Abstract
The evolution of sexual reproduction remains a fascinating enigma in biology. Theoretically, populations of sexual organisms investing half of their resources into producing male offspring that don’t contribute to reproduction should grow at only half the rate of their asexual counterparts. This demographic disadvantage due to male production is known as the twofold cost of sex. However, the question of whether this cost is truly twofold for sexual females remains unanswered. The cost of producing males should decrease when the number of male offspring is reduced. Here, we report a case where the cost of males is actually less than twofold. By measuring the numbers of sexual strain coexisting with asexual strain among thrips, our survey revealed that the sexual strain showed female-biased sex ratios and that the relative frequency of sexual strain is negatively correlated with the proportion of males in the sexual strain. Using computer simulations, we confirmed that a female-biased sex ratio evolves in sexual individuals due to the coexistence of asexual individuals. Our results demonstrate that there is a cost of producing males that depends on the number of males. We therefore conclude that sexual reproduction can evolve with far fewer benefits than previously assumed.
Highlights
The evolution of sexual reproduction remains a fascinating enigma in biology
By measuring the numbers of sexual strain coexisting with asexual strain among thrips, our survey revealed that the sexual strain showed female-biased sex ratios and that the relative frequency of sexual strain is negatively correlated with the proportion of males in the sexual strain
We confirmed that a femalebiased sex ratio evolves in sexual individuals due to the coexistence of asexual individuals
Summary
Our field survey showed that sex ratios were significantly biased toward females in many populations of the sexual strain (17 of the 32 populations, p < 0.05 based on binomial tests after Bonferroni corrections). Because phylogenetic relationships among populations often lead to statistically erroneous conclusions during correlation analysis[22], we removed the phylogenetic effect using an independent contrast method by creating a neighbor-joining tree of the populations based on genetic distances (FST) (see Method and Supplementary Fig. S1 for details). Even after this treatment, the coefficient was significantly negative (Fig. 1b; linear regression analysis without intercept; n = 16 independent pairs; F1,15 = 15.88, p = 0.001; Pearson’s product-moment correlation, r = −0.613, p = 0.012).
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