Sexual size dimorphism is associated with reproductive life history trait differentiation in coexisting sepsid flies

Abstract

Organismal life histories evolve as syndromes, resulting in correlated evolutionary differentiation of key traits that ultimately aid in discerning species. Reproductive success depends both on the absolute body size of an individual and its size relative to the opposite sex: sexual size dimorphism. In an attempt to further elucidate their coexistence and ecological diversification, we compared standard life history (first reproduction, clutch size, egg size) and associated reproductive trait differentiation of 15 widespread European sepsid fly species (Diptera: Sepsidae) under laboratory common garden conditions. Despite relatively uniform body sizes, sexual dimorphism ranged from female-to male-biased, and development time varied twofold across species. We expected, and found, the abundant and relatively large species (Sepsis cynipsea, punctum, thoracica) with often male-biased SSD to lay larger but fewer eggs and show fast-developing, fast-reproducing life histories with aggressive (coercive) mating behavior characterized by short mating latencies and male conflict. In contrast, the smaller and more dispersed species with female-biased SSD (S. flavimana, orthocnemis, violacea) laid smaller but more eggs, showing a generally slower life history with long and delayed copulation and oviposition, high mating reluctance fostering extensive inter-sexual conflict, and more elaborate male (pre-)copulatory courtship. Two Saltella species were exceptional, being large, developing slowly, nevertheless copulating soon after adult emergence, profusely and briefly. The documented life history differentiation seems partly driven by sexual selection leading to male-biased dimorphism, rather than undetermined ecological selection, but regardless appears insufficient to explain the coexistence and diversification of these sepsid species in European pastoral landscapes.

A role for sex‐determination genes in life history evolution? Doublesex mediates sexual size dimorphism in the gazelle dung beetle

“…Consequently, sexual size dimorphism and sexual bimaturism are widespread in anisogamous organisms and contribute greatly to intraspecific variation in nature (Fairbairn, 1997). However, although a large body of literature documents the role of behaviour and sexual as well as ecological selection in driving variation in size, age at maturity, sexual size dimorphism and sexual bimaturism (Badyaev, 2002; Blanckenhorn, 2005; Blanckenhorn et al., 2020; Hirst et al., 2015; Shine, 1989; Stillwell et al., 2010; Temeles et al., 2000), the developmental genetic mechanisms underpinning intraspecific body size variation remain far less well understood (but see e.g. : Millington et al., 2021; Rideout et al., 2016; Rohner et al., 2017; Shingleton, 2011; Stillwell & Davidowitz, 2010).…”

Secondary Sexual Trait Melanization in “Black” Scavenger Flies: Nutritional Plasticity and Its Evolution

“…However, in several species and even populations within species, intense sexual selection on male body size led to the evolution of male-biased sexual size di-morphism ). These shifts in overall sexual size dimorphism coincide with the presence of aggressive male-male interactions and territoriality, exaggerated male morphology and allometry, and reduced investment into copulatory courtship (Puniamoorthy et al 2012a,2 0 1 2 b; Blanckenhorn et al 2020Blanckenhorn et al , 2021. This variation here allows for testing of whether the evolution of more intense sexual selection on males and territorial behavior coevolve with nutritional plasticity and sexual dimorphism in melanization.…”

“…Taxa that evolved more male-biased sexual size dimorphism also evolved less melanized forefemora in males (PGLS: F 1, 10 p 12:98, P p :005). This suggests an evolutionary link between cuticular melanism in males and more intense sexual selection or territorial behavior (both of which coincide with male-biased sexual size dimorphism; Blanckenhorn et al 2020). In contrast, there was no relationship between sexual dimorphism in body size and cuticular melanization in females (PGLS: F 1, 10 p 3:05, P p :111).…”

“…In the latter case, I expected close relatives of S. thoracica to show moderate levels of plasticity and for the sigmoidal reaction norm (i.e., polyphenic development) to be derived from a (quasi)linear reaction norm. Furthermore, I tested whether the association between reduced melanization and mating success found in S. thoracica extends to the macroevolutionary scale by contrasting average melanization of the forefemur with previously documented variation in mating systems and reproductive behavior among taxa (Puniamoorthy et al 2012a(Puniamoorthy et al , 2012bBlanckenhorn et al 2020). If reduced male melanization increases mating success, the degree of melanism is expected to coevolve with the degree of male-biased sexual size dimorphism-aproxy for the degree of sexual selection and male-male aggressive behavior in these species (see Blanckenhorn et al 2020Blanckenhorn et al , 2021.…”

Evolution and plasticity of morph‐specific integration in the bull‐headed dung beetle Onthophagus taurus

“…Phenotypic integration can be adaptive if natural selection favors the co‐occurrence of several traits via correlational selection or selection on certain functional trait combinations on the evolutionary, static, and ontogenetic levels (Cheverud, 1982, 1984; Klingenberg, 2014). Examples include evolutionary integration of ecological variation, life history, physiology, and behavior within and among species (Blanckenhorn et al., 2020; Réale et al., 2010; Ricklefs & Wikelski, 2002), developmental integration between morphology and behavior among individuals (Beckers, Kijimoto, & Moczek, 2017), or the ontogenetic integration between functionally related skeletal structures (Zelditch, 1988). In addition to such adaptive scenarios, trait covariation can also arise as a by‐product of shared developmental processes involved in the development of different traits, or arise neutrally by mere genetic correlation via pleiotropy or linkage (Lande, 1982; Lynch & Walsh, 1998).…”