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We incubated eggs of the Japanese gecko Gekko japonicus at three temperatures,and measured yolk testosterone (T) and 17p-estradiol (E2) levels at three time points in embryonic development (oviposition,1/3 of incubation,and 2/3 of incubation),to test the hypothesis that G.japonicus is among reptilian species where females can control over offspring sex by means of yolk allocation.Eggs incubated at 24 ℃ and 32 ℃ produced mostly females,and eggs incubated at 28 ℃ mixed-sex hatchlings.Female-producing eggs were larger than male-producing eggs.Clutches in which eggs were incubated at the same temperature produced mostly same-sex siblings.Yolk T level at laying was negatively related to eggs mass,and yolk E2/T ratio was positively related to egg mass.Results of two-way ANOVA with incubation temperature and stage as the factors show that: yolk E2 level was higher at 32 ℃ than at 24 ℃; yolk T level was higher,whereas yolk E2/T ratio was smaller,at 28 ℃ than at 24 ℃; yolk E2 and T levels were higher at 2/3 than at 1/3 of incubation.Our data validate the hypothesis tested,and show that incubation temperature affects the dynamics of developmental changes in yolk steroid hormones,influences of yolk steroid hormones on offspring sex are secondary relative to incubation temperature effects,and offspring sex in G.japonucus correlates with an interaction between incubation temperature and yolk steroid hormones.