections in maize plants. To identify possible alternative hosts, we collected samples from ornamental and native plants located in Cerrado regions with a high incidence of corn stunt disease. We also monitored the disease’s progression over time and calculated the area under the disease progression curve (AUDPC). Additionally, we sampled healthy, asymptomatic maize plants growing near symptomatic ones and analyzed all the samples using qPCR to detect the pathogen. Our results showed no Css infection in the sampled alternative host species and no asymptomatic infections in the nearby maize plants. The incidence of maize stunting varied significantly among the sample years and counties. In Aparecida do Rio Negro, the infection rate was six times higher in 2020 compared to 2019, while Porto Nacional showed an 11-fold increase. During surveillance from March to July, the infection rate at the Sierra site went from less than 3% in March 2019 to 6% in July (F1,4 = 415.84; p = 0.0003). MBS infection increased significantly, while S. kunkelii remained stable below 3%. In 2020, MBS increased from 3% to more than 12%. A similar trend was observed at the Sede site, where MBS increased from 9% in 2019 to 11% in 2020. An increase in the AUDPC was observed both within individual years and between 2019 and 2020, indicating a worsening trend in disease severity. Overall, our findings reinforce the need for preventive measures in managing maize crop fields.

Leptodelphax maculigera (Hemiptera: Delphacidae) Harbors the Corn Stunt Complex Pathogens.

The African planthopper Leptodelphax maculigera (Hemiptera: Delphacidae) has been recently reported in many places in Brazil in association with maize. Its occurrence in maize production fields in Brazil has brought concerns to the corn production chain regarding the possibility of this planthopper to be a vector for maize bushy stunt phytoplasma (MBSP), corn stunt spiroplasma (Spiroplasma kunkelii), maize rayado fino virus (MRFV), and maize striate mosaic virus (MSMV). The phytoplasma and spiroplasma, which are bacteria belonging to the class Mollicutes, and the two viruses are associated with the corn stunt disease complex. Given the presence of the African planthopper species and the corn stunt complex in Brazil, we further investigated the abundance of this planthopper species in the State of Santa Catarina, Brazil, and whether the planthopper can carry the four pathogens. We inspected 12 maize production fields in different municipalities in the state for 20 weeks, using two yellow sticky traps for each maize field. The sticky traps were replaced weekly. A total of 130 specimens of L. maculigera were captured, with a great discrepancy in quantity among locations and weeks. We detected the mollicute MBSP and the viruses MRFV and MSMV in L. maculigera, whereas S. kunkelii was absent in the assessed African planthopper samples. The molecular detection of the phytoplasma and the viruses in field-collected African planthoppers is strong evidence that this insect species has the ability to acquire those pathogens through feeding from the phloem of diseased maize plants. Nonetheless, transmission capacity needs to be experimentally proven to assert L. maculigera as a vector for the corn-stunting pathogens.

Susceptibility of corn hybrids to corn stunt complex transmitted by the corn leafhopper, Dalbulus maidis (DeLong & Wolcott, 1923) in Brazil

Corn (Zea mays L.) production plays a key role in the global agricultural scenario. Corn is currently the most produced cereal in the world, with more than 1.1 billion tons harvested in 2021. The main pest that affects corn in Latin America is the corn leafhopper (Dalbulus maidis) (DeLong & Wolcott, 1923). This insect causes direct damage to Zea mays plants through suction of the corn sap, but its importance is greater due to the transmission of two phytopathogens of the mollicute class: the phytoplasma (MBSP – maize bushy stunt phytoplasma) that causes the red stunting; and spiroplasma (Spiroplasma kunkelii) which causes pale stunting; in addition to a virus, the corn stripe virus (MRFV – maize rayado fino virus). Stunting is a systemic disease that affects the physiology, nutrition, and development of the corn crop, significantly reducing its productivity. Therefore, the use of resistant hybrids is the main alternative to reduce the incidence and damage caused by corn stunting. This work aimed to evaluate the susceptibility of maize hybrids to the stunting complex transmitted by the corn leafhopper in three different sowing dates. The experiment was carried out with 63 corn hybrids in three different sowing times, the first being on 11/24/2022, the second on 12/30/2022 and the third on 02/06/2023. The hybrids SHSuper CONV, NTX454 VT PRO2 and FS670 PWU were the ones that had the lowest rates of stunting severity, showing a high resistance to this disease complex. The hybrids AG9025 PRO3, DKB240 PRO3 and K9300 PRO3 were the ones that presented the highest disease severity scores (6 to 5.7 on the scale used), indicating a high susceptibility to the stunt pathogens complex. The use of less susceptible hybrids is one of the main components for managing stunting diseases complex in the corn crop in South America, being one of the most important features, together with potential yield, as a parameter for growers to select the hybrid to be used.

