Microarthropod contributions to fitness variation in the common moss Ceratodon purpureus.

The evolution of sustained plant-animal interactions depends critically upon genetic variation in the fitness benefits from the interaction. Genetic analyses of such interactions are limited to a few model systems, in part because genetic variation may be absent or the interacting species may be experimentally intractable. Here, we examine the role of sperm-dispersing microarthropods in shaping reproduction and genetic variation in mosses. We established experimental mesocosms with known moss genotypes and inferred the parents of progeny from mesocosms with and without microarthropods, using a pooled sequencing approach. Moss reproductive rates increased fivefold in the presence of microarthropods, relative to control mesocosms. Furthermore, the presence of microarthropods increased the total number of reproducing moss genotypes, and changed the rank-order of fitness of male and female moss genotypes. Interestingly, the genotypes that reproduced most frequently did not produce sporophytes with the most spores, highlighting the challenge of defining fitness in mosses. These results demonstrate that microarthropods provide a fitness benefit for mosses, and highlight the potential for biotic dispersal agents to alter fitness among moss genotypes.

Forest passerines as a novel dispersal vector of viable bryophyte propagules.

Animal dispersal influences the community structure and diversity of a wide variety of plant taxa, yet the potential effects of animal dispersal in bryophytes (hornworts, liverworts, and mosses) is poorly understood. In many communities, birds use bryophyte-abundant niche space for foraging and gathering nest material, suggesting that birds may play a role in bryophyte dispersal. As highly motile animals with long migratory routes, birds potentially provide a means for both local and long-distance bryophyte dispersal in a manner that differs greatly from passive, aerial spore dispersal. To examine this phenomenon, we collected and germinated bryophyte propagules from the legs, feet and tails of 224 birds from 34 species within a temperate forest community. In total we found 1512 spores, and were able to germinate 242 bryophyte propagules. In addition, we provide evidence that topical (externally-carried) spore load varies by bird species and behaviour. Tail feather spore abundance is highest in bark and foliage gleaning species and is positively correlated with tarsal length. Together, these data suggest that a variety of forest birds exhibit the potential to act as dispersal vectors for bryophyte propagules, including an abundance of spores, and that understanding the effects of animal behaviour on bryophyte dispersal will be key to further understanding this interaction.

Rapid population sex-ratio changes in the moss Ceratodon purpureus.

PREMISE OF THE STUDY: Sex-ratio variation occurs widely in dioecious plants, but the mechanisms of population sex-ratio bias are poorly understood. In bryophytes, sex ratios are often female biased, and little information is available about how and when bias forms. METHODS: To test whether population sex-ratio variation can emerge during the gametophytic phase and is not purely a product of spore sex ratios, we created artificial populations of the moss Ceratodon purpureus, with male- and female-biased sex ratios, and placed half under a stress treatment. We hypothesized that male-majority populations would become female-biased and that stress would increase this transition. After 18 mo, when sporophytes were initially forming, we used sex-specific molecular markers to determine population sex ratios. KEY RESULTS: Female-majority populations did not differ significantly from their original bias, whereas male-majority populations became significantly more female biased. The plants had only just produced their first spores, so these sex-ratio changes occurred during the gametophytic generation, as a result of sex-specific growth or survival. Sporophytes occurred only in populations with female-biased final sex ratios, which suggests that females in male-majority populations may have invested energy in ramets rather than in sporophyte production. The stress treatment was mild and had no effect on sex ratio. CONCLUSIONS: Our results suggest that female bias can be generated during the gametophytic generation, before plants reach sexual maturity. These results, combined with those of previous work, suggest that both the gametophytic and the sporophytic stages drive population sex ratios in C. purpureus, thus indicating that multiple mechanisms operate to create biased population sex ratios.

Sex-specific volatile compounds influence microarthropod-mediated fertilization of moss.

Sexual reproduction in non-vascular plants requires unicellular free-motile sperm to travel from male to female reproductive structures across the terrestrial landscape. Recent data suggest that microarthropods can disperse sperm in mosses. However, little is known about the chemical communication, if any, that is involved in this interaction or the relative importance of microarthropod dispersal compared to abiotic dispersal agents in mosses. Here we show that tissues of the cosmopolitan moss Ceratodon purpureus emit complex volatile scents, similar in chemical diversity to those described in pollination mutualisms between flowering plants and insects, that the chemical composition of C. purpureus volatiles are sex-specific, and that moss-dwelling microarthropods are differentially attracted to these sex-specific moss volatile cues. Furthermore, using experimental microcosms, we show that microarthropods significantly increase moss fertilization rates, even in the presence of water spray, highlighting the important role of microarthropod dispersal in contributing to moss mating success. Taken together, our results indicate the presence of a scent-based 'plant-pollinator-like' relationship that has evolved between two of Earth's most ancient terrestrial lineages, mosses and microarthropods.

