Abstract: 【Aim】The study of sex ratio variation is important for understanding the causes and consequences of selective pressures. The pollinating fig wasps are widely used in researches on sex ratio and local mate competition (LMC) because of theoretical assumptions of their biological and ecological feature resemblance. Our study aimed to explore the mechanism of sex ratio adjustment and evolution in fig wasps. 【Methods】 We conducted controlled introduction experiments on Ficus semicordata in Xishuangbanna, and the first receptive day was defined firstly for each experimental fig by observing the behavior of the pollinator (Ceratosolen gravelyi). Then we tested the effects of foundress number, time interval of foundress entry and density of nonpollinating fig wasps (Sycoscapter trifemmensis) on clutch size (the number of adult wasps) and the offspring sex ratio of pollinating fig wasps, as well as the relationship between clutch size and the offspring sex ratio in each treatment. 【Results】 When foundress number increased, the mean clutch size per foundress significantly decreased (P<0.001) and the offspring sex ratio increased significantly (P<0.001), and the offspring sex ratio was negatively related to the clutch size per foundress for individual figs (P<0.05). As the foundress entry interval prolonged, the clutch size per foundress decreased and the offspring sex ratio (P=0.86) was less female-biased, and the similar trends were found between 2 foundresses (P=0.87) and 3 foundresses treatments (P=0.49). The density of non-pollinating fig wasps significantly reduced clutch size (P<0.001) and increased the offspring sex ratio of pollinating fig wasps (P<0.001), and a significantly negative relationship was found for individual figs between clutch size and the offspring sex ratio (P<0.001). 【Conclusion】 This study simulated the natural condition of interaction between figs and fig wasps, so the results can help us understanding the pattern and mechanism of sex ratio allocation in natural condition, and the evolution mechanisms of mutualistic symbiosis between figs and fig wasps as well.

Key words: Pollinating fig wasps, mutualism, local mate competition, clutch size, sex ratio