The mission of the IUCN SSC BryophyteSpecialist Group is to promote the explorationof bryological diversity across all geographicscales and its long-term conservation.

Pleurozia gigantea has a widespread distribution in the tropics, occurring in Africa, south and east Asia, and the Pacific Ocean region.Within its distribution range the species occurs in geographically isolated areas of hyper-oceanic or montane climate. Although the number of mature individuals is not known, it is very likely much higher than the thresholds for a threatened listing under criteria C and D. Globally, the population is likely to be declining, but probably not at a rate to qualify the species for listing as threatened.

SYNTHESYS offers access to European Natural History museums.Twenty partner institutions from 12 European countries form SYNTHESYS, and offer coordinated access to their vast natural history collections and state-of-the art analytical facilities and qualified support from in-house researchers and curators. The institutions also co-operate in Network activities to improve the collections’ management, long-term preservation and accessibility. A Joint research activity develops tools to enhance the quality of and increase access to digitalized collections and data within Natural History institutions. SYNTHESYS was initiated by CETAF and receives funding by the European Union’s Research Framework Programs. It recently  secured EC funding for a third period (SYNTHESYS3 in FP7; Grant no. 312253), which started on 1st September 2013 and lasts for four years. Scientists in based in European member or associate states apply for grants covering research costs and subsistence at one or several partner institutions with a proposal that is assessed by a committee based on scientific quality.

Land-use changes including agricultural intensification are major drivers of biodiversity loss. As other countries, Switzerland introduced agri-environment schemes (AES) to counteract declining agro-biodiversity. Among numerous bryophytes of the agricultural landscape, arable bryophytes have adapted to recurrent tillage. They are short-lived above-ground, while their diaspores may build up a persistent diaspore bank in the soil. We monitored the field hornwort and the Carolina hornwort, characteristic species of the Central European arable bryophyte flora over a period of almost three decades, and studied effects of Biodiversity Promoting Areas (BPAS), a key element of Swiss AES, on arable bryophyte diversity. Hornwort occurrences strongly declined, and stubble-fields left unprocessed until late autumn, the favourite habitat of arable specialist bryophytes, nearly disappeared from 1991 to 2018. The loss of suitable habitats follows from decreased cereal cultivation and direct post-harvest management. Different BPA-fallows, designed to promote birds, flowering plants, or ecosystem services, harboured arable bryophyte specialists, but their richness decreased with fallow age. Diaspore banks contained arable bryophyte specialists not occurring above-ground and may serve as a reservoir during unsuitable cultivation. Targeted modifications of current AES, aiming to maintain habitats favourable for arable bryophytes, are necessary to safeguard the typical arable bryophytes in the Swiss arable landscape.

Ähnlich wie die Wildkräuter in Äckern sind auch die typischen, hochspezialisieren Ackermoose stark gefährdet und im Rückgang begriffen. Biodiversitätsförderflächen könnten dazu beitragen, ihren Rückgang zu stoppen. Entscheidend für den Erhalt der Ackermoose im Schweizer Kulturland ist jedoch, dass genügend und sorgfältig ausgewählte Getreidestoppelfelder nicht vor Ende Oktober gepflügt oder anderweitig bearbeitet werden. 

Background and aims Plants have evolved an unrivalled diversity of reproductive strategies, including variation in the degree of sexual versus clonal reproduction. This variation has important effects on the dynamics and genetic structure of populations. We examined the association between large-scale variation in reproductive patterns and intraspecific genetic diversity in two moss species where sex is manifested in the dominant haploid generation and sex expression is irregular. We predicted that in regions with more frequent realised sexual reproduction, populations should display less skewed sex ratios, more often express sex, and have higher genetic diversity than in regions with largely clonal reproduction.

Methods We assessed reproductive status and phenotypic sex in the dioicous long-lived Drepanocladus trifarius and D. turgescens, in 248 and 438 samples across two regions in Scandinavia with frequent or rare, respectively, realised sexual reproduction. In subsets of the samples, we analysed genetic diversity using nuclear and plastid sequence information and identified sex with a sex-specific molecular marker in non-reproductive samples.

