Intraspecies variation of Metarhizium brunneum against the green peach aphid, Myzus persicae, provides insight into the complexity of disease progression.

BACKGROUND: Intensive application of chemical insecticides is required for aphid pest control. Among the biorational alternatives, entomopathogenic fungi are the most sustainable biocontrol agents; those of the order Hypocreales attack and cause fungal disease in arthropod hosts, with variations in host susceptibility attributed to both fungal and host characteristics. We evaluated inter- and intraspecies variations in Metarhizium spp. virulence and differences in fungal disease progression on adult and nymph stages of the green peach aphid, Myzus persicae (Sulzer), a parthenogenetically reproducing insect species. RESULTS: Minor interspecies diversity was detected between the generalist Metarhizium species examined. Interestingly, significant intraspecies diversity was observed between Metarhizium brunneum isolates Mb7 and MbK. Infected adult aphids demonstrated similar disease progression for both isolates, mortality rates of more than 80% and fivefold reduction in fecundity. However, nymph mortality was detected only following MbK infection, with 50% mortality and significant reduction in molting rates. Confocal laser scanning microscopy demonstrated the variation in the disease stages of conidial adhesion and hemocoel colonization on each examined day post inoculation for each isolate. Significantly faster disease progression was observed in MbK-infected versus Mb7-infected nymphs, the latter demonstrating a higher percentage of uninfected nymphs accompanied with aphid molting. CONCLUSIONS: The observed intraspecies variation suggests that altered conidial adhesion to the nymph cuticle is a major factor affecting virulence. We prove the role of nymph ecdysis as a defense mechanism disrupting fungal infection. Because significant differences were observed between closely related isolates, this study emphasizes the importance of appropriate isolate selection for biological control. © 2021 Society of Chemical Industry.

ANIMAL/VEGETAL DIFFERENCES IN CORTICAL GRANULE EXOCYTOSIS DURING ACTIVATION OF THE FROG EGG.

One of the more striking morphological events during egg activation is exocytosis of the cortical granules. In the frog egg, the wave of cortical granule exocytosis takes about 100 sec to traverse the animal half, and travels slower in the vegetal half. We examined cortical granule exoctyosis during activation with respect to this animal/vegetal difference. In eggs which were acquiring the ability to be activated (recovering from CO2 -intoxication or undergoing meiotic maturation), animal half cortical granules became capable of responding to activating stimuli prior to vegetal half ones. Since Ca2+ is involved in exocytosis, we examined the effect of Ca2+ on cortical granule breakdown in vitro. There was no difference in sensitivity to Ca2+ of cortical granules from immature vs. mature eggs, but animal half cortical granules were more sensistive to Ca2+ than vegetal half ones. Finally, we found that prick-activation of eggs at the vegetal pole was frequently unsuccessful but would occur when external Ca2+ was raised. These experiments show that there are regional differences in the frog egg with respect to cortical granule responsiveness, and they suggest that the differences are due to Ca2+ sensitivity.