D. Nicholas McLetchie

Sex expression, skewed sex ratios, and microhabitat distribution in the dioecious desert moss Syntrichia caninervis (Pottiaceae)

The moss Syntrichia caninervis is the dominant soil bryophyte in a blackbrush (Coleogyne ramosissima) community in the southern Nevada Mojave Desert, with a mean cover of 6.3%. A survey of the 10-ha study site revealed an expressed ramet sex ratio of 14♀ : 1♂ (N = 890), with 85% of ramets not expressing sex over their life span, and an expressed population sex ratio of 40♀ : 2♂ : 1♀♂ (female : male : mixed-sex, N = 89), with 52% of populations not expressing sex. A greater incidence of sex expression was associated with shaded microsites, higher soil moisture content, and taller ramets. Shaded microsites had higher surface soil moisture levels than exposed microsites. In the exposed microhabitat, surface soil moisture was positively correlated with ramet height but not with sex expression. Male ramets and populations were restricted to shaded microhabitats, whereas female ramets and populations were found in both shaded and exposed microhabitats, suggesting gender specialization. The rarity of mature sporophytes, found in 0% of the ramets sampled and in only 3% of the populations, is probably due to the rarity of mixed-sex populations. We hypothesize that mixed-sex populations are rare because of factors relating to male rarity and that the differential cost of sex expression reduces the clonal growth capacity of male individuals.

Sex Expression, Plant Size, and Spatial Segregation of the Sexes Across a Stress Gradient in the Desert Moss Syntrichia caninervis

Abstract Spatial variation in population sex ratios is common in bryophytes, but the ecological correlates of this variation are not well documented. The dioecious desert moss Syntrichia caninervis presents an example of this variation occurring at the regional scale (kilometers) as well as the microscale (centimeters). Here, our goal is to correlate variation in plant traits and sex expression levels to relevant ecological variation, and specifically to investigate whether spatial segregation of the sexes (SSS) exists at the level of the patch. Five sporophytic patches of S. caninervis were sampled at 10 cm intervals along a transect extending from shrub understory into the exposed intershrub region, thus representing a stress gradient of light intensity and moisture availability. Along a gradient from understory to intershrub microsites, individual plant length and biomass declined significantly, with plants under the canopy twice as large as those in the more exposed microsites. Sex expression declined from 76% under the shrub canopy to < 5% in the intershrub region, with the number of inflorescences per individual declining from 1.5 to near zero along this transect. Of the 778 individuals sampled, only 11 males were recovered, and all of these males occurred within 15 cm of the shrub canopy line. Nevertheless, the high number of female and nonexpressing individuals coupled with a rarity of male individuals resulted in no association between gender and microsite. However, using the distribution of fertilized female inflorescences as a surrogate measure of historical male presence indicated that males were significantly clustered near the shrub canopy line, indicative of spatial SSS. We hypothesize that males are less stress tolerant than females and that this condition may be an outgrowth of the sexes allocating differential resources to sexual reproduction.

Age and sex-specific rates of leaf regeneration in the Mojave Desert moss Syntrichia caninervis

The extremely skewed female-biased sex ratio in the desert moss Syntrichia caninervis was investigated by assessing the regeneration capacity of detached leaves. Juvenile, green, yellow-green, and brown leaves equating to approximately 0, 2, 6, and 12 yr of age, respectively, were detached from individuals of S. caninervis collected from 10 field populations and grown in a growth chamber for 58 d at a light intensity of 33-128 μmol · m(-2) · s(-1). Younger leaves (0-2 yr old) tended to have a greater viability, regenerate more quickly, extend their protonemal filaments farther, produce shoots (gametophores) more quickly, produce more shoots, and accumulate a greater biomass than older leaves (6 and 12 yr old). Among younger leaf classes, regenerating female leaves were more likely to produce a shoot than male leaves and produced more shoots than male leaves. The sexes did not differ significantly in time until protonemal emergence, linear extension of protonemata, or rate of biomass accumulation. However, protonemata of male leaves tended to emerge more quickly and produce a greater total biomass, ultimately consisting mostly of protonemata, than did female leaves. The more rapid proliferation of shoots by female leaf regenerants may help to explain the rarity of males in this species.