Key results Contrary to our predictions, sex ratios did not differ between regions, genetic diversity did not differ in D. trifarius and was higher in the region with rare sexual reproduction in D. turgescens. Supporting our predictions, relatively more samples expressed sex in D. trifarius in the region with frequent sexual reproduction. Overall, samples were mostly female. Degree of sex expression and genetic diversity differed between sexes.

Conclusions Sex expression levels, regional sex ratios and genetic diversity were not directly associated with regional frequency of realised sexual reproduction, and relationships and variation patterns differed between species. We conclude that a combination of species-specific life histories such as longevity, overall degree of successful sexual reproduction and recruitment, and historic factors, are important to explain this variation. Our data on haploid-dominated plants significantly complement plant reproductive biology.

Background and Aims. Roughly half of the species of bryophytes have separate sexes (dioecious) and half are hermaphroditic (monoecious). This variation has major consequences for the ecology and evolution of the different species. In some sexually reproducing dioecious bryophytes, sex ratio has been shown to vary with environmental conditions. This study focuses on the dioecious wetland moss Drepanocladus trifarius, which rarely produces sexual branches or sporophytes and lacks apparent secondary sex characteristics, and examines whether genetic sexes exhibit different habitat preferences, i.e. whether sexual niche partitioning occurs.

Methods. A total of 277 shoots of D. trifarius were randomly sampled at 214 locations and 12 environmental factors were quantified at each site. Sex was assigned to the individual shoots collected in the natural environments, regardless of their reproductive status, using a specifically designed molecular marker associated with female sex.

Key Results. Male and female shoots did not differ in shoot biomass, the sexes were randomly distributed with respect to each other, and environmental conditions at male and female sampling locations did not differ. Collectively, this demonstrates a lack of sexual niche segregation. Adult genetic sex ratio was female-biased, with 28 females for every male individual.

Conclusions. The results show that although the sexes of D. trifarius did not differ with regard to annual growth, spatial distribution or habitat requirements, the genetic sex ratio as nevertheless significantly female-biased. This supports the notion that factors other than sex-related differences in reproductive costs and sexual dimorphism can also drive the evolution of biased sex ratios in plants

Patterns of sex expression and sex ratios are key features of the life histories of organisms. Bryophytes are the only haploid-dominant land plants. In contrast with seed plants, more than half of bryophyte species are dioecious, with rare sexual expression and sporophyte formation and a commonly female-biased sex ratio. We asked whether variation in sex expression, sex ratio and sporophyte frequency in ten dioecious pleurocarpous wetland mosses of two different families was best explained by assuming that character states  evolved: (1) in ancestors within the respective families or (2) at the species level as a response to recent habitat conditions. Lasso regression shrinkage identified relationships between family membership and sex ratio and sporophyte frequency, whereas environmental conditions were not correlated with any investigated reproductive trait. Sex ratio and sporophyte frequency were correlated with each other. Our results suggest that ancestry is more important than the current environment in explaining reproductive patterns at and above the species level in the studied wetland mosses, and that mechanisms controlling sex ratio and sporophyte frequency are phylogenetically conserved. Obviously, ancestry should be considered in the study of reproductive character state variation in plants.

The Species Survival Commission (SSC), a science-based network comprising over 9,000 volunteer experts deployed across more than 130 Specialist Groups, Red List Authorities and Task Forces, all working together towards achieving the vision of “A world that values and conserves present levels of biodiversity,” presents its annual report for 2014. 

We investigated whether the European near-endemic moss Drepanocladus lycopodioides has declined in its core distributionarea in the southern Baltic Sea region and explored potential explanations for this. First, we re-visited sites with documentedrecords from 1854 to 1957 in the Stockholm archipelago and nearby mainland in southern Sweden. Second, we comparedthe numbers and distributions of Swedish occurrences, their habitats, and substrates up to 1950 (289 herbarium records),and from 1951 onwards (347) using different map layers. Third, we explored whether inter-annual variation in reproductiveperformance was related to moisture conditions at five sites on an island in the Stockholm archipelago during nine years. Weestimated moisture conditions based on precipitation data and dry days during the time when fertilization occurs in the yearprior to sporophyte maturation. Drepanocladus lycopodioides has strongly decreased in intensively managed agriculturallandscapes but remains frequent in natural or semi-natural habitats on limestone. Sporophyte development varied betweenyears and depended on the weather condition during the fertilization period. Climate forecasts for the study area predict longerdry spells during late spring to late summer, which coincides with the timing of sexual branch initiation and fertilization inthe species. We expect that sporophyte production will likely decrease in the future, which will affect the species’ abilities tore-colonize spots where it vanishes during years with poor growth conditions.

