Spatial Patterns and Sex Ratios in Dioecious and Monoecious Mosses of the Genus Splachnum

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Abstract
Spatial autocorrelation analysis was applied to natural populations of monoecious Splachnum ampullaceum and dioecious S. sphaericum from Isle Royale, Michigan [USA]. Male and female gametophores of both species are highly clumped, resulting in locally biased sex ratios. In our study populations, ratios were typically biased 2:1 in favor of females. Directed dispersal of spores, which could produce such patterns, is rejected as an explanation for significant spatial autocorrelation because the unique spore dispersal mechanism of Splachnum ensures that both sexes will be dispersed to any given point. Moreover, in S. ampullaceum, protenema from single spores have the ability to produce either male or female shoots. In experimental cultures of S. ampullaceum, S. sphaericum, and S. rubrum, low light, pH, and nutrition favored production of males and females. Therefore, preferential production of male or female gametophores in appropriate microhabitats may be responsible for the clumping of the sexes observed in natural populations. Estimation of neighborhood sizes and areas in S. ampullaceum and S. sphaericum indicated that populations of both species were panmictic. Frequency distributions of minimum gamete dispersal distances were strongly leptokurtic and skewed right. Average minimum gamete dispersal distances in both species were very short, averaging less than 5 mm.