Spatiotemporal fluctuations in natural selection acting on the gall‐parasitic aphid Tetraneura sorini

Abstract

The measurement of the selection gradient is crucial for understanding the magnitude of selection acting directly on a trait and predicting the evolutionary trajectory of that trait. This study evaluated the selection gradient acting on the morphology of the gall-parasitic aphid Tetraneura sorini during the galling process and compared the strength among populations. Gall formers (first instars) frequently fight with conspecifics or heterospecifics for usurping incipient galls using their well-developed hind legs. First instars that successfully acquired galls were found within galls, whereas those that failed were found dead on leaf surfaces. Selection gradients were estimated using logistic stepwise regression and partial least square (PLS) regression. Calculated selection differentials indicated that first instars that secured galls were larger in body size than failed individuals through all populations. However, selection gradients on weapon traits varied largely among populations or among years in the same population. We confirmed microevolutionary changes in the relationship between traits, which accorded with the expectation from changes in the selection gradients. When gall formers were transferred onto developing buds individually, individuals that successfully induced galls had smaller body size than failed individuals. Available evidence suggests that the selection gradient on body size becomes higher with an increasing proportion of T. sorini in the Tetraneura species community. Thus, we concluded that more intense fighting with conspecifics leads to stronger selective pressure on body size, but that selective pressure for each trait is variable depending on differences in the tactics and species composition among populations.

Female–female competition leads to female‐biased sex allocation and dimorphism in brood sex composition in a gall‐forming aphid

“…Several foundresses start galling on the same leaf and foundresses who failed to induce their own galls attack those who are inducing an incipient gall (Akimoto & Yamaguchi, ). Muramatsu and Akimoto () examined the body sizes of foundresses from nine populations, and in all of these, they detected significant directional selection for larger body size. Mothers who produce a larger number of bigger females could have more numerous granddaughters that survive the galling process and reproduce because of their better fighting ability.…”

“…The T. sorini foundress is the largest in body size of the Tetraneura species, with well‐developed hind legs, and thus, it has an overwhelming advantage in aggressive interactions. Muramatsu and Akimoto () showed that where the density of other Tetraneura species (host species) is lower, T. sorini foundresses more frequently fight with conspecifics, such that foundresses are subject to stronger directional selection for larger body sizes.…”

“…Foundresses (gall formers) frequently fight with one another over the possession of incipient galls (Akimoto & Yamaguchi, ). Larger foundresses are more likely to win these fights (Muramatsu & Akimoto, ). Therefore, one can assume that mothers can achieve greater reproductive returns by investing a larger proportion of resources in individual sexual females and producing larger foundresses (Figure ).…”

“…Males and females are collectively called "sexuals." Tetraneura sorini foundresses are more likely to fail to induce galls than those of other Tetraneura species, but can act as a "parasite" and usurp incipient galls induced by other Tetraneura species (Akimoto & Yamaguchi, 1997;Muramatsu & Akimoto, 2016). The T. sorini foundress is the largest in body size of the Tetraneura species, with welldeveloped hind legs, and thus, it has an overwhelming advantage in aggressive interactions.…”

Frequency‐dependent selection acting on the widely fluctuating sex ratio of the aphid Prociphilus oriens

“…During fighting, larger body size is overwhelmingly advantageous (Whitham, ; Aoki & Makino, ; Akimoto & Yamaguchi, ). In Tetraneura sorini , foundresses are reportedly subject to directional selection for larger body size in all populations examined (Muramatsu & Akimoto, ). Therefore, where unrelated foundresses frequently fight, mothers can gain higher fitness returns by investing more resources in individual females, thereby producing larger foundresses.…”