First report of maize rayado fino virus in corn fields in Santa Catarina, Brazil.

Maize (Zea mays L.) is the main cereal food of humans and animals in Brazil. In 2020 and 2021, a severe infestation of corn leafhoppers (Dalbulus maidis; Hemiptera: Cicadellidae) was observed in Santa Catarina State (South of Brazil). Subsequently, symptoms of chlorotic stripes limited in leaf veins started to appear in maize plants. Given the similarity of symptoms and the presence of high populations of corn leafhoppers in corn production areas, 30 plants in reproductive stage showing systemic symptoms were collected in summer and autumn from commercial fields of five municipalities in Santa Catarina: Campos Novos (27°23'18.0"S, 51°12'52.7"W), Lages (27°47'17.8"S, 50°18'16.9"W), Mafra (26°06'42"S, 49°48'25"W), Fraiburgo (27°01'36"S, 50°55'19"W), and Abelardo Luz (26°34'02"S, 52°20'02"W). The young leaves of these samples were used for molecular analyses targeting the maize rayado fino virus (MRFV; Tymoviridae: Marafivirus). Total nucleic acids were extracted using TRIzol® (Invitrogen, USA), following the manufacturer's instructions. These were used as a template for cDNA synthesis with the enzyme MMLV-RT (Promega, USA), following the manufacturer's instructions. The polymerase chain reaction (PCR) was performed using Gotaq® DNA polymerase (Promega, USA) and MRFV-09/MRFV-10 primers (Hammond et al. 1997). All PCR products were subjected to electrophoresis in 1% agarose gel and were visualized under ultraviolet light. Twenty-eight of the 30 tested plants were MRFV-positive, showing a fragment with an expected size of ~633 bp. To confirm our results, all MRFV-positive samples were sent for sequencing (GenBank accession numbers OM763708 - OM763710 and ON730784 - ON730806) and submitted to BLASTn search (https://blast.ncbi.nlm.nih.gov/Blast.cgi), resulting in identities ranging from 96.21% to 99.21% with the isolate "Brazil 26" of MRFV, which was detected in 2005 in São Paulo, Brazil (GenBank accession nº: AF186178) (Hammond and Bedendo 2005). A second set of primers was used to validate the first PCR, confirming MRFV infection (data not shown).Moreover, whitish streaks and leaf reddening were observed on the leaves of some plants; therefore, the identification for phytoplasmas (Candidatus Phytoplasma asteris) and spiroplasmas (Spiroplasma kunkelii) from the corn stunt complex was performed. For this, previously extracted nucleic acids from each sample were used as templates for a multiplex PCR using the primers CSSR6/CSSF2 and R16F2n/R16R2 (Gundersen and Lee 1996; Barros et al. 2001). Two plants were infected with only spiroplasma, 17 samples were infected with Spiroplasma and MRFV, and three samples were infected by these three pathogens. An increasing incidence of corn stunt has been observed in commercial fields in Santa Catarina in recent years. Mollicutes are commonly found and mostly studied as causal agents of corn stunt disease. On the contrary, despite being present in Brazil since the 1970s, the virus is less studied because its contribution to the corn stunt complex is still unknown (Hammond and Bedendo 2001). In this report, indications that the virus is expanding to different regions in southern Brazil were observed, which raises an opportunity for further evaluation and its consideration in monitoring programs. Moreover, to the best of our knowledge, this is the first report of MRFV in Santa Catarina, Brazil.

Achaparramiento o lapeado en maíz

Abstract This factsheet for farmers in Nicaragua provides information on corn stunt (Spiroplasma kunkelii), a disease of maize ( Zea mays ), transmitted by the leafhopper insect vector Sisimitalia infulata . It includes details about the disease's symptoms and transmission, the biology of the insect vector, management and a photo of a plant showing the symptoms. INTA, Granada

Insecticide seed treatment against corn leafhopper: helping protect grain yield in critical plant growth stages.