Sex-specific morphological and physiological differences in the moss Ceratodon purpureus (Dicranales).

Background and Aims: Dioecy and sexual dimorphism occur in many terrestrial plant species but are especially widespread among the bryophytes. Despite the prevalence of dioecy in non-vascular plants, surprisingly little is known about how fine-scale sex-specific cell and leaf morphological traits are correlated with sex-specific physiology and population sex ratios. Such data are critical to understanding the inter-relationship between sex-specific morphological and physiological characters and how their relationship influences population structure. In this study, these data types were assessed to determine how they vary across three populations within one moss species and whether fine-scale morphological traits scale up to physiological and sex ratio characteristics. Methods: Twenty cell-, leaf- and canopy-level traits and two photochemical measurements were compared between sexes and populations of the dioecious moss Ceratodon purpureus . Field population-expressed sex ratios were obtained for the same populations. Key Results: Male and female plants differed in cell, leaf and photochemical measures. These sexual dimorphisms were female biased, with females having larger and thicker leaves and greater values for chlorophyll fluorescence-based, leaf photochemistry measurements than males. Female traits were also more variable than male traits. Interestingly, field population sex ratios were significantly male biased in two study populations and female biased in the third study population. Conclusions: The results demonstrate that the larger morphology and the greater physiological output of female C. purpureus gametophytes compared with males occurs across populations and is likely to have significant effects on resource allocation and biotic interactions. However, this high level of dimorphism does not explain population sex ratio variation in the three study populations tested. This research lays the groundwork for future studies on how differential sex-specific variation in cell and leaf traits influences bryophyte plant fitness.

Passive warming reduces stress and shifts reproductive effort in the Antarctic moss, Polytrichastrum alpinum.

BACKGROUND AND AIMS: The Western Antarctic Peninsula is one of the most rapidly warming regions on Earth, and many biotic communities inhabiting this dynamic region are responding to these well-documented climatic shifts. Yet some of the most prevalent organisms of terrestrial Antarctica, the mosses, and their responses to warming have been relatively overlooked and understudied. In this research, the impacts of 6 years of passive warming were investigated using open top chambers (OTCs), on moss communities of Fildes Peninsula, King George Island, Antarctica. METHODS: The effects of experimental passive warming on the morphology, sexual reproductive effort and stress physiology of a common dioicous Antarctic moss, Polytrichastrum alpinum ,: were tested, gaining the first species-specific mechanistic insight into moss responses to warming in the Antarctic. Additionally community analyses were conducted examining the impact of warming on overall moss percentage cover and sporophyte production in intact Antarctic moss communities. KEY RESULTS: Our results show a generally greater percentage moss cover under warming conditions as well as increased gametangia production in P. alpinum Distinct morphological and physiological shifts in P. alpinum were found under passive warming compared with those without warming: warmed mosses reduced investment in cellular stress defences, but invested more towards primary productivity and gametangia development. CONCLUSIONS: Taken together, results from this study of mosses under passive warming imply that in ice-free moss-dominated regions, continued climate warming will probably have profound impacts on moss biology and colonization along the Western Antarctic Peninsula. Such findings highlight the fundamental role that mosses will play in influencing the terrestrialization of a warming Antarctica.

Moss antheridia are desiccation tolerant: Rehydration dynamics influence sperm release in Bryum argenteum.

