Involvement of apoptosis in host-parasite interactions in the zebra mussel

Abstract

The question of whether cell death by apoptosis plays a biological function during infection is key to understanding host-parasite interactions. We investigated the involvement of apoptosis in several host-parasite systems, using zebra mussels Dreissena polymorpha as test organisms and their micro- and macroparasites. As a stress response associated with parasitism, heat shock proteins (Hsp) can be induced. In this protein family, Hsp70 are known to be apoptosis inhibitors. Mussels were diagnosed for their respective infections by standard histological methods; apoptosis was detected using the TUNEL methods on paraffin sections and Hsp70 by immunohistochemistry on cryosections. Circulating hemocytes were the main cells observed in apoptosis whereas infected tissues displayed no or few apoptotic cells. Parasitism by intracellular bacteria Rickettsiales-like and the trematode Bucephalus polymorphus were associated with the inhibition of apoptosis whereas ciliates Ophryoglena spp. or the trematode Phyllodistomum folium did not involve significant differences in apoptosis. Even if some parasites were able to modulate apoptosis in zebra mussels, we did not see evidence of any involvement of Hsp70 on this mechanism.

Figure 1. Density of apoptotic cells in zebra mussels infected or not by microparasites and their gonadal development stage.

(A) Mean number of apoptotic cells per mm² of tissue in non-infected zebra mussels, infected by ciliates Ophryoglena spp. (Oph), or by intracellular bacteria Rickettsiales-like organisms (RLO), or co-infected by these two parasites (Oph-RLO) (+ S.D.). (B) Percentages of mussels at each gamete development stage and gonadal index values (white dots). The host gender was also taken into account. Significant differences between groups of a same gender are indicated by different letters. The star indicates a significant difference between non-infected organisms (one-way ANOVA, Duncan’s post hoc test).

Figure 2. Density of apoptotic cells in zebra mussels infected or not by macroparasites.

(A) Mean number of apoptotic cells per mm² of tissue in non-infected zebra mussels or infected by the trematode Phyllodistomum folium (+ S.D.). The Gonadal index value is also indicated for each group. (B) Mean number of apoptotic cells per mm² of tissue in non-infected zebra mussels or infected by the trematode Bucephalus polymorphus (+ S.D.). The Gonadal index value is only indicated for the non-infected group since it cannot be calculated for the infected one (i.e. castration). Different letters indicate significant differences between groups (t-test).

Figure 3. Hsp70 expression in zebra mussels infected or not by microparasites.

Hsp 70 expression (Surface density, Sv) in each experimental group of the microparasite study. Both host sex and parasite species (Ophryoglena spp., Oph; Rickettsiales-like organisms, RLOs; co-infections, Oph-RLO) were taken into account. The center line shows the median, 25^th-75^th percentile (within box), minimum/maximum value (error bar). The small circles represent atypical values and the stars the extreme values.

Figure 4. Hsp70 expression in zebra mussels infected or not by macroparasites.

Hsp 70 expression (Surface density, Sv) in each experimental group of the macroparasite studies. (A) Comparison of non-infected vs P. folium-infected zebra mussels, males or females. (B) Comparison of non-infected vs B. polymorphus-infected zebra mussels. Center lines show the median, 25^th-75^th percentile (within box), minimum/maximum value (error bar). The small circles represent atypical values and the stars the extreme values.