Proximate sources of sexual size dimorphism in insects: locating constraints on larval growth schedules

Abstract

Different levels of sexual size dimorphism (SSD) have usually been explained by selective forces operating in the adult stage. Developmental mechanisms leading to SSD during the juvenile development have received less attention. In particular, it is often not clear if the individuals of the ultimately larger sex are larger already at hatching/birth, do they grow faster, or do they grow for a longer time. In the case of insects, the question about sexually dimorphic growth rates is still open because most previous studies fail to adequately consider the complexity of larval growth curve, the existence of distinct larval instars in particular. Applying an instar-specific approach, we analysed ontogenetic determination of female-biased SSD in a number of distantly related species of Lepidoptera. The species studied showed a remarkable degree of similarity: SSD appeared invariably earlier than in the final instar, and tended to accumulate during development. The higher weight of the females was shown to be primarily a consequence of longer development within several larval instars. There was some evidence of higher instantaneous growth rates of females in the penultimate instar but not in the final instar. Egg size, studied in one species, was found not to be sexually dimorphic. The high across-species similarity may be seen as an indication of constraints on the set of possible mechanisms of size divergence between the two sexes. The results are discussed from the perspective of the evolution of insect body size in general. In particular, this study confirms the idea about limited evolvability of within-instar growth increments. An evolutionary change towards larger adult size appears always to be realised via moderate changes in relative increments of several larval instars, whereas a considerable change in just one instar may not be feasible.

Sources of variance in immunological traits: evidence of congruent latitudinal trends across species

“…Additionally, simultaneous rearing enabled us to extract haemolymph synchronously in terms of both absolute time and development stage of the larvae. The strict synchronisation of larval development (within species) was achieved by adjusting rearing temperature in a unified manner during the pre-experimental development, mainly through keeping the larvae at +4°C for 1 or 2 days before the experiment (Tammaru and Esperk, 2007;Tammaru et al, 2010) so that the larvae moulted into their final larval instar simultaneously. To minimise any possible error related to the synchronising low-temperature treatments per se, all larvae were kept at +4°C for at least 12 h before the experiment.…”

“…Approximation of differential growth rate (DGR) was based on the mass increment accumulated during the second day of the final larval instar. This is considered the most adequate characteristic of the 'free' growth period in the beginning of an instar; that is, the growth that is not yet affected by physiological preparations for pupation (Ayres and MacLean, 1987;Esperk and Tammaru, 2004;Tammaru et al, 2010). From the mass data, DGR was calculated as (final mass 1/3 -initial mass 1/3 )/time, where time was in days (Meister et al, 2017), with the cube root transformation having been suggested to linearise larval growth curves of lepidopteran larvae (Tammaru and Esperk, 2007).…”

Sexual Dimorphism Using Geometric Morphometric Approach

“…Recent analyses demonstrate that many lepidopteran species exhibit female-biased sexual size dimorphism 73% of 48 species in Reference [57]. Size and shape differences are established during the larval period [58,59] by developmental and physiological mechanisms (e.g., number of larval instars and hormonal regulation). Because females of many species are capital breeders (i.e., they allocate larval resources for reproduction), and large size is related directly to fecundity [60][61][62], selection for large female body size appears to be driven by natural selection for increased fecundity [63].…”

Small-scale variation in sexual size dimorphism and sex ratio in the aquatic mothAcentria ephemerellaDenis and Schiffermüller, 1775 (Lepidoptera: Crambidae)

“…Sexual size dimorphism (SSD) is a common phenomenon in many insect species (Blanckenhorn 2005;Stillwell, Blanckenhorn, Teder, Davidowitz, and Fox 2010;Tammaru, Esperk, Ivanov, and Teder 2010). The ultimate and proximate causes of SSD variability have been studied intensively in the last decade Tammaru et al 2010).…”

“…The ultimate and proximate causes of SSD variability have been studied intensively in the last decade Tammaru et al 2010). The SSD shows strong inter-and intra-specific variability largely reflecting different reproductive adaptations of the sexes (Abouheif and Fairbairn 1997;Blanckenhorn et al 2007;Allen, Zwaan, and Brakefield 2011).…”