Heat shock alters pea aphid–Buchnera interactions: negative allometry of gene densities Nousheen Parven 1,2 , Izumi Yao 1 , Takashi Kanbe 1 & Shin-ichi Akimoto 1 * 1 Department of Ecology and Systematics, Graduate School of Agriculture, Hokkaido University, Sapporo, Japan, and 2 Department of Zoology, University of Dhaka, Dhaka, Bangladesh Accepted: 6 October 2020 Key words: Acyrthosiphon pisum, bacteriocyte, elongation factor 1-alpha, 16S-rRNA gene, sterility, symbiont, real-time PCR, transgenerational effect, temperature, Hemiptera, Aphididae Abstract In the face of global climate change, the understanding of how aphid-symbiont relationships are affected by heat shock is critical. We evaluated the effects of heat shock on the pea aphid, Acyrthosi- phon pisum Harris (Hemiptera: Aphididae), and its obligate endosymbiont Buchnera aphidicola Munson et al. by means of quantitative PCR in treated aphids and their offspring. First-instar aphids received a single heat shock (35 °C for 6 h), repetitive heat shocks (repeat of the single heat shock for 3 days), or a control treatment (constant 20 °C). We evaluated the impacts on aphid body length and Buchnera and aphid gene densities, estimated from the number of copies of bacterial 16S-rRNA and nuclear elongation factor 1-alpha (EF1a) genes, respectively. Heat shock negatively affected aphid body length and Buchnera and EF1a gene densities. Heat-shocked aphids contained lower den- sities of Buchnera and EF1a genes than control aphids when body length was kept constant. When Buchnera and EF1a gene densities were represented on a log–log scale, Buchnera densities increased with EF1a densities in all treatments, but Buchnera densities showed negative allometry with EF1a densities. Compared to control aphids, heat-shocked aphids contained lower Buchnera densities rela- tive to EF1a densities. Some heat-shocked aphids became sterile if their Buchnera gene density was lower than a threshold (ca. 42 000 copies). The offspring of aphids subjected to a single heat shock recovered the number of Buchnera, but the offspring of aphids subjected to repetitive heat shocks exhibited markedly lower Buchnera and EF1a densities. Thus, heat shock negatively affects both aphid and Buchnera cell proliferation, in the heat shock-treated generation as well as in their off- spring, but the impact is more severe on Buchnera. Because the symbiont supplies essential amino acids, vitamins, and an essential protein, this could reduce aphid development and reproduction and possibly leads to extinction of local populations. Introduction Climate change under global warming will reportedly increase temperature variability, leading to a substantial increase in daily maximum temperature, with extreme heat events (Easterling et al., 2000; IPCC, 2013; Ma et al., 2015). Global warming is expected to alter the distribu- tion, phenology, and life-history traits of insects in the coming decades (Hoffmann et al., 2013; Kingsolver et al., 2013; Auad et al., 2015; Peng et al., 2019). Thus, increase in the frequency, intensity, and duration of high temperatures has become an area of focus for ecological research (Hansen et al., 2006; Bauerfeind & Fischer, 2014). The detrimental effects of high temperatures on the sur- vival, development, and reproduction of aphid species have been well documented (Lamb et al., 1987; Wang et al., 1997; Chen et al., 2000; Shingleton et al., 2003; Ma et al., 2004; Kuo et al., 2006; Russell & Moran, 2006; Lu & Kuo, 2008; Moran & Yun, 2015). Heat stress has harmful effects on aphids, primarily because of the heat sensitivity of the aphid obligate endosymbiont Buchnera aphidicola Munson et al., whose density in the host aphid decreases under high-temperature conditions (Montllor et al., 2002; Chen et al., 2009; Burke et al., 2010; Lu et al., 2014). Buch- nera supplies essential amino acids, vitamins, and a specific protein [symbionin, a member of the GroEL *Correspondence: Shin-ichi Akimoto, Department of Ecology and Systematics, Graduate School of Agriculture, Hokkaido University, Sapporo, Japan. E-mail: akimoto@res.agr.hokudai.ac.jp 462 © 2021 The Netherlands Entomological Society Entomologia Experimentalis et Applicata 169: 462–471, 2021 DOI: 10.1111/eea.13039