Consequences of population structure for sex allocation and sexual conflict

Both sex allocation and sexual conflict can be modulated by spatial structure. However, how the interplay between the type of dispersal and the scale of competition simultaneously affects these traits in sub-divided populations is rarely considered.

We investigated sex allocation and sexual conflict evolution in meta-populations of the spider mite Tetranychus urticae evolving under budding (pairing females from the same patch) or random (pairing females from different patches) dispersal and either local (fixed sampling from each subpopulation) or global (sampling as a function of subpopulation productivity) competition.

Females evolving under budding dispersal produced less female-biased offspring sex ratios than those from the random dispersal selection regimes, contradicting theoretical predictions. In contrast, the scale of competition did not strongly affect sex allocation. Offspring sex ratio and female fecundity were unaffected by the number of mates, but female fecundity was highest when their mates evolved under budding dispersal, suggesting these males inflict less harm than those evolving under random dispersal.

This work highlights that population structure can impact the evolution of sex allocation and sexual conflict. Moreover, selection on either trait may reciprocally affect the evolution of the other, for example via effects on fecundity.

Methods
In a fully crossed design, using experimental evolution, we placed replicate populations of T. urticae in 4 selection regimes with either local or global competition, and random versus budding dispersal (see details in Rodrigues et al. 2020 JEB) This design enabled us to follow evolution of both sex ratio and sexual conflict under different population structures. Each regime was replicated three times (GB-1, GB-2, GB-3, GR-1, GR-2, GR-3, LB-1, LB-2, LB-3, LR-1, LR-2 and LR-3). For each replicate population, each generation comprised a total of 96 mated adult females, being assigned in pairs to 48 bean leaf patches.

Four experiments were performed (see details in Rodrigues et al. 2020 JEB):

1. Sex allocation during experimental evolution

The sex allocation of females was measured directly in the replicate populations of each selection regime at generations 12, 17, 20 and 31.

2. Sex allocation in a common environment

In this assay, all regimes were each exposed to a common environment for 1 generation to equilibrate maternal effects before measuring the offspring sex-ratios of females that mated randomly with males from their selection regime.

3. Sex allocation in response to patch fecundity

We measured the fecundity and sex allocation of single females from our selection regimes in response to the presence of eggs laid by sterile females on the same patch. Because the eggs of the sterilised females do not hatch, we can distinguish the offspring of the focal female (adult individuals) from that of the sterilised one (unhatched eggs) within a single patch.

4. Sexual conflict

The impact of mating with males evolved under the ‘Global Budding’ and ‘Global Random’ selection regimes on the fecundity of females from the ancestral population was compared.

Statistical analysis

All analyses were carried out using the R statistical package (v. 3.0.3) and JMP13. We used Generalised Linear Mixed Models with a beta-binomial error structure and logit link function, and quasi-poisson or negative binomial error structures and log link function, to analyse the effect of selection regime on sex ratio and mean offspring production, respectively. Maximal models were simplified by sequentially eliminating non-significant terms (p < 0.05) from the highest- to the simplest-order interaction, with the highest p-value to establish a minimal model. The significance of the explanatory variables in the minimal models was established using chi-squared tests. A posteriori contrasts with Bonferroni corrections were done to interpret the effect of selection regime when significant. Details can be found in Rodrigues et al. 2020 JEB.