Bryophyte population sex ratios are predominately female-biased, at least with respect to plants expressing sexual structures. One hypothesis to explain this bias is that males produce sexual structures less often than females, but occur at similar frequencies, a hypothesis termed the “shy male hypothesis.” Another nonexclusive possibility is that offspring sex ratios (as sporelings) are biased and populations retain this bias. To test these hypotheses, we examined sex ratios in expressing and nonexpressing shoots for the cosmopolitan moss Bryum argenteum collected in the field, and in shoots grown from spores in the lab. An examination of 154 collections of B. argenteum from native habitats and urban settings in the USA revealed that populations were significantly female-biased (>80% female). Male rarity was most pronounced in aridland regions of the Mojave Desert and California chaparral; males were significantly more common in altered urban habitats and in high elevation native habitats. When all shoots from clumps representing three mixed-sex, sporophytic populations were grown to sex expression, male nonexpressing shoots were not found to be significantly more abundant than expected based on the field expressing shoot sex ratio, lending little support to the “shy male hypothesis.” Offspring sex ratios derived from sporelings were not significantly different from 1∶1, thus not explaining the sharply female-biased population ratios observed in the field. We propose that factors between spore germination and adult maturation, including clonal dynamics, are causing the female-biased population and within-clump sex ratio imbalance of B. argenteum.
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Vol. 113 • No. 4