Wolbachia infection facilitates adaptive increase in male egg size in response to environmental changes

Abstract

Under challenging conditions such as maladapted biotic and abiotic conditions, females can plastically adjust their egg size (gamete or zygote size) to counteract fitness declines early in life. Recent evidence suggests that endosymbionts may enhance this egg-size plasticity. Possible endosymbionts' modification of impact of multiple stressors is not well explored. Therefore, this study aims to test (1) whether Wolbachia infection influences the plasticity of parental investment in egg size under suboptimal environmental conditions and (2) whether the plasticity depends on the sex of eggs. We used three lines of the azuki bean beetle (Callosobruchus chinensis): a line coinfected with the wBruCon and wBruOri Wolbachia strains, a cured line infected solely with the wBruCon, and an uninfected (cured) line. These lines were subjected to either a control environment or a simulated climate change environment (elevated temperature and carbon dioxide levels, eT&CO₂) to examine Wolbachia infection effects on parental investment in their offspring (egg size) and its subsequent impact on offspring fitness, including survival, development, and adult lifespan under starvation. After two days of eT&CO₂ exposure, coinfected parents increased male egg size only. Larger eggs developed faster in both sexes and exhibited higher survival. However, offspring adult lifespan was not influenced by egg size but by environment, sex, Wolbachia infection, and development time: eT&CO₂ reduced male lifespan but not female lifespan, the singly-infected line females lived longer than coinfected and uninfected line females, and shorter development time linked to longer lifespan. The negative correlation between development time and lifespan was higher under eT&CO₂ but not sex-specific. This study is the first to demonstrate sex-specific egg size plasticity associated with Wolbachia infection in species with sex determination systems other than haplodiploid.

Keywords: Inclusive fitness; Insect; Modified atmosphere; Parent age; Plasticity; Reproduction; Sex difference; Thermal stress.

Fig. 1

Experiment design for studying egg size plasticity. A pair of beetles from coinfected, singly-infected (Con only), or uninfected lines was allowed to oviposit for three consecutive days under either control conditions or elevated temperature and CO₂ (eT&CO₂) conditions (eight pairs for each combination of line and environmental condition). Beans with a single hatched egg were isolated to track the timing of adult emergence and mortality. Beetle size relative to bean size is smaller.

Fig. 2

Effects of infection status on egg size across environments, parents’ ages (or parents’ exposure times) and sexes. Mean egg size (with 95% confidence intervals) is shown for coinfected, singly-infected (Con only), and uninfected lines in control environment (in blue) or elevated temperature and CO₂ (eT&CO₂) (in red). Different alphabets indicate significant differences within beetle lines.

Fig. 3

Size distributions (upper panel: data density; lower panel: frequency) of hatched eggs that developed into adults (“survived”) and those that did not survive to adulthood (“dead”).

Fig. 4

Development time (log-transformed) from egg to adult reared in control environment or eT&CO₂ (elevated temperature and CO₂) environment. (a) Effect of egg length. Solid lines represent linear regression fits, with shaded bands indicating 95% confidence intervals. (b) Effects of parents’ age and offspring sex. Offspring (mean) from same parents relate to solid lines. Effect of beetle line was non-significant. Different alphabets indicate significant differences.

Fig. 5

(a) Lifespan (log₁₀-transformed)(under starvation) of offspring adults developed from eggs laid in different environments. Egg length did not affect lifespan under starvation (p = 0.400). (b) Correlation of log₁₀(offspring adult lifespan under starvation) with log₁₀(development time) (p < 0.001) and two-way interaction effects on log₁₀(offspring adult lifespan under starvation) [environment × sex p < 0.001; environment × log₁₀(development time) p = 0.048]. (c) Female and male offspring adult lifespan of beetle lines with different Wolbachia infection statuses [coinfected, singly-infected (Con only), and uninfected] laid and reared in control or eT&CO₂ (elevated temperature and CO₂) environment (line × sex p = 0.008). Different alphabets indicate significant differences. Jittering in (b,c) for visibility.