PREMISE OF THE STUDY: Free-living sperm of mosses are known to be partially desiccation tolerant. We hypothesized that mature moss antheridia should also tolerate desiccation and that rehydration to partial turgor (prehydration) or rehydration to full turgor (rehydration) before immersion in water is required for full recovery from any damaging effects of prior desiccation. METHODS: Bryum argenteum (silvery-thread moss) was grown in continuous culture for several months, produced mature perigonia (clusters of antheridia), and these were subjected to a slow rate of drying (∼36 h from full turgor to desiccation) and equilibration with 50% relative humidity. Perigonia were prehydrated (exposed to a saturated atmosphere) or rehydrated (planted upright in saturated media) for 0, 45, 90, 135, 180, and 1440 min, then immersed in sterile water. Time to first sperm mass release, number of antheridia releasing sperm masses, and the integrity of the first sperm mass released were assessed. KEY RESULTS: Rehydration of dried antheridia for at least 3 h before immersion in water resulted in antheridia functioning similar to control undried antheridia. Compared with rehydration, prehydration was not effective in the recovery of antheridia from desiccation. CONCLUSIONS: For the first time, moss antheridia are shown to be fully desiccation tolerant at a functional level, capable of releasing fully functional sperm following a slow drying event provided the antheridia are allowed to rehydrate at least 3 h before immersion in water.

Distribution drivers and physiological responses in geothermal bryophyte communities.

PREMISE OF STUDY: Our ability to explain community structure rests on our ability to define the importance of ecological niches, including realized ecological niches, in shaping communities, but few studies of plant distributions have combined predictive models with physiological measures. METHODS: Using field surveys and statistical modeling, we predicted distribution drivers in geothermal bryophyte (moss) communities of Lassen Volcanic National Park (California, USA). In the laboratory, we used drying and rewetting experiments to test whether the strong species-specific effects of relative humidity on distributions predicted by the models were correlated with physiological characters. KEY RESULTS: We found that the three most common bryophytes in geothermal communities were significantly affected by three distinct distribution drivers: temperature, light, and relative humidity. Aulacomnium palustre, whose distribution is significantly affected by relative humidity according to our model, and which occurs in high-humidity sites, showed extreme signs of stress after drying and never recovered optimal values of PSII efficiency after rewetting. Campylopus introflexus, whose distribution is not affected by humidity according to our model, was able to maintain optimal values of PSII efficiency for 48 hr at 50% water loss and recovered optimal values of PSII efficiency after rewetting. CONCLUSIONS: Our results suggest that species-specific environmental stressors tightly constrain the ecological niches of geothermal bryophytes. Tests of tolerance to drying in two bryophyte species corresponded with model predictions of the comparative importance of relative humidity as distribution drivers for these species.

Tolerance to environmental desiccation in moss sperm.

• Sexual reproduction in mosses requires that sperm be released freely into the environment before finding and fertilizing a receptive female. After release from the male plant, moss sperm may experience a range of abiotic stresses; however, few data are available examining stress tolerance of moss sperm and whether there is genetic variation for stress tolerance in this important life stage. • Here, we investigated the effects of environmental desiccation and recovery on the sperm cells of three moss species (Bryum argenteum, Campylopus introflexus, and Ceratodon purpureus). • We found that a fraction of sperm cells were tolerant to environmental desiccation for extended periods (d) and that tolerance did not vary among species. We found that this tolerance occurs irrespective of ambient dehydration conditions, and that the addition of sucrose during dry-down improved cell recovery. Although we observed no interspecific variation, significant variation among individuals within species in sperm cell tolerance to environmental desiccation was observed, suggesting selection could potentially act on this basic reproductive trait. • The observation of desiccation-tolerant sperm in multiple moss species has important implications for understanding bryophyte reproduction, suggesting the presence of a significant, uncharacterized complexity in the ecology of moss mating systems.

Species-specific interactions in avian-bryophyte dispersal networks.

Studies from seed plants have shown that animal dispersal fundamentally alters the success of plant dispersal, shaping community composition through time. Our understanding of this phenomenon in spore plants is comparatively limited. Though little is known about species-specific dispersal relationships between passerine birds and bryophytes, birds are particularly attractive as a potential bryophyte dispersal vector given their highly vagile nature as well as their association with bryophytes when foraging and building nests. We captured birds in Gifford Pinchot National Forest to sample their legs and tails for bryophyte propagules. We found 24 bryophyte species across 34 bird species. We examined the level of interaction specificity: (i) within the overall network to assess community level patterns; and (ii) at the plant species level to determine the effect of bird behaviour on network structure. We found that avian-bryophyte associations are constrained within the network, with species-specific and foraging guild effects on the variety of bryophytes found on bird species. Our findings suggest that diffuse bird-bryophyte dispersal networks are likely to be common in habitats where birds readily encounter bryophytes and that further work aimed at understanding individual bird-bryophyte species relationships may prove valuable in determining nuance within this newly described dispersal mechanism.

Measuring volatile emissions from moss gametophytes: A review of methodologies and new applications.

