Sex ratio strategies and the evolution of cue use

Abstract

Quantitative tests of sex allocation theory have often indicated that organism strategies deviate from model predictions. In pollinating fig wasps, Lipporrhopalum tentacularis, whole fig (brood) sex ratios are generally more female-biased than predicted by local mate competition (LMC) theory where females (foundresses) use density as a cue to assess potential LMC. We use microsatellite markers to investigate foundress sex ratios in L. tentacularis and show that they actually use their clutch size as a cue, with strategies closely approximating the predictions of a new model we develop of these conditions. We then provide evidence that the use of clutch size as a cue is common among species experiencing LMC, and given the other predictions of our model argue that this is because their ecologies mean it provides sufficiently accurate information about potential LMC that the use of other more costly cues has not evolved. We further argue that the use of these more costly cues by other species is due to the effect that ecological differences have on cue accuracy. This implies that deviations from earlier theoretical predictions often indicate that the cues used to assess environmental conditions differ from those assumed by models, rather than limits on the ability of natural selection to produce 'perfect' organisms.

The mechanism of sex ratio adjustment in a pollinating fig wasp

“…The determination of cue use patterns in such species is of general importance to evolutionary biologists because of its implications for our understanding of whether there are constraints upon the precision of adaptation (see also Herre 1987;West & Herre 1998;Herre et al 2001). Deviations in fit from the quantitative predictions of classical LMC theory (see Hamilton 1967) have been hypothesized to be a consequence of foundresses using cues other than Moore et al 2005). Of course, with regard to whether adaptation is constrained, this means the question becomes how closely foundress strategies approximate theoretical optima for a given clutch size.…”

“…Moore et al's (2005) model predicts that such foundresses should exhibit a negative logarithmic relationship between clutch size and sex ratio. Hence, to maintain the optimal sex ratio as clutch size increases, they should produce mostly male offspring at the start of oviposition bouts, followed by mostly females with a few males interspersed (see also Stubblefield & Seger 1990).…”

“…The study of such sex allocation strategies has developed into a highly productive field in evolutionary biology, in part because their close relation to organism fitness also allows quantitative tests of theory (e.g. Orzack 1990;Orzack et al 1991;Flanagan et al 1998;Moore et al 2002Moore et al , 2005 and so can provide insight into whether there are constraints upon the precision of adaptation (see Herre 1987;West & Herre 1998;Herre et al 2001). An area receiving particular attention has been the evolution of sex ratio strategies (proportion of male offspring produced) in spatially structured mating populations.…”

“…L. tentacularis foundress first (winged) and second (wingless) clutches differ in size (see Gibernau et al 1996 for similar in B. psenes). The extent to which other foundresses depositing offspring affects potential LMC depends on their relative contribution to the brood ( Frank 1985;Stubblefield & Seger 1990;Moore et al 2005), so this will reduce the accuracy of the information foundress density supplies about potential LMC and therefore affect whether its use as a cue evolves. We also note here that the accuracy of foundress density as a cue is likely to be reduced by sequential entry to figs in the field: this will lead to unequal brood contributions if oviposition site limitation occurs in multi-foundress figs, and, assuming physical interaction is required, underestimation of actual foundress density if re-emergences occur (see Greeff & Compton 1996;Kathuria et al 1999;Kinoshita et al 2002;Moore et al 2002).…”

Evolution of the scale and manner of brother competition in pollinating fig wasps

“…In another study where the focus was on sex ratio (Kjellberg et al 2005), we found no evidence for females leaving figs and we believe that the foundresses were overlooked in the Ware & Compton (1992) study cited by Moore et al (2003). When not all females contribute to the brood, or do so in unequal proportions, the optimal sex ratio is more female biased than with equal contributions (Frank 1985;Moore et al 2005). Below, we use data showing higher than expected sex ratios to argue that overcounting is not likely to be a problem, and if it is, it seems unlikely that there will be a systematic difference between fighting and nonfighting species.…”