The corn leafhopper, Dalbulus maidis (Hemiptera: Cicadellidae), spreads maize stunt pathogens and requires timely and effective crop protection. We determined the interaction between maize phenology and the vector feeding/infection period by stunt pathogens with the residual efficacy of neonicotinoid insecticidal seed treatments. Greenhouse- and field-grown maize plants, seed-treated with clothianidin or imidacloprid insecticides, were infested during seven growth stages with corn leafhoppers reared under controlled conditions on maize plants displaying infection symptoms by both spiroplasma (corn stunt spiroplasma, Spiroplasma kunkelii) and phytoplasma (maize bushy phytoplasma) pathogens. In the greenhouse and field settings, seed treatment reduced the stunt disease symptoms and corn yield loss during the VE-V4 maize growth stages and caused no phytotoxicity. The neonicotinoid seed treatment reduced 20-60% of the yield losses from the corn stunt disease until the V4 growth stage. Infestation by infective corn leafhoppers in the V12 maize growth stage caused a 25-30% yield loss irrespective of seed treatment, yet no stunt disease symptom was evident. Nonetheless, corn yield losses and visual stunt symptoms as rated by a nine-category ordinal scale were strongly correlated (r=0.79, P < 0.01). These results reinforce that maize plants are more susceptible to leafhopper stunt disease during the VE-V4 growth stages (emergence to the fourth-leaf stage). Seed treatment helps reduce the damage in the early growth stages (VE-V2), although supplemental control measures depending on leafhopper population density may be needed from VE-V12 to protect yield losses from the maize stunt condition. © 2021 Society of Chemical Industry.

Infection with Spiroplasma kunkelii on temperate and tropical x temperate maize in Argentina and development of a tool to evaluate germplasm

Infection with Spiroplasma kunkelii on temperate and tropical x temperate maize in Argentina and development of a tool to evaluate germplasm

Pest Risk Analysis and Quarantine Precautions for Maize Importation into Ethiopia: A Case of Eleven Source Countries

Maize improvement program in Ethiopia utilizes a large amount of germplasm from external sources. Seed samples were imported from eleven different countries Mexico, Kenya, Zimbabwe, India, Norway, Colombia, Thailand, Zambia, South Africa, Nigeria and Egypt from 2011 to 2020. During maize importation, regular inspection was carried out at Holetta Agricultural Research Center for freedom from pests of insects, pathogens and weed seeds. In order to be effective in this important regulatory pest management undertaking, this review attempts to provide a concerted approach for effecting pre- and post-entry regulatory measures for maize importation into Ethiopia based on pest risk analysis (PRA) to safeguard the country. A total of 18 pests (including 12 arthropods, 3 fungi, 1 bacterium, 1 virus and 1 Spiroplasma) are of quarantine concern when importing maize seed into Ethiopia from the eleven major germplasm source-countries considered in this PRA. Three fungi (Cocliobolus, Fusarium and Mycosphaerella), one bacterium bacterial wilt of maize (Pantoea stewartii) and the virus maize chlorotic dwarf virus and corn stunt Spiroplasma (Spiroplasma kunkelii) and also twelve arthropods are of quarantine concern to the country. The number and species of pests of quarantine concern for the country vary depending on country of origin the highest in Mexico with 11 pests followed by India (7 pests) and then Colombia (6 pests). The rest consists of only 1–5 pests of quarantine importance for Ethiopia. Therefore, this review describes quarantine precautions in the import control scenarios and presents PRA based on pathway analysis for importing maize seed into Ethiopia from eleven major source-countries. Generally, the importers of maize into Ethiopia need to strictly consider the information provided in this paper before, during and after arrival of maize samples for research.

Characterization of components of resistance to Corn Stunt disease.

