Inheritance of resistance to the papaya ringspot virus-watermelon strain from two different accessions of winter squash Cucurbita maxima Duch.
Inheritance of resistance to the papaya ringspot virus-watermelon strain (= PRSV-W, syn. watermelon mosaic virus-1 = WMV-1) was studied for two Cucurbita maxima accessions – ABL-010 and Redlands Trailblazer – considered to be resistant to the potyvirus. ABL-010 (ABL) derives its resistance from Brazilian accessions of C. maxima. Redlands Trailblazer (RT) is an Australian cultivar of C. maxima, the resistance of which traces back to C. ecuadorensis. In both cases, virus could be recovered from the inoculated (symptomless) plants via back inoculation onto susceptible C. pepo cultivar Caserta, indicating a tolerance type of resistance. Both ABL and RT were crossed to each other and to the standard PRSV-W-susceptible cultivar Buttercup (B). For each one of the crosses (ABL ×> B), (ABL × RT) and (RT × B), parental lines, F1, F2 and backcrosses to both the parents (BC1P1 and BC1P2) were tested for PRSV-W resistance after mechanical inoculation. Seedlings were inoculated at the first true leaf stage with a known PRSV-W isolate in 0.02 M phosphate buffer (pH 7.0) plus sodium sulphite 0.01%. Plants were scored for virus symptoms 28 days after inoculation; symptom severity being rated with a scale from 0 (= no symptoms) to 5 (= plants with severe mosaic and leaf distortions). Broad and narrow-sense heritabilities of PRSV-W resistance were 0.44/0.36, 0.59/0.59 and 0.58/0.58 for crosses (ABL × B), (ABL × RT) and (RT × B), respectively. Resistance imparted by ABL in cross with susceptible B could be explained by the action of three genes with partial dominance. Resistance imparted by RT in cross with B was found to be under control of at least two genes with additive effects. Susceptible plants were found in segregating populations of the cross ABL × RT (transgressive segregation), indicating that at least one of the loci involved in the control of resistance in ABL was not allelic to their counterpart loci in RT. The resistance imparted by RT appears to be of higher level than that imparted by ABL.
Inhibition of mechanical transmission of virus by sap, and infection of Cucurbita ecuadorensis with papaya ringspot virus type W
Dilutions of Cucurbita ecuadorensis sap were mixed with dilutions of sap from C. maxima infected with papaya ringspot virus type W (PRV-W), and the mixtures mechanically inoculated to seedlings of C. pepo cv. Small Sugar or Cucumis sativus cv. Supermarket. Plants inoculated with C. maxima sap plus C. ecuadorensis sap (at 1/8 when C. pepo was the test plant, or 1/16 when C. sativus was the test plant) showed reduced infection compared with plants not receiving C. ecuadorensis sap. The reduction caused by C. ecuadorensis sap was greater at a lower virus concentration (1 g infected C. maxima/400 mL sodium-potassium phosphate buffer 0.1 mol/L) than at a higher virus concentration (1 g/25 mL). It was concluded that C. ecuadorensis sap contains an inhibitor of infection, that tissue derived from this species should be diluted at least 1 g/ 10 mL buffer before use in inoculations, and that C. pepo, which was 5 times more sensitive than C. sativus, should be used as the test seedling in reisolations. When C. ecuadorensis, C. maxima and the F1 (C. maxima x C. ecuadorensis) were inoculated on their cotyledons with a severe isolate of PRV-W, symptoms were expressed on at least 1 plant of each species and the F1, and the virus was reisolated in seedlings of C. pepo cv. Small Sugar from the uninoculated leaves of all plants. Severity of symptoms and the proportion of plants infected at reisolation were least for C. ecuadorensis, intermediate for the F1 and most for C. maxima. This is the first report of infection of C. ecuadorensis by PRV-W.
Coexistence of Virulence and Antibiotic Resistance Genes in Pasteurella multocida Isolated from Diseased Rabbits
Pasteurella multocida (P. multocida)is considered a predominant pathogenic bacterial agent causing respiratory manifestations (snuffles) in rabbits with considerable economic losses and unfavorable prognosis in Egypt. A few recent P. multocida strains exhibit resistance to the most commonly used antibiotics in the veterinary field. Therefore, the present study was carried out to investigate the prevalence of both virulence and antibiotic resistance genes among P. multocida isolated from diseased rabbits in Sharkia Governorate, Egypt. Only 10 out of 100 tested rabbits` nasal swabs were finally confirmed positive for P. multocida of serogroup A by polymerase chain reaction (PCR). Antibiotic susceptibility testing of the recovered isolates revealed that they were all multidrug resistant (MDR) with a predominance of resistance to amoxicillin, ampicillin, amoxicillin/clavulanic acid, neomycin and tetracycline (100% each), followed by kanamycin and streptomycin (90% each). All the recovered isolates were further subjected to PCR screening of some common virulence and antibiotic resistance genes of interest . With the exception of toxA gene, the other virulence associated genes (ptfA, Omp87 and nanB) were found among all the examined isolates. Totally, all MDR P. multocida isolates contained at least one antibiotic resistance gene with aphA1 being the most prevalent (100%), followed by ermX gene (40%). Antibiotic resistance genotyping demonstrated the presence of multiple antibiotic resistance genes among majority of the isolates (40%) with only one isolate harboring 4 genes encoding identical resistance phenotypes. Evidentially, all MDR P. multocida isolates possessed at least 3 virulence genes accompanied by the attendance of antibiotic resistance genes. These findings evidenced that rabbits are potential sources of pathogenic P. multocida strains harboring virulence and antibiotic resistance genes. Therefore, it is evident that there is an urgent need for the judicious use of antibiotics in rabbits` treatment systems to successfully mitigate the propagation of drug resistance across P. multocida species.