Highlights d Females without male mates evolved to produce increasing numbers of male flowers d Evolved females continued to sire progeny in competition with re-introduced males d Evolved females enjoyed reproductive assurance and an ability to outcross d Observed rates of evolution in the experiment were among the highest yet recorded

The suppression of recombination during sex-chromosome evolution is thought to be favoured by linkage between the sex-determining locus and sexually antagonistic loci, and leads to the degeneration of the chromosome restricted to the heterogametic sex. Despite substantial evidence for genetic degeneration at the sequence level, the phenotypic effects of the earliest stages of sex-chromosome evolution are poorly known.Here, we compare the morphology, viability and fertility between XY and YY individuals produced by crossing seed-producing males in the dioecious plant Mercurialis annua, which has young sex chromosomes with limited XÀY sequence divergence.We found no significant difference in viability or vegetative morphology between XY and YY males. However, electron microscopy revealed clear differences in pollen anatomy, and YY males were significantly poorer sires in competition with their XY counterparts. Our study suggests either that the X chromosome is required for full male fertility in M. annua, or that male fertility is sensitive to the dosage of relevant Y-linked genes.We discuss the possibility that the maintenance of male-fertility genes on the X chromosome might have been favoured in recent population expansions that selected for the ability of females to produce pollen in the absence of males.

Total word count: 5144 15 Number of figures: 2 16 Number of tables: 2 17 Supporting information: 1 18 2 Summary 19The suppression of recombination during sex-chromosome evolution is thought to be 20 favoured by linkage between the sex-determining locus and sexually-antagonistic loci, and 21 leads to the degeneration of the chromosome restricted to the heterogametic sex. Despite 22 substantial evidence for genetic degeneration at the sequence level, the phenotypic effects of 23 the earliest stages of sex-chromosome evolution are poorly known. Here, we compare the 24 morphology, viability and fertility between XY and YY individuals produced by crossing 25 seed-producing males in the dioecious plant Mercurialis annua L., which has young sex 26 chromosomes with limited X-Y sequence divergence. We found no significant difference in 27 viability or vegetative morphology between XY and YY males. However, electron 28 microscopy revealed clear differences in pollen anatomy, and YY males were significantly 29 poorer sires in competition with their XY counterparts. Our study suggests either that the X 30 chromosome is required for full male fertility in M. annua, or that male fertility is sensitive to 31 the dosage of relevant Y-linked genes. We discuss the possibility that the maintenance of 32 male-fertility genes on the X chromosome might have been favoured in recent population 33 expansions, which selected for the ability of females to produce pollen in the absence of 34 males. 35

Background and Aims Plant reproductive traits are widely understood to be responsive to the selective pressures exerted by pollinators, but there is also increasing evidence for an important role of antagonists such as herbivores in shaping these traits. Many dioecious species show leaky sex expression, with males and females occasionally producing flowers of the opposite sex. Here, we asked to what extent leakiness in sex expression in M. annua might also be plastically responsive to simulated herbivory. This is important because enhanced leakiness in dioecious populations could lead to a shift in both the mating system and in the conditions for transitions between combined and separate sexes. Methods We examined the effect of simulated herbivory on the sexual expression of males and females of Mercurialis annua (Euphorbiaceae) in two experiments in which different levels of simulated herbivory led to enhanced leakiness in both sexes. Key Results We showed that leaky sex expression in both males and females of the wind-pollinated dioecious herb Mercurialis annua is enhanced in response to simulated herbivory, increasing the probability and the degree of leakiness in both sexes. We also found that leakiness was greater in larger females but not in larger males. Conclusions We discuss hypotheses for a possible functional link between herbivory and leaky sex expression, and consider what simulated herbivory-induced leakiness might imply for the evolutionary ecology of plant reproductive systems, especially the breakdown of dioecy and the evolution of hermaphroditism.