Mosses inhabit nearly all terrestrial ecosystems and engage in important interactions with nitrogen-fixing microbes, sperm-dispersing arthropods, and other plants. It is hypothesized that these interactions could be mediated by biogenic volatile organic compounds (BVOCs). Moss BVOCs may play fundamental roles in influencing local ecologies, such as biosphere-atmosphere-hydrosphere communications, physiological and evolutionary dynamics, plant-microbe interactions, and gametophyte stress physiology. Further progress in quantifying the composition, magnitude, and variability of moss BVOC emissions, and their response to environmental drivers and metabolic requirements, is limited by methodological and analytical challenges. We review several sampling techniques with various analytical approaches and describe best practices in generating moss gametophyte BVOC measures. We emphasize the importance of characterizing the composition and magnitude of moss BVOC emissions across a variety of species to better inform and stimulate important cross-disciplinary studies. We conclude by highlighting how current methods could be employed, as well as best practices for choosing methodologies.

Inter-sexual competition in a dioecious grass.

Spatial segregation of the sexes (SSS) occurs in many dioecious angiosperms, but little data are available on the fitness advantages, if any, for males and females. We examined whether reciprocally transplanted male and female seedlings of Distichlis spicata, a dioecious grass species that exhibits extreme SSS, differed in their responses to microhabitats and competition treatments. Plants grown without conspecific competitors grew equally well in both male- or female-majority habitats, suggesting that male and female plants do not have differential resource needs at the juvenile life-history stage. However, plants subject to intra-sexual competition were significantly larger than plants subject to inter-sexual competition, suggesting that niche partitioning may occur in D. spicata.

Long-lived sperm in the geothermal bryophyte Pohlia nutans.

Non-vascular plants rely on sperm to cross the distance between male and female reproductive organs for fertilization and sexual reproduction to occur. The majority of non-vascular plants have separate sexes, and thus, this distance may be a few millimetres to many metres. Because sperm need water for transport, it has been assumed that sperm lifespans are short and that this type of sexual reproduction limits the expansion of non-vascular plants in terrestrial environments. However, little data is available on the lifespan of sperm in non-vascular plants, and none is available for bryophytes, the group thought to have first colonized terrestrial habitats. Here, we documented the lifespan of sperm of Pohlia nutans, collected from a geothermal spring's area, and tested the effects of variation under environmental conditions on this lifespan. Surprisingly, 20 per cent of the sperm were still motile after 100 h, and sperm lifespan was not significantly affected by temperature variation between 22 and 60 degrees C. Lifespan was significantly affected by sperm dilution and temperatures above 75 degrees C. These results suggest the need to reconsider the importance of sperm motility in bryophyte fertilization.

Species-specific effects of passive warming in an Antarctic moss system.

Polar systems are experiencing rapid climate change and the high sensitivity of these Arctic and Antarctic ecosystems make them especially vulnerable to accelerated ecological transformation. In Antarctica, warming results in a mosaic of ice-free terrestrial habitats dominated by a diverse assemblage of cryptogamic plants (i.e. mosses and lichens). Although these plants provide key habitat for a wide array of microorganisms and invertebrates, we have little understanding of the interaction between trophic levels in this terrestrial ecosystem and whether there are functional effects of plant species on higher trophic levels that may alter with warming. Here, we used open top chambers on Fildes Peninsula, King George Island, Antarctica, to examine the effects of passive warming and moss species on the abiotic environment and ultimately on higher trophic levels. For the dominant mosses, Polytrichastrum alpinum and Sanionia georgicouncinata, we found species-specific effects on the abiotic environment, including moss canopy temperature and soil moisture. In addition, we found distinct shifts in sexual expression in P. alpinum plants under warming compared to mosses without warming, and invertebrate communities in this moss species were strongly correlated with plant reproduction. Mosses under warming had substantially larger total invertebrate communities, and some invertebrate taxa were influenced differentially by moss species. However, warmed moss plants showed lower fungal biomass than control moss plants, and fungal biomass differed between moss species. Our results indicate that continued warming may impact the reproductive output of Antarctic moss species, potentially altering terrestrial ecosystems dynamics from the bottom up. Understanding these effects requires clarifying the foundational, mechanistic role that individual plant species play in mediating complex interactions in Antarctica's terrestrial food webs.

Density-dependent self-fertilization and male versus hermaphrodite siring success in an androdioecious plant.