Corn Stunt is an important disease in the Americas due to it high prevalence and the yield reductions that can cause when present. However, changes in the presence of this disease across years hampers the effective identification of resistant genotypes to this disease. To avoid the limitations of phenotypic selection under natural pressure, this research aimed to devise an effective strategy to screen disease-resistant genotypes in the absence of high and constant natural pressures. To do so, we investigated the presence of antixenosis and antibiosis as components of resistance to the vector Dalbulus maidis as well as resistance to the pathogen Spiroplasma kunkelii under artificial inoculation conditions in four maize hybrids. The hybrids shown differences in their levels of resistance and target organisms, either the insect vector or the pathogen. Antixenosis and antibiosis to D. maidis were observed in DK72-10. Resistance to S. kunkelii by DK79-10 was seen as a delayed onset of symptoms, and DKB390 showed antixenosis to D. maidis and resistance to S. kunkelii. An association between symptom severity and yield reduction was found, but not between accumulation of pathogen S. kunkelii and symptom severity nor yield. In conclusion, the proposed methodology was efficacious and can aid in the screening of resistant genotypes in breeding programs to reduce the impact of Corn Stunt disease, ensuring that hybrids with good resistance level will be planted by farmers whenever disease occurs.

Corn Stunt Disease: An Ideal Insect-Microbial-Plant Pathosystem for Comprehensive Studies of Vector-Borne Plant Diseases of Corn.

Over 700 plant diseases identified as vector-borne negatively impact plant health and food security globally. The pest control of vector-borne diseases in agricultural settings is in urgent need of more effective tools. Ongoing research in genetics, molecular biology, physiology, and vector behavior has begun to unravel new insights into the transmission of phytopathogens by their insect vectors. However, the intricate mechanisms involved in phytopathogen transmission for certain pathosystems warrant further investigation. In this review, we propose the corn stunt pathosystem (Zea mays–Spiroplasma kunkelii–Dalbulus maidis) as an ideal model for dissecting the molecular determinants and mechanisms underpinning the persistent transmission of a mollicute by its specialist insect vector to an economically important monocotyledonous crop. Corn stunt is the most important disease of corn in the Americas and the Caribbean, where it causes the severe stunting of corn plants and can result in up to 100% yield loss. A comprehensive study of the corn stunt disease system will pave the way for the discovery of novel molecular targets for genetic pest control targeting either the insect vector or the phytopathogen.

Differential transmission of phytoplasma and spiroplasma to maize caused by variation in the environmental temperature in Brazil

The effects of temperature in winter-spring (average 17.0 °C) and spring-summer (average 23.0 °C) seasons on the transmission and latent period of maize bushy stunt phytoplasma (MBS-phytoplasma) and corn stunt spiroplasma (CSS) (Spiroplasma kunkelii) caused by the corn leafhopper, Dalbulus maidis (DeLog and Wolcott) (Hemiptera: Cicadellidae) were evaluated in two experiments in a screenhouse in Sete Lagoas, Minas Gerais State, Brazil. Approximately 200 mollicute-free adults of D. maidis were confined on popcorn (Zea mays L. cv. IAC125) plants infected with spiroplasma and the same number of mollicute-free leafhoppers were confined on plants infected with phytoplasma. They remained on the source plants for a 6 d acquisition access period (AAP). Next, leafhoppers were fed healthy popcorn seedlings. Transmission tests were performed with one leafhopper per seedling for each mollicute, with 12 replicates, at 12, 20, 30, 40 d since the first day of AAP. In addition, at 40 d, one leafhopper with phytoplasma and another with spiroplasma were confined together on 24 popcorn seedlings. The inoculation access period (IAP) was 3 d. The temperature was recorded daily during the experimental periods. In Experiment 1, during the 40 d latent period, only 41.6% and 58.3% of plants, respectively, exposed to phytoplasma or to both mollicutes presented MBS-phytoplasma symptoms. In Experiment 2, during the 20 d latent period, some popcorn plants presented MBS-phytoplasma (16.7%) and CSS (33.3%) symptoms, whereas almost 100% of plants at the 30 and 40 d latent period presented disease symptoms. Plant symptoms and results from PCR tests indicated that MBS-phytoplasma was more tolerant to low temperatures than S. kunkelii.

The enemy within: phloem-limited pathogens.

