Biased sex ratios among reproductive individuals are common in plants, but
the underlying mechanisms, as well as the evolutionary consequences, are
not well understood. The classical theory of Düsing and Fisher predicts an
equal primary sex ratio at seed production, based on the selective
advantage of the rare sex. Biased sex ratios among reproductive plants can
arise from sexual dimorphism in survival and flowering. Sex ratio biases
can also be present from the seed stage; in these cases, assumptions of
Düsing’s and Fisher’s theory, for example, random mating or demographic
equilibrium, are thought to be violated. We investigated mechanisms
leading to female-biased sex ratios in the arctic-alpine dwarf
willow Salix herbacea L. We studied sex ratios in three natural
populations over three years as well as in 29 crosses (full-sib families)
under controlled conditions over four growth periods. We tested whether
sex ratio was associated with habitat parameters (elevation and snowmelt
time), or with germination, survival or flowering, and whether females and
males differed in size or flowering that may cause observation bias. We
detected a strong and consistent female bias, both in natural populations
(sex ratio [proportion of females]: 0.71-0.82) and in our controlled
experiment (overall sex ratio: 0.70-0-72). Female bias became more
pronounced with increasing elevation. Our data did not support sexual
dimorphism in size or flowering. Family sex ratios varied largely (from
0.25 to 1), including many female-biased families, unbiased families and
two male-biased families. Families with lower germination, seedling
establishment, survival or flowering did not have stronger female bias,
indicating that intrinsically higher survival or flowering in females does
not explain overall female bias. Synthesis: Our results suggest that sex
ratio bias in S. herbacea is already present in seeds and does not arise
through intrinsic differences between sexes. Candidate mechanisms that can
lead to both overall female bias and variation in sex ratio among families
are meiotic drive or cyto-nuclear interactions. The pioneer habit of Salix
may lead to non-equilibrium population dynamics that allow for the
long-term persistence of variable genetic sex ratio distortion systems
that arise from genetic conflict.