The evolution of threshold traits: effects of selection on fecundity and correlated response in wing dimorphism in the sand cricket

Abstract

The quantitative genetic basis of traits can be determined using a pedigree analysis or a selection experiment. Each approach is valuable and the combined data can contribute more than either method alone. Analysis using both sib analysis and selection is particularly essential when there are likely to be nonlinearities in the functional relationships among traits. A class of traits for which this occurs is that of threshold traits, which are characterized by a dichotomous phenotype that is determined by a threshold of sensitivity and a continuously distributed underlying trait called the liability. In this case, traits that are correlated with the liability may show a nonlinear relationship due to the dichotomy of expression at the phenotypic level. For example, in wing dimorphic insects fecundity of the macropterous (long‐winged) females appears in part to be determined by the allocation of resources to the flight muscles, which are almost invariably small or absent in the micropterous (short‐winged, flightless) females. Pedigree analysis of the cricket Gryllus firmus has shown that wing morph, fecundity and the trade‐off between the two have additive genetic (co)variance. It has also been shown that selection on proportion macroptery produced an asymmetric correlated response of fecundity. The present paper details the results of direct selection on fecundity and the correlated response in proportion macroptery. Selection for increased fecundity resulted in increased fecundity within both wing morphs and a correlated decrease in proportion macroptery. Similarly, selection for decreased fecundity resulted in a decrease within morphs and a correlated increase in the proportion of macropterous females. This provides additional evidence that the trade‐off between fecundity and wing morphology has a genetic basis and will thus modulate the evolution of the two traits.

Dimorfismo sexual alar em Aedes scapularis (Diptera: Culicidae)

“…Como exemplos pode-se citar a correlação entre dimorfismo sexual e diferenciação ecológica entre sexos em Diptera (Atchley 1971) e a relação entre determinantes genéticos da forma alar e a fecundidade de machos e fêmeas de Orthoptera (Roff et al 1999). Roff & Fairbairn (1991) ressaltam que a evolução do dimorfismo sexual de asas em insetos é informativa para entendimento da evolução da capacidade dispersional de machos e fêmeas.…”

Life-history plasticity in female threespine stickleback

“…That is, allometric slopes (model II, reflecting error in both x and y variables) relating clutch mass to body mass are 41 in many populations we have studied (Figure 6). Such an increase is expected under lifetime allocation models of reproductive effort (Roff, 2002), and thus represents adaptive plasticity. All of our data bearing on this phenomenon are from cross-sectional studies of females of multiple ages collected at one time, and we know of only one study (Lee et al, 2012) that has tracked individual stickleback across multiple years.…”

“…Reproduction at age 1 would be favored if it resulted in a lifetime reproductive output exceeding that of females that delayed breeding to age 2 (Roff, 2002). Because female size has the greatest influence on clutch size, reproduction at age 1 should be favored only when it does not substantially diminish survival to, or size at, age 2 (Hutchings, 1999).…”

“…The mass of the eggs spawned in a single clutch is a common proxy for the relative amount of energy devoted to reproduction by a female during the time interval required to produce the clutch (Roff, 2002). Scaled to female body mass, this metric constitutes one of the most widely used indices of reproductive effort (gonadosomatic index: Gunderson and Dygert, 1988; relative clutch mass; Heins and Baker, 1993).…”

“…As opposed to reproductive effort, clutch size is relatively easily defined, and relatively easily quantified if done so just before reproduction. Plasticity for clutch size has been documented in a wide array of poikilotherms (Roff et al, 1999: Klemetsen et al, 2003, including numerous fish (Kennedy et al, 2008;Evans et al, 2010). Little clutch-to-clutch size fluctuation is reported in ectotherms, but there is evidence for long-term plasticity cued by energy acquisition during the prespawning period when the gonads are developing.…”