How to become larger: ontogenetic basis of among‐population size differences in a moth

Abstract

Evolutionary studies on animal body size have primarily focussed on selective pressures operating during the adult life. In contrast, ontogenetic pathways leading to differently sized adults have received less attention. In the present study, based on a common garden experiment, we report considerable genetic differences in body size among European populations of Ematurga atomaria (L.) (Lepidoptera: Geometridae). In terms of body mass, the moths from a southern (Georgian) population are twice as large as their northern (Estonian) conspecifics. Detailed monitoring of larval growth schedules revealed that the size difference arises through a longer development period of the Georgian larvae, with no difference in the number of instars. Differential (instantaneous) growth rates of the larvae do not differ between the populations. Eggs and newly hatched larvae are larger in the Georgian population but the difference vanishes in the second instar. The larger size of the Georgian moths is regained through higher relative mass increments during each of the three final instars. Such gradual 'accumulation of the difference' confirms the idea about constraints on substantial evolutionary changes in growth patterns within a single instar. The larger Georgian moths were found to be considerably more fecund which implies a strong selection for large female size. It remains unclear which counteracting selective pressures have favored the smaller size of Estonian conspecifics. As the associated difference in egg size appears not to be carried over to larger larval size, the adaptive value of larger eggs is not likely in contributing to the prospective large adult size. The larger eggs of the Georgian population should have an adaptive value per se, or represent a mechanistic consequence of larger maternal body size.

Ecology of female mating failure/lifelong virginity: a review of causal mechanisms in insects and arachnids

“…Carryover effects, defined as consequences of larval hostplant quality on reproductive output of adult females, are more important in capital breeders, which rely exclusively on larval accumulated resources, than in income breeders, which feed as adults or receive male nutrients while in copula (Jervis et al, 2005). In capital breeders, carryover effects of plant quality translate into large size of females at pupation/adult emergence, which correlate positively with fitness components, including fecundity (Rhainds, 2015;Davis et al, 2016), lifespan (Holm et al, 2016;Meister et al, 2018), mating contest outcomes (Bath et al, 2015;Joel et al, 2017), and mating success (Rhainds, 2010;Gwynne & Lorch, 2013;de Cock et al, 2014).…”

Comparative analysis of larval growth in Lepidoptera reveals instar‐level constraints

“…Within-instar growth patterns also appear relatively conservative as divergent selection on adult body size may only slightly change relative size increments in any particular instar Meister, Hämäläinen, Valdma, Martverk, & Tammaru, 2018). Moreover, the relative size increment often remains constant across larval instars, which is an empirically broadly supported pattern known as Dyar's Rule (Berg & Merritt, 2009;Dyar, 1890;Hutchinson, McNamara, Houston, & Vollrath, 1997).…”

“…Moreover, the relative size increment often remains constant across larval instars, which is an empirically broadly supported pattern known as Dyar's Rule (Berg & Merritt, 2009;Dyar, 1890;Hutchinson, McNamara, Houston, & Vollrath, 1997). However, there are deviations from the rule (Hutchinson et al, 1997;Kivelä, Friberg, et al, 2016;Meister et al, 2018), so quantification of conservative instar-level patterns in insect growth, as well as deviations from such patterns, may allow the identification of evolutionary constraints, which is needed for advancing our understanding of life-history evolution.…”

Weak and inconsistent associations between melanic darkness and fitness‐related traits in an insect

“…Indeed, in capital breeding insects (Davis et al ., ), realized fecundity of females is largely determined by adult body mass, for which pupal mass is a reasonable proxy (Honěk, ; Tammaru et al ., , ; Rhainds et al ., ). Ematurga atomaria is clearly a capital breeder (Javoiš et al ., ) and shows a strong correlation between body size and fecundity (Meister et al ., ). Moreover, the principle of L‐shaped reaction norms for body size and development time at maturation (see Teder et al ., ; for insect data) also allows one to reliably associate long developmental periods with inferior fitness.…”