Sex ratios and the shy male hypothesis in the moss Bryum argenteum (Bryaceae)

Abstract 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.

Do the Sexes of the Desert Moss Syntrichia caninervis Differ in Desiccation Tolerance? A Leaf Regeneration Assay

Disparate sex ratios are a widespread pattern in dioecious bryophytes, with female‐biased ratios especially prevalent in arid environments. The absence of male plants in environments experiencing high desiccation pressure prompted the hypothesis that male plants may be less desiccation tolerant than female plants in the desert moss Syntrichia caninervis. This hypothesis was investigated by exposing detached leaves to consecutive wet/rapid‐dry treatments and monitoring viability, protonemal emergence time, shoot production, growth rate of secondary protonemata, and microbial infection frequency over a 56‐d period. The desiccation treatment consisted of exposure of mature 1‐yr‐old leaves to zero, two, four, and six wet/rapid‐dry cycles. Hydrated leaves were then allowed to regenerate. Desiccation stress level was significantly correlated to reduced protonemal emergence, reduced growth rates, and reduced shoot production. Female detached leaves produced protonemata more quickly, and these protonemata grew twice as rapidly and eventually produced more shoots than male detached leaves. Male leaves were also more subject to mortality and microbial infection, although these trends were not statistically significant. No \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Trade‐offs and Spatial Life‐History Strategies in Classical Metapopulations

\mathrm{sex}\,\times \mathrm{desiccation}\,

Microhabitat and Sex Distribution in Marchantia inflexa, a Dioicous Liverwort

\end{document} stress interactions occurred in the stress responses measured. The disparity in growth rates between female and male leaf regeneration, under both stressed and nonstressed conditions, may play a significant role in male rarity. We conclude (i) that the leaf regeneration assay works well as a response variable for desiccation tolerance (DT) studies and (ii) that sex‐based DT, at least with respect to responses to rapid drying cycles in the lab, while not indicated in Syntrichia, may yet operate under field conditions.

Plant size, sex expression and sexual reproduction along an elevation gradient in a desert moss

BACKGROUND AND AIMS Expected life history trade-offs associated with sex differences in reproductive investment are often undetected in seed plants, with the difficulty arising from logistical issues of conducting controlled experiments. By controlling genotype, age and resource status of individuals, a bryophyte was assessed for sex-specific and location-specific patterns of vegetative, asexual and sexual growth/reproduction across a regional scale. METHODS Twelve genotypes (six male, six female) of the dioecious bryophyte Bryum argenteum were subcultured to remove environmental effects, regenerated asexually to replicate each genotype 16 times, and grown over a period of 92 d. Plants were assessed for growth rates, asexual and sexual reproductive traits, and allocation to above- and below-ground regenerative biomass. KEY RESULTS The degree of sexual versus asexual reproductive investment appears to be under genetic control, with three distinct ecotypes found in this study. Protonemal growth rate was positively correlated with asexual reproduction and sexual reproduction, whereas asexual reproduction was negatively correlated (appeared to trade-off) with vegetative growth (shoot production). No sex-specific trade-offs were detected. Female sex-expressing shoots were longer than males, but the sexes did not differ in growth traits, asexual traits, sexual induction times, or above- and below-ground biomass. Males, however, had much higher rates of inflorescence production than females, which translated into a significantly higher (24x) prezygotic investment for males relative to females. CONCLUSIONS Evidence for three distinct ecotypes is presented for a bryophyte based on regeneration traits. Prior to zygote production, the sexes of this bryophyte did not differ in vegetative growth traits but significantly differed in reproductive investment, with the latter differences potentially implicated in the strongly biased female sex ratio. The disparity between males and females for prezygotic reproductive investment is the highest known for bryophytes.