Canadian Forest Service Publications
Impact of rearing temperature on encapsulation and the accumulation of transcripts putatively involved in capsule formation in a parasitized lepidopteran host. 2018. Seehausen, L.; Naumann, P.-H.; Béliveau, C.; Martel. V.; Cusson, M. J. Insect Physiol. 244-249.
Issued by: Laurentian Forestry Centre
Catalog ID: 39133
CFS Availability: PDF (request by e-mail)
Encapsulation and melanisation are innate immune reactions of insects against foreign intruders such as parasitoids. In an earlier study, we observed that immature life stages of the endoparasitoid Tranosema rostrale (Hymenoptera: Ichneumonidae) parasitizing Choristoneura fumiferana (Lepidoptera: Tortricidae) larvae experienced higher mortality due to encapsulation and melanisation when reared at high (30 °C) than at lower (10 °C, 20 °C) temperatures. Downregulation of T. rostrale polydnavirus genes in parasitized hosts and up regulation of two genes involved in the spruce budworm’s melanisation process were identified as likely contributors to parasitoid mortality at high temperature. However, levels of transcripts of genes involved in the spruce budworm’s cellular encapsulation process were not measured inasmuch as candidate genes, in the spruce budworm, had not yet been identified. In addition, our assessment of temperature-dependent encapsulation and melanisation of foreign objects in spruce budworm larvae was only partial. To fill these knowledge gaps, we injected Sephadex™ beads into unparasitized spruce budworm larvae and assessed their encapsulation/melanisation after the insects had been held at three different temperatures (10, 20, and 30 °C), and we identified spruce budworm genes putatively involved in the encapsulation process and quantified their transcripts at the same three temperatures, using a qPCR approach. As expected, both encapsulation and melanisation of Sephadex™ beads increased as a function of temperature. At the molecular level, three of the five genes examined (Integrin β1, Hopscotch, Stat92E) clearly displayed temperature-dependent upregulation. The results of this study further support the hypothesis that a temperature-dependent increase in the encapsulation response of C. fumiferana against T. rostrale is due to the combined effects of reduced expression of polydnavirus genes and enhanced expression of host immune genes.
Plain Language Summary
To rebuff attacks by parasitoids interfering with its development, the spruce budworm (SBW) is capable of encapsulating parasitoid eggs or making them turn black, thereby killing them. In this study, the researchers demonstrated that the process of encapsulating the eggs of the parasitoid Tranosema rostrale with SBW blood cells increased with the temperature.
More precisely, the study confirms that the relationship between temperature and encapsulation can be explained by the combined effect of two factors: a decrease in the expression of virus genes (virus transmitted to the SBW by T. rostrale) and an increase in the expression of genes associated with the SBW’s immune response as a function of temperature.
The parasitoid wasp T. rostrale uses its stinger to lay its eggs under the skin of the SBW larvae. Once invaded by T. rostrale, the SBW ceases to develop at the sixth and final larval stage in reaction to a virus called polydnavirus (PDV), which is produced in the wasp’s ovary.