Kin-mediated plasticity in alternative reproductive tactics

Conditional strategies occur when the fitness payoff an individual receives from expressing a given phenotype (from a range of two or more possible phenotypes) is contingent upon that individual’s environmental circumstances. This conditional strategy model underlies many cases of alternative reproductive tactics, in which individuals of one sex employ different means to obtain reproductive opportunities. How genetic relatedness and indirect fitness effects could affect the expression of alternative reproductive tactics remains unexplored. Here, we address this gap using the Acarid mite Rhizoglyphus echinopus, in which large males develop into aggressive ‘fighters’ and small males develop into non-aggressive ‘scramblers.’ Because fighters kill their rivals whereas scramblers do not, fighters should incur a greater indirect fitness cost when competing with their relatives, and thus the expression of the fighter phenotype could be reduced in the presences of relatives. To test this, we raised mites in full-sibling or mixed-sibship groups and found that fighters were more common in full-sibling groups, not less common as we predicted. This apparently counter-intuitive result could be explained if relatedness and cue variability are interpreted as a signal of population density, since fighters are more common at low densities in this species. Alternatively, our results may indicate that males compete more intensely with relatives than non-relatives in this species. Our study provides the first evidence of kin-mediated plasticity in the expression of alternative reproductive tactics.

条件策略是指，个体从表达某一特定表型（可从两种及以上可选表型中选择）所获得的适合度收益，取决于该个体所处的环境条件。该条件策略模型是诸多替代性生殖策略案例的理论基础——即同一性别个体可通过不同途径获取繁殖机会。目前，遗传亲缘关系与间接适合度效应如何影响替代性生殖策略的表达，仍是尚未探索的研究空白。
本研究以刺足根螨（Rhizoglyphus echinopus）为实验材料，填补这一研究空白。该物种存在雄性二型现象：体型较大的雄性会发育为具有攻击性的"格斗者"，体型较小的雄性则发育为无攻击性的"逃窜者"。由于格斗者会击杀竞争对手，而逃窜者不会，因此格斗者在与亲缘个体竞争时，需承担更高的间接适合度代价，故而在亲缘个体存在的环境中，格斗者表型的表达可能会受到抑制。
为验证这一假设，我们将螨虫以全同胞组或混交同胞组的方式饲养，结果发现格斗者在全同胞组中的占比更高，而非我们预测的更低。这一看似违背直觉的结果可通过以下逻辑解释：若将亲缘关系与线索变异性解读为种群密度的信号——鉴于该物种中格斗者在低密度种群中更为常见。此外，本研究结果也可能表明，该物种中雄性个体与亲缘个体间的竞争强度高于非亲缘个体。
本研究首次证实了亲缘介导的表型可塑性对替代性生殖策略表达的调控作用。