Status-dependence and morphological trade-offs in the expression of a sexually selected character in the mite, Sancassania berlesei

Abstract

In the male dimorphic mite Sancassania berlesei, fighter males kill rivals with a pair of armoured legs whereas scrambler males are benign with unmodified legs. In an adaptive response mediated by colony pheromones, fighter expression increases at low colony density. Under the status-dependent evolutionarily stable strategy (ESS) model we expected heavier final instar nymphs to become fighters. This was supported in group reared nymphs. In individually reared nymphs fighter expression was experimentally suppressed using two concentrations of colony pheromone. Here, male morph expression again depended on tritonymphal body mass and contact is therefore unnecessary for individuals to judge their status. Fighter suppression was greater in the higher pheromone treatment, but morph determination remained status dependent. The weight and length of fighters was lower than scramblers of same-weight final instar nymphs, indicating a developmental trade-off, and a cost not recouped at the adult stage.

Phenotypic plasticity in the developmental integration of morphological trade-offs and secondary sexual trait compensation

“…Knell et al 2004). In taxa where exaggerated structures become fixed at a particular moment in development, and in particular those developing adult morphology in a closed system such as holometabolous insects, the stage is set for resource competition (Klingenberg & Nijhout 1998;Nijhout & Emlen 1998;Emlen 2001;Radwan et al 2002;Moczek & Nijhout 2004). Equally, in such systems, the opportunity for selection to act on precise integration of traits that enable exaggerated structures to be borne and displayed is also clear.…”

“…A number of recent studies have demonstrated that intra-organism competition occurs over resources devoted to somatic structures (Klingenberg & Nijhout 1998;Nijhout & Emlen 1998;Emlen 2001;Radwan et al 2002;Moczek & Nijhout 2004). These morphological trade-offs have, in particular, been demonstrated in species where alternative reproductive tactics give rise to dramatically divergent morphologies among males, i.e.…”

“…These morphological trade-offs have, in particular, been demonstrated in species where alternative reproductive tactics give rise to dramatically divergent morphologies among males, i.e. where there is extreme phenotypic plasticity (Nijhout & Emlen 1998;Emlen 2001;Radwan et al 2002;Moczek & Nijhout 2004). These studies demonstrate that when resources are finite, as in the case of holometabolous insects (Nijhout & Emlen 1998;Emlen 2001;Moczek & Nijhout 2004) or quiescent mites (Radwan et al 2002), investment in exaggerated morphology places demands on other traits reducing their final size or reducing overall body size.…”

“…where there is extreme phenotypic plasticity (Nijhout & Emlen 1998;Emlen 2001;Radwan et al 2002;Moczek & Nijhout 2004). These studies demonstrate that when resources are finite, as in the case of holometabolous insects (Nijhout & Emlen 1998;Emlen 2001;Moczek & Nijhout 2004) or quiescent mites (Radwan et al 2002), investment in exaggerated morphology places demands on other traits reducing their final size or reducing overall body size. Resource competition of this kind is thought to be particularly acute in closed systems, and in the holometabola to be strongest between traits developing from spatially close areas of the epithelium (Nijhout & Emlen 1998;Emlen 2001).…”

“…In many such species, the dimorphism is an extreme case of phenotypic plasticity: a facultative consequence of the conditions under which the individual developed (Emlen 1994;Gross 1996;Hunt & Simmons 1997;Tomkins 1999;Radwan et al 2002). These species are ideal for examining developmental trade-offs (Nijhout & Emlen 1998;Emlen 2001;Radwan et al 2002) and also have great potential for exploring the developmental integration of traits that are selected alongside the principal target of elaboration. Where developmental reprogramming divides males between morphologies that differ in trait exaggeration, the adaptations individuals have to bearing exaggerated traits will become particularly apparent.…”