The growing impact of phloem-limited pathogens on high-value crops has led to a renewed interest in understanding how they cause disease. Although these pathogens cause substantial crop losses, many are poorly characterized. In this review, we present examples of phloem-limited pathogens that include intracellular bacteria with and without cell walls, and viruses. Phloem-limited pathogens have small genomes and lack many genes required for core metabolic processes, which is, in part, an adaptation to the unique phloem environment. For each pathogen class, we present multiple case studies to highlight aspects of disease caused by phloem-limited pathogens. The pathogens presented include Candidatus Liberibacter asiaticus (citrus greening), Arsenophonus bacteria, Serratia marcescens (cucurbit yellow vine disease), Candidatus Phytoplasma asteris (Aster Yellows Witches' Broom), Spiroplasma kunkelii, Potato leafroll virus and Citrus tristeza virus. We focus on commonalities in the virulence strategies of these pathogens, and aim to stimulate new discussions in the hope that widely applicable disease management strategies can be found.

A Nightmare for Males? A Maternally Transmitted Male-Killing Bacterium and Strong Female Bias in a Green Lacewing Population

For maternally transmitted microbes, a female-biased host sex ratio is of reproductive advantage. Here we found a strong female bias in a field population of the green lacewing, Mallada desjardinsi (Insecta; Neuroptera). This bias was attributed to the predominance of individuals harboring a maternally inherited male-killing bacterium that was phylogenetically closely related to the plant-pathogenic Spiroplasma phoeniceum and Spiroplasma kunkelii. Among 35 laboratory-reared broods produced by wild-caught females, 21 broods (60%)—all infected with Spiroplasma—consisted of only females (940 individuals). Among 14 broods consisting of both males and females (516 and 635 individuals, respectively), 4 broods were doubly infected with Spiroplasma and Rickettsia, 6 broods were singly infected with Rickettsia, and 3 broods were uninfected (remaining one brood was unknown). Mortality during embryonic and larval development was prominent in all-female broods but not in normal sex ratio broods. Following antibiotic treatment on all-female broods, mortality was significantly reduced and the sex ratio was restored to 1:1. Strong expression and high prevalence of this male-killer is remarkable considering its low density (~10−5–10−4 cells per host mitochondrial gene copy based on quantitative PCR). In addition, a bacterium closely related to Rickettsia bellii was present in 25 of 34 broods (73.5%), irrespective of the sex ratio, with the infection density comparable to other cases of endosymbiosis (~10−2–10−1 cells per mitochondrial gene copy). Higher density of Rickettsia than Spiroplasma was also demonstrated by electron microscopy which visualized both Spiroplasma-like cells and Rickettsia-like cells inside and outside the ovarian cells.

Spiralin Diversity Within Iranian Strains of Spiroplasma citri

The first-cultured and most-studied spiroplasma is Spiroplasma citri, the causal agent of citrus stubborn disease, one of the three plant-pathogenic, sieve-tube-restricted, and leafhopper vector-transmitted mollicutes. In Iranian Fars province, S. citri cultures were obtained from stubborn affected citrus trees, sesame and safflower plants, and from the leafhopper vector Circulifer haematoceps. Spiralin gene sequences from different S. citri isolates were amplified by PCR, cloned, and sequenced. Phylogenetic trees based on spiralin gene sequence showed diversity and indicated the presence of three clusters among the S. citri strains. Comparison of the amino acid sequences of eleven spiralins from Iranian strains and those from the reference S. citri strain GII-3 (241 aa), Palmyre strain (242 aa), Spiroplasma kunkelii (240 aa), and Spiroplasma phoeniceum (237 aa) confirmed the conservation of general features of the protein. However, the spiralin of an S. citri isolate named Shiraz I comprised 346 amino acids and showed a large duplication of the region comprised between two short repeats previously identified in S. citri spiralins. We report in this paper the spiralin diversity in Spiroplasma strains from southern Iran and for the first time a partial internal duplication of the spiralin gene.