Models of mating-system evolution emphasize the importance of frequency-dependent interactions among mating partners. It is also known that outcross siring success and the selfing rate in self-compatible hermaphrodites can be density dependent. Here, we use array experiments to show that the mating system (i.e., the outcrossing rate) and the siring success of morphs with divergent sex allocation strategies are both density dependent and frequency dependent in androdioecious populations of the wind-pollinated, annual plant Mercurialis annua. In particular, the outcrossing rate is a decreasing function of the mean interplant distance, regulated by a negative exponential pollen fall-off curve. Our results indicate that pollen dispersed from a male inflorescence are over 60% more likely to sire outcrossed progeny than equivalent pollen dispersed from hermaphrodites, likely due to the fact that males, but not hermaphrodites, disperse their pollen from erect inflorescence stalks. Because of this difference, and because males of M. annua produce much more pollen than hermaphrodites, the presence of males in the experimental arrays reduced both the selfing rate and the outcross siring success of hermaphrodites. We use our results to infer a density threshold below which males are unable to persist with hermaphrodites but above which they can invade hermaphroditic populations. We discuss our findings in the context of a metapopulation model, in which males can only persist in well-established populations but are excluded from small, sparse populations, for example, in the early stages of colonization.

Sexual systems and measures of occupancy and abundance in an annual plant: testing the metapopulation model.

The need for reproductive assurance during dispersal, along with the pressure of local mate competition, means that the importance of frequent or repeated colonization is implicit in the sexual-system evolution literature. However, to date there have been few empirical tests of the association between colonization and the sexual system in plants. Here we provide such a test by comparing occupancy and abundance of populations of the European plant Mercurialis annua across regions characterized by different sexual systems. Specifically, we predicted that monomorphic, hermaphroditic populations, which are thought to have evolved under selection for reproductive assurance during repeated bouts of colonization, would be smaller and their suitable habitat less frequently occupied than dimorphic populations, where males co-occur with either females or hermaphrodites. We show that both of these predictions are upheld. We evaluate our results against competing hypotheses for the occupancy-abundance relationship and conclude that they are most consistent with the metapopulation model for sexual-system variation in M. annua.

Life on the Edge-the Biology of Organisms Inhabiting Extreme Environments: An Introduction to the Symposium.

Life persists, even under extremely harsh conditions. While the existence of extremophiles is well known, the mechanisms by which these organisms evolve, perform basic metabolic functions, reproduce, and survive under extreme physical stress are often entirely unknown. Recent technological advances in terms of both sampling and studying extremophiles have yielded new insight into their evolution, physiology and behavior, from microbes and viruses to plants to eukaryotes. The goal of the "Life on the Edge-the Biology of Organisms Inhabiting Extreme Environments" symposium was to unite researchers from taxonomically and methodologically diverse backgrounds to highlight new advances in extremophile biology. Common themes and new insight that emerged from the symposium included the important role of symbiotic associations, the continued challenges associated with sampling and studying extremophiles and the important role these organisms play in terms of studying climate change. As we continue to explore our planet, especially in difficult to reach areas from the poles to the deep sea, we expect to continue to discover new and extreme circumstances under which life can persist.

New sequence-tagged site molecular markers for identification of sex in Distichlis spicata.

Sex-linked molecular markers have become valuable tools for understanding sex ratio evolution and sex-specific physiology in pre-reproductive plants. To develop new accurate methods for sexing Distichlis spicata juveniles and nonflowering individuals, we converted a random amplified polymorphic DNA-polymerase chain reaction marker that co-segregated with the female phenotype into a set of sequence-tagged site markers. We tested the marker pair on known males and females from populations in Oregon and California. A single band was obtained for all female samples but never for males.

Sex-specific variation in the interaction between Distichlis spicata (Poaceae) and mycorrhizal fungi.

Associations between mycorrhizal fungi and plants can influence intraspecific competition and shape plant population structure. While variation in plant genotypes is known to affect mycorrhizal colonization in crop systems, little is known about how genotypes affect colonization in natural plant populations or how plant sex might influence colonization with mycorrhizal fungi in plant species with dimorphic sexual systems. In this study, we analyzed mycorrhizal colonization in males and females of the wetland dioecious grass Distichlis spicata, which has spatially segregated sexes. Our results suggest that D. spicata males and females interact with mycorrhizal fungi differently. We discuss the implications for the role of this sex-specific symbiotic interaction in the maintenance of the within-population sex ratio bias of D. spicata.