We investigated whether the European near-endemic moss Drepanocladus lycopodioides has declined in its core distribution area in the southern Baltic Sea region and explored potential explanations for this. First, we re-visited sites with documented records from 1854 to 1957 in the Stockholm archipelago and nearby mainland in southern Sweden. Second, we compared the numbers and distributions of Swedish occurrences, their habitats, and substrates up to 1950 (289 herbarium records), and from 1951 onwards (347) using different map layers. Third, we explored whether inter-annual variation in reproductive performance was related to moisture conditions at five sites on an island in the Stockholm archipelago during nine years. We estimated moisture conditions based on precipitation data and dry days during the time when fertilization occurs in the year prior to sporophyte maturation. Drepanocladus lycopodioides has strongly decreased in intensively managed agricultural landscapes but remains frequent in natural or semi-natural habitats on limestone. Sporophyte development varied between years and depended on the weather condition during the fertilization period. Climate forecasts for the study area predict longer dry spells during late spring to late summer, which coincides with the timing of sexual branch initiation and fertilization in the species. We expect that sporophyte production will likely decrease in the future, which will affect the species’ abilities to re-colonize spots where it vanishes during years with poor growth conditions.

Sex ratio variation is a common but unexplained phenomenon of many species with chromosomal sex determination, including many bryophytes. Expressed sex ratio variation could be related to environmental conditions in a few mosses investigated to date. However, many bryophyte populations are non-fertile during their entire life cycle and intraspecific genetic sex ratio variation remains highly unexplored in natural populations. Drepanocladus lycopodioides, a pleurocarpous wetland moss with a distribution largely confined to Western Eurasia, rarely to occasionally forms sexual organs. It belongs to the majority of bryophytes that exhibits a female bias in expressed sex ratio. We applied a novel approach to sex individual shoots irrespective of their reproductive state using a specifically designed female-targeting molecular marker. We demonstrated that the bias in sex expression corresponds to a genetic female bias in the European adult population. Here, we investigated three regional populations of D. lycopodioides in its core distribution area. We asked whether haplotype diversity (H), sex expression (SE), genetic sex ratios, and sporophyte frequency varied within and among regions, whether these characteristics were related with each other, and / or to environmental parameters. Levels of H differed among regions and were positively related to habitat patch size. H was unequally partitioned between the sexes and was associated with estimated regional sporophyte frequency. Recorded plot-wise sporophyte frequency was generally very low in all regions. Overall genetic sex ratio was female-biased in all regions. Sex expression and genetic sex ratio varied strongly within regions (SE 0 –75%), with 27% of the plots lacking sex organs and 78% of the plots one-sexed, but differences among regions were non-significant. While no sex expression occurred in habitats deeper than 25cm, genetic sex ratio was not related to the measured environmental parameters.

En stor del av världspopulationen av grov gulmossa finns i Sverige. För att bevara arten behövs kunskap. Irene Bisang och Lars Hedenäs har studerat grov gulmossa under många år och de har hittat betydligt fler honor än hanar. Öppna och kalkrika miljöer måstebevaras för artens långsiktiga fortlevnad.

Premise of research. Maintenance of dioecious and monoecious sexual systems at nearly equal frequencies, infrequent sexual expression, and distinctly female-skewed sex ratios among the dioecious species are reproductive characteristics of bryophytes, which are otherwise unusual among embryophytes. Most sex ratio assessments to date have relied on gametophytes forming sexual organs, and how these reflect genetic genders is largely unresolved.

Methodology. For the European wetland moss Drepanocladus lycopodioides, we ask whether the adult expressed sex ratio is more strongly female biased than the “true” population sex ratio based on genetically male and female plants, i.e., whether males exhibit a lower sex expression rate than females (shy males). We assess expressed sex ratio on the basis of sex expression in individually scored herbarium specimens. We directly and on a large geographic scale assess nonexpressed sex ratio, for the second time in a moss, by sexing individual shoots from nonexpressing specimens using a molecular sex marker.