Presence of &lt;I&gt;Dalbulus maidis&lt;/I&gt; (Hemiptera: Cicadellidae) and of &lt;I&gt;Spiroplasma kunkelii&lt;/I&gt; in the Temperate Region of Argentina

"Corn stunt" is one of the main corn (Zea mays L.) diseases in the Americas and Dalbulus maidis (DeLong & Wolcott) is the key vector of the pathogen Spiroplasma kunkelii Whitcomb. In Argentina, the corn-producing area is in the temperate region, where vector and pathogen prevalence levels are unknown. In this study, the prevalence and distribution of D. maidis and S. kunkelii in the temperate region of Argentina and D. maidis overwintering ability in this region were determined. Surveys were conducted in 2005-2006 and 2006-2007 seasons to determine D. maidis and S. kunkelii presence, and in winter 2006 to determine the vector overwintering ability. The highest S. kunkelii prevalence and incidence levels were found in the transition area from the temperate to the subtropical region, related to the highest D. maidis prevalence and insects sampled per location. D. maidis adults were found in volunteer corn plants and spontaneous vegetation in autumn and winter months, which were inoculative for the pathogen S. kunkelii. This overwintering ability was related to detection of D. maidis insects in corn crops at early growth stages in the following growing season. This work emphasizes that corn stunt disease is present in the temperate region of Argentina, and this highlights the need to develop proper agronomic practices like monitoring insect vector populations and controlling voluntary plants. This study also indicates that further research is needed to understand the potential yield reduction caused by this pathogen on symptomless plants and population dynamics of the insect vector.

An Analysis of the Genomic Variability of the Phytopathogenic Mollicute Spiroplasma kunkelii

Corn stunt disease has become a factor limiting maize production in some areas of the Americas in recent years. Although resistant maize genotypes have been developed in the past, this resistance has been unstable over time or in some geographical locations. To better understand disease components that could affect the stability of host resistance, we assessed the genome variability of the etiologic agent, Spiroplasma kunkelii. Isolates were obtained from a number of areas, and characterized molecularly by amplification of several regions of the spiroplasma chromosome and sequencing of specific gene fragments. The degree of polymorphism between isolates of different geographic origins was low, and the level of genomic variability was similar within isolates of different countries. Polymorphism among isolates was found in viral insertions and in the sequence of Skarp, a gene that encodes a membrane protein implicated in attachment to insect cells. The results suggest that the genome composition of this species is highly conserved among isolates. Hence, it is unlikely that the instability of maize resistance is due to generation of new pathotypes of S. kunkelii. Instead, other components of this complex pathosystem could account for the breakdown of resistance.

Exitianus obscurinervis (Hemiptera: Cicadellidae), a New Experimental Vector of Spiroplasma kunkelii

"Corn stunt" caused by the mollicute Spiroplasma kunkelii (Whitcomb) is potentially one of the most severe diseases affecting the corn (Zea mays L.) crop in the Americas, and the leafhopper Dalbulus maidis (DeLong & Wolcott) is considered its most important vector. However, other insects seen quite frequently in corn crops might well be its vectors in Argentina To identify any leafhoppers species other than D. maidis that can transmit S. kunkelii, transmission assays were conducted, using individuals of Exitianus obscurinervis (Stål) collected in field and reared under controlled conditions. S. kunkelii was transmitted to corn plants by E. obscurinervis. The pathogen was transmitted to seven of the 11 plants, which showed characteristic corn stunt symptoms, and the presence of the pathogen was confirmed by DAS-ELISA. The presence of S. kunkelii in the E. obscurinervis individuals used in transmission experiments was confirmed by polymerase chain reaction and electron microscopy. The current study shows the existence of a new experimental vector of S. kunkelii, the leafhopper E. obscurinervis, which acquired spiroplasmas from infected plants and inoculated it to healthy plants.

The S1 ribosomal protein family contains a unique conservative domain

The number of amino acid residues contained in the S1 ribosomal protein of various bacteria varies in a wide range: from 111 to 863 residues in Spiroplasma kunkelii and Treponema pallidum, respectively. The architecture of this protein is traditionally (in particular, because of unknown spatial structure) represented as repeated S1 domains, the copy number of which depends on the protein length. The data on the copy number and boundaries of these domains is available in specialized databases, such as SMART, Pfam, and PROSITE; however, these data can be rather different for the same object. In this work, we used the approach utilizing analysis of predicted secondary structure (PsiPred program). This allowed us to detect the structural domains in S1 protein sequences; their copy number varied from one to six. Alignment of the S1 proteins containing different numbers of domains with the S1 RNA-binding domain of Escherichia coli polynucleotide phosphorylase provided for discovering a domain within this family displaying the maximal homology to the E. coli domain. This conservative domain migrates along the chain, and its location in the proteins with different numbers of domains follows a certain pattern. Similar to the S1 domain of polynucleotide phosphorylase, residues Phe19, Phe22, His34, Asp64, and Arg68 in this conservative domain are clustered on the surface to form an RNA-binding site.

