Spider Mites Singly Infected With Either Wolbachia or Spiroplasma Have Reduced Thermal Tolerance

Abstract

Heritable symbionts play an essential role in many aspects of host ecology in a temperature-dependent manner. However, how temperature impacts the host and their interaction with endosymbionts remains largely unknown. Here, we investigated the impact of moderate (20°C) and high (30 and 35°C) temperatures on symbioses between the spider mite Tetranychus truncatus and two maternally inherited endosymbionts (Wolbachia and Spiroplasma). We found that the thermal tolerance of mites (as measured by survival after heat exposure) was lower for mites that were singly infected with either Wolbachia or Spiroplasma than it was for co-infected or uninfected mites. Although a relatively high temperature (30°C) is thought to promote bacterial replication, rearing at high temperature (35°C) resulted in losses of Wolbachia and particularly Spiroplasma. Exposing the mites to 20°C reduced the density and transmission of Spiroplasma but not Wolbachia. The four spider mite strains tested differed in the numbers of heat shock genes (Hsps) induced under moderate or high temperature exposure. In thermal preference (Tp) assays, the two Wolbachia-infected spider mite strains preferred a lower temperature than strains without Wolbachia. Our results show that endosymbiont-mediated spider mite responses to temperature stress are complex, involving a combination of changing endosymbiont infection patterns, altered thermoregulatory behavior, and transcription responses.

Keywords: Spiroplasma; Tetranychus truncatus; Wolbachia; thermal preference; thermal tolerance.

FIGURE 1

Survival rates of four spider mite strains over 8 days under different temperatures. (A) Wolbachia and Spiroplasma co-infected strains (w+s+), (B) singly Wolbachia-infected strains (w+), (C) singly Spiroplasma-infected (s+), and (D) uninfected stains (w-s-).

FIGURE 2

Effect of low (20°C) and high (30 or 35°C) temperatures on Wolbachia-induced CI. (A) Visual differences in normal eggs (upper panels) and stunted eggs from CI (lower panels). (B) Egg hatch rates in four crosses between different spider mite strains at either kept under different temperatures. Horizontal bars indicate the medians based on n = 18–99 per treatment. Mann–Whitney test, **p < 0.01, ***p < 0.001.

FIGURE 3

Wolbachia and Spiroplasma densities in female adult spider mites maintained under low (20°C) and high (30 or 35°C) temperatures for up to 5 days. Wolbachia density in co-infected strains (A) and Wolbachia-infected strains (B). Spiroplasma density in co-infected strains (C) and Spiroplasma-infected strains (D). Data are shown as the mean ± SEM. Different letters indicate significant differences between each treatment at the same time point (p < 0.05; ns, no significant).

FIGURE 4

Infection frequencies of two endosymbionts in female adult spider mites reared for four generations under different temperature conditions. Infection rate of Wolbachia in co-infected strains (A) and Wolbachia-infected strains (B). Infection rate of Spiroplasma in co-infected strains (C) and Spiroplasma-infected strains (D).

FIGURE 5

Transcription response of four female adult spider mite strains following 6 h of different temperature exposures. (A) Principal components analysis (PCoA) of gene expression patterns of four spider mite strains under 20, 25, 30, and 35°C. (B) Number of differentially expressed genes (DEGs) with fold change > 2 and an FDR-adjusted p-value of < 0.05 for four spider mite strains under different temperatures. (C) Significantly differently expressed Hsp genes were analyzed by comparing each of 20, 30, and 35°C vs. the control (25°C) based on a threshold of > 2-fold change and an FDR-adjusted p-value of < 0.05. The gray squares in the heat map indicate that gene expression was absent. Asterisks indicate statistically significant differences (*p < 0.05; **p < 0.01; and ***p < 0.001).

FIGURE 6

Thermal preference (Tp) of the four spider mite strains. (A) Average Tp of the four spider mite strains. Each dot on the graph indicates the mean of six individuals in each replicate. Long bar indicates the mean of all dots in the strain, and error bars indicate SE. Different letters indicate significant differences between four spider mite strains (p < 0.05). (B) Relative proportions of the four spider mite strains observed at a given temperature.