Pivotal results. On the basis of the female and male frequencies in these two data sets and the overall proportion of expressing specimens, we estimate the European population sex ratio as 2.6 : 1 (female to male). All three sex ratios are significantly female skewed and do not significantly differ from each other, indicating that there is no gender difference in sex expression rates.

Conclusions. These results and previous data for Drepanocladus trifarius show that males are not shy in the two wetland mosses of markedly different habitats.

The motile spermatozoids of bryophytes can swim up to a few centimetres. They require free water to fertilize the egg of the female organs, which sit on plants separate from the males in more than 50% of the species. When the sex organs are produced on different plants, this presents a serious obstacle to successful fertilization. The problems are overcome by a variety of mechanisms. Some of the more spectacular include spermatozoid transport up to at least several decimetres by water movement, by water drops spread from splash-cups surrounding the male organs up to two meters, by ejection up to15 centimetres into the air, or by having dwarf males that grow directly on the female plants. Here we report on another special kind of spermatozoid transfer that we came across during fieldwork in 2014, namely by micro-arthropods. Bryophyte fertilization mediated by animals was suggested more than a century ago, and was recently shown to occur in experimental settings. However, our observation is likely one of the first made directly in nature.

Sex ratio variation is common among organisms with separate sexes. In bryophytes, sex chromosome segregation at meiosis suggests a balanced progeny sex ratio. However, most bryophyte populations exhibit female-biased phenotypic sex ratios based on the presence of reproductive structures on gametophytes. Many bryophyte populations do not form sexual organs, and genotypic sex ratio variation in such populations is mostly unknown. We tested sex expression, and phenotypic and genotypic sex ratios against environmental parameters in natural populations of the unisexual wetland moss Drepanocladus lycopodiodes at 11 sites in each of three regions in southern Sweden. We identified sex in 660 individual ramets, based on sexual structures, when present, or with a specifically designed molecular marker, when absent. All regions exhibited a female bias in phenotypic and genotypic sex  atios. Sex ratio biases and sex expression differed between regions. Sex ratios were less male-biased in larger patches. Wetter patches exhibited a stronger female bias in genotypic sex ratio and lower sex expression. This is the first evidence of environmental effects on genotypic sex ratio in mosses. A higher frequency of females in wet patches could be due to higher female resource demands for sporophyte production or higher male sensitivity to wetness. A higher incidence of females than males in moister sites aligns with female flowering plants, but differs from reproductive bryophytes in drier environments. Taken together with previous results, our data indicate that sex ratio variation and its drivers differ among species, their life histories and environments.

A new find of the Critically Endangered arable specialist moss Ephemerum recurvifolium in Switzerland, new to Ct. Bern

Die Vorkommen von Hornmoosen in den

Äckern des Mittellandes haben seit den frühen 1990er Jahren stark abgenommen (Abb. 2). Die Feldbewirtschaftung, insbesondere das frühe Pflügen und die Bearbeitung von Stoppelfeldern im Herbst, erklären wesentlich die Abnahme der Hornmoose. Damit verschwinden die bevorzugten Standorte für Hornmoose und andere Ackermoos-Spezialisten. Die Einführung des ÖLN, unter anderem wegen der Bodenschutzauflage, die seit 2015 eine Einsaat zeitig nach der Ernte verlangt, hatte also keinen positiven Einfluss auf die Hornmoosvorkommen. Eine hohe Luftfeuchtigkeit im Sommer fördert andererseits die Hornmoosentwicklung, aber dieser Witterungseffekt ist deutlich geringer als der Bewirtschaftungseffekt.

Ähnlich wie die Wildkräuter in Äckern sind auch die typischen, hoch-spezialisieren Ackermoose gefährdet und im Rückgang begriffen. Im Schweizer Mittelland sind ihre bevorzugten Lebensräume, bis in den Spätherbst unbearbeitete Getreidestoppelfelder, fast vollständig verschwunden. Biodiversitätsförderflächen (BFF) können dazu beitragen, ihren Rückgang zu stoppen, allerdings nur so lange nicht die dichte Vegetationsbedeckung und Streuauflagen in den älteren BFF die Ackermoose verdrängen. Entscheidend für den Erhalt der Ackermoose im Schweizer Kulturland ist deshalb die Förderung von kurzlebigen BFF und vor allem von Getreidestoppelfeldern, in denen die Bearbeitungszeitpunkte sorgfältig angepasst sind.

We asked how arable management and the introduction of AES-measures affect the specialised bryophyte flora of arable land, whether these specialist species do occur in existing BPAs, and how weather conditions affect the development of arable bryophyte populations. 

We  propose tailored amendments of certain directives of the Swiss Agri-environmental Schemes (AES) to safeguard the characteristic arable specialist bryophytes in the Swiss arable landscape. 

Agricultural intensification represents one of the major drivers for the dramatic loss of biodiversity worldwide. To halt the decline of farmland biodiversity, Switzerland adopted agri-environment schemes (AES) in 1998. Here, we monitored the occurrence, abundance and habitats of two species of arable bryophyte specialists, the Field hornwort (Anthoceros agrestis) and the Carolina hornwort (Phaeoceros carolinianus), in 28 crop fields in the intensively cultivated Swiss Plateau from 1991–2018, to investigate the effects of arable management, AES directives, and weather conditions on their performance. The target species are characteristic of the specialized short-lived arable bryophyte flora of Central Europe that depend on bare substrate in low-intensively cultivated and regularly ploughed fields. Trends in their occurrence thus reflect in many respects the status of the arable bryophytes in cultivated fields. Hornwort occurrence significantly declined between 1991 and 2018. A strong decrease in stubble fields that remain unmanaged after harvest, the favourite habitat for many arable specialists in the study area, largely accounted for the decline. Stubble fields nearly disappeared in the study area because of a gradual reduction in the cultivation area of cereals and the increasing practice of immediate post-harvest tillage. The latter is common in intensive arable farming and was accentuated by AES directives amended in 2005. Hornwort occurrences were positively affected by high air humidity during summer, but weather effects were subordinate to management effects. We propose tailored amendments of AES regulations, that aremented at selected sites, to maintain the characteristic arable specialist bryophytes in the Swiss Plateau: crop rotation with adequate proportions of cereals that are regularly ploughed but not before the end of October, no post-harvest processing of stubble fields, and optimization of the existing instrument ‘Biodiversity Promoting Areas’, e.g., short-term fallows in crop fields. Late-autumn or overwintering stubbles and short-term fallows will benefit many other organisms that depend on extensively managed open habitats, for example arableflowers, farmland breeding birds and specialized arthropods

Red List Category & Criteria:

Endangered B2ab(iii) ver 3.1

Implications of long-term monitoring of arable bryophytes for the conservation of these species in Switzerland

Mission statement: The mission of the IUCN SSC Bryophyte Specialist Group is to promote the exploration of bryological diversity across all geographic scales and its long-term  onservation.

Mission statement: The mission of the IUCN SSC Bryophyte Specialist Group is to promote the exploration of bryological diversity across all geographic scales and its long-term  onservation.

The mission of the IUCN SSC BryophyteSpecialist Group (BSG) is to promote theexploration of bryological diversity acrossall geographic scales and its long-termconservation.

The mission of the IUCN SSC BryophyteSpecialist Group (BSG) is to promote theexploration of bryological diversity acrossall geographic scales and its long-termconservation

The mission of the IUCN SSC BryophyteSpecialist Group (BSG) is to promote the exploration of bryological diversity acrossall geographic scales and its long-term conservation.

Introduction: Codonoblepharon forsteri is a rare epiphytic moss characteristically associated with water-filled holes in trees. This study aims to review its range and population and assess effects of climate change.

Methods: An inventory of sites from where C. forsteri has been recorded was compiled. Extent of Occurrence (EOO) and Area of Occupancy (AOO) were calculated. Population size was estimated, using an occupied tree as an ‘individual-equivalent’ of the moss. Climatic conditions of its current distribution were characterised, and an ensemble model of its distribution generated. The latter was projected onto present and future climatic layers.

Results: C. forsteri has been recorded from 205 sites in 18 countries, in Europe, N Africa and SW Asia. It has been undergoing an overall decline. Most sites have few occupied trees, and a world population of 1000–10,000 individual-equivalents is estimated. Model projections suggest the species will experience a range increase of +0.27–0.78 by 2050 and +0.34–0.97 by 2070, especially in the NW, in particular across France and the UK. Range loss is predicted to be -0.16–0.23 in 2050 and -0.18–0.32 in 2070, affecting the driest areas of the current range around the Mediterranean, especially in N Africa.

Conclusions: C. forsteri has a relatively large EOO but a relatively small AOO, likely the product of its habitat specialism. A major reason for recent declines appears to be widespread abandonment of traditional ‘pollarding’ of trees. The potential climatic range of the species will shift significantly northwards over the next few decades

The hornwort Anthoceros agrestis is emerging as a model system for the study of symbiotic interactions and carbon fixation processes. It is an annual species with a remarkably small and compact genome. Single accessions of the plant have been shown to be related to the cosmopolitan perennial hornwort Anthoceros punctatus. We provide the first detailed insight into the evolutionary history of the two species. Due to the rather conserved nature of organellar loci, we sequenced multiple accessions in the Anthoceros agrestis–A. punctatus complex using three nuclear regions: the ribosomal spacer ITS2, and exon and intron regions from the single-copy coding genes rbcS and phytochrome. We used phylogenetic and dating analyses to uncover the relationships between these two taxa. Our analyses resolve a lineage of genetically near-uniform European A. agrestis accessions and two non-European A. agrestis lineages. In addition, the cosmopolitan species Anthoceros punctatus forms two lineages, one of mostly European accessions, and another from India. All studied European A. agrestis accessions have a single origin, radiated relatively recently (less than 1 million years ago), and are currently strictly associated with agroecosystem habitats.

Background and AimsWe examined the relationship between reproductive allocation and vegetative growth in three monoicous sexual systems of bryophytes. The sexual systems show a gradient of increasing distance between the sexes, from gonioautoicous to cladautoicous to rhizautoicous. Here, we investigated the following two hypotheses: (1) reproductive allocation differs between sexes and sexual systems, and male reproductive allocation increases with increasing distance between male and female gametangia; and (2) reproductive allocation is negatively related to vegetative growth.MethodsWe sampled the three sexual systems, represented by three moss species of the genus Fissidens in the Atlantic Forest of Southeastern Brazil. Ramets were washed in the laboratory; the reproductive structures were detached from the vegetative ramets and sorted regarding sex and individual, dried at 70 °C for 72 h, and weighed in an ultramicrobalance. We calculated the mean reproductive and vegetative mass and reproductive allocation and used generalized linear models to test our predictions.Key ResultsReproductive allocation differed between species and sexes. It was higher in the rhizautoicous than in the cladautoicous and gonioautoicous species. Mean reproductive allocation was greater in males than in females of the rhizautoicous species, greater in females than males of the cladautoicous species, and did not differ between the sexes in the gonioautoicous species. Estimates of reproductive and vegetative mass were positively related in females of the rhizautoicous species. Vegetative mass was not related to reproductive allocation in the gonioautoicous species, but negatively related to reproductive allocation in the male and female branchlets of the cladautoicous species and in the female ramets of the rhizautoicous species.ConclusionsThe reproductive allocation patterns differ between the rhizautoicous species and the ‘truly’ monoicous species, with shorter intersexual distances, which implies that our hypotheses were supported only in part. We suggest that the hypotheses should be reformulated and tested further by comparing ‘truly’ monoicous species with dioicous species and by including other genera.

Many species exhibit a strongly female-biased phenotypically expressed sex ratio that likewise is in some cases genetic and in others possibly due to varying responses to environmental conditions. The "shy male" hypothesis lacks support in explaining most of this female bias. Examples of distinct male bias in expressed sex ratios also exist. Sex ratios based on genetic information on non-expressing plants is known for a very limited number of species.  Some species, perhaps more than we realize, have sexual plasticity. That is, they have different gender expressions in different years, possibly dependent on age or available energy resources. This can be due to hormonal expressions of the same or neighboring plants. When sexual reproduction fails, asexual reproduction by specialized propagules can compensate, and this is especially true for dioicous species. Because of the energy cost of producing sporophytes, males might have the energy needed for producing asexual structures. In addition, clonal growth and fragmentation can help the species spread. A modeling study suggests that disturbance level (weather, pollution, fire, etc) affects genders differentially, hence maintaining both sexes in the long term. Epiphytes are frequently isolated on a tree with only one sex present. They can benefit from asexual reproduction and have a higher than average percent of propguliferous taxa.

Males and females can differ in non-sexual ways, including size, biomass, branching, maturation rate, chlorophyll content, and photosynthetic rate. Large female and small male plants (dwarf males) are known among bryophytes, but not the converse, except in non-sporophytic Diphyscium. Most dwarf males develop on the leaves or tomentum of females of the species. Dwarf males are often missed in surveys and this omission can cause misleading results in sex ratio determination. Spores of some species develop dwarf males on females of the species but normal males on other substrates. Dwarfism can increase the success of fertilization while decreasing the competition for resources with the females.

Bryophytes are isosporous, but some species exhibit anisospory; some exhibit false anisospory due to abortion of spores. The anisosporous condition seems to present a potential advantage for fertilization when it is correlated with the presence of dwarf males. On the other hand, this strategy reduces the dispersal of the larger female spores compared to that of the smaller male spores. This is less of a problem if nearly all females get fertilized. Many anisosporous and false anisosporous conditions occur in species with no dwarf males (Mogensen 1981). This causes us to seek other explanations for their presence, including abortion related to water, space, and nutrient limitations within the capsule. The abortions can provide room for remaining developing spores while maintaining protection and resources for them.

The liverwort genus Sphaerocarpos was the first genus in which sex chromosomes were known in plants. Many bryophytes possess sex chromosomes (X & Y chromosomes, or designated U & V to refer to their haploid condition) which may play a role in gender determination. Bryophytes can be monoicous (bisexual) or dioicous (unisexual). Gametangia in monoicous bryophytes can be autoicous (♂ & ♀ gametangia in separate clusters), paroicous (♂ & ♀ gametangia in separate groupings but one cluster), or synoicous (♂ & ♀ gametangia intermixed in same cluster). Monoicy frequently has arisen through hybridization and polyploidy. Transitions from monoicy to dioicy and vice versa have happened multiple times. There have been more changes from monoicy to dioicy than the reverse in mosses, whereas the opposite was the case in hornworts. McDaniel et al. suggested that dioicy works best when there are advantages to both sexes for being separate. Sperm dispersal begins with bursting of the antheridium, often accompanied by movement with surface tension of water drops. In thallose liverworts, sperm are often expelled explosively into the air. Sperm dispersal is usually accomplished by movement through a water film or by splashing and is sometimes aided by gravity. But some species have their sperm dispersed by invertebrates, including insects and mites. Dispersing sperm are known to survive as much as 200 hours and travel distance is known up to 230 cm. Travel distance and weather seem to be the most important factors in determining the success of fertilization in bryophytes

Monoicy (both sexes on same individual) frequently has arisen through hybridization and polyploidy (multiple sets of chromosomes). Barriers to hybridization and to selfing in bryophytes are poorly known. These include external barriers such as spatial/geographic isolation, ecological isolation, and seasonal isolation. Internal barriers include gametic isolation, genetic incompatibility, hybrid sterility, and reduced fitness. Nevertheless, hybridization seems to have played a major role in the evolution of monoicy due to lack of these barriers in many species. Formation of gametangia and especially sporophyte formation incur reproductive costs measurable in reduced future vegetative and reproductive performance. Overall investment in sexual reproduction may vary among species, in some cases being greater in males and in others greater in females, depending on if assessed at the pre- or postfertilization stage.

Tradeoffs occur between dispersal ability of small spores and success of establishment of large spores. Fragments and vegetative diaspores are most successful at colonizing over short distances and are more likely to succeed than spores. Asexual reproduction can keep the species going for many years in the absence of sexual reproduction. Tradeoffs occur also among asexual reproduction, sexual reproduction, and vegetative performance. These tradeoffs vary among species.

The dominant haploid state of bryophytes limits their ability to store recessive alleles, but autopolyploidy, somatic mutations, vegetative reproduction, and independent assortment at meiosis contribute to diversity. Despite their clonal nature, bryophytes still exhibit considerable genetic variation.

The