Effects of Corn Stunt Spiroplasma on Survival of &lt;I&gt;Dalbulus maidis&lt;/I&gt; (Hemiptera: Cicadellidae) Parasitized or Preyed upon by &lt;I&gt;Gonatopus bartletti&lt;/I&gt; (Hymenoptera: Dryinidae) or Infected by &lt;I&gt;Metarhizium anisopliae&lt;/I&gt;

The effects of corn stunt spiroplasma (CSS), Spiroplasma kunkelii, on survival of corn leafhopper Dalbulus maidis (Delong & Wolcott) (Hemiptera: Cicadellidae), exposed to parasitism, fungal disease, or predation were investigated in the laboratory. Adult corn leafhoppers with or without CSS were exposed separately to parasitism or predation by Gonatopus bartletti, a parasitoid that acts also as a predator, or to infection by Metarhizium anisopliae, a pathogenic fungus, to evaluate the effects of CSS on corn leafhopper survival confronted with these natural enemies. CSS alone did not affect corn leafhopper survival, but differentially mediated survival of leafhoppers exposed to parasitism or predation by G. bartletti, or infection with M. anisopliae. CSS did not affect the survival of corn leafhoppers parasitized by G. bartletti: Survivorship of leafhoppers carrying CSS and parasitized by G. bartletti was ≈9%, whereas in the absence of CSS and parasitized it was ≈17%, but the difference was not significant. Similarly, CSS did not affect the survival of corn leafhoppers infected with M. anisopliae: Survivorship of leafhoppers carrying CSS and infected with M. anisopliae was ≈64%, whereas in the absence of CSS and infected with M. anisopliae it was ≈73%, but the difference was not significant. In contrast, CSS reduced the survival rate of corn leafhoppers exposed to predation by G. bartletti, though the effect was mediated by the incubation period of CSS. Corn leafhoppers exposed to predation and carrying CSS for 10 or 20 d had significantly lower survivorship rates, ≈54 and ≈47%, respectively, compared with those of leafhoppers carrying CSS for 2 d or free of CSS, ≈91 and ≈78%, respectively. The lower survival rate of corn leafhoppers carrying CSS and exposed to predation by G. bartletti, particularly of leafhoppers with longer CSS incubation periods, together with the lower survival rate of CSS in leafhoppers parasitized by G. bartletti documented in earlier studies, are suggested as factors limiting the abundance of the vector and plant pathogen in their shared area of origin.

Habitat of the Corn Leafhopper (Hemiptera: Cicadellidae) During the Dry (Winter) Season in Mexico

Although the corn leafhopper Dalbulus maidis (DeLong and Wolcott) is the most important vector of maize pathogens in Latin America, little is known about how and where it overwinters (passes the dry season), particularly in Mexico. The objectives of this study were (1) to monitor the abundance of D. maidis adults throughout the dry season in maize and maize-free habitats and (2) to determine where and how D. maidis adults, exposed or nonexposed to the maize pathogen Spiroplasma kunkelii Whitcomb, overwinter in a maize-free habitat. Work for the first objective was done during the two consecutive dry seasons of 1999-2000 and 2000-2001; the second objective was done during the dry seasons of 2003-2004 and 2005-2006. During the dry winter seasons, D. maidis was prevalent as long as maize was present in irrigated areas. The leafhopper was found in 52 of the 58 irrigated maize fields sampled in Mexico at the end of the dry seasons of 1999-2000 and 2000-2001. However, leafhopper adults were not found in nonirrigated maize-free habitats at high elevation during the dry winter season (February, March, and April), although leafhopper adults were prevalent on perennial wild grasses in January after maize harvest. Additional experiments revealed, however, that corn leafhopper adults, although few in number, survived the entire dry season in these nonirrigated maize-free fields. Also, no detectable difference in survival existed between leafhoppers exposed and those not exposed to S. kunkelli during the two dry seasons in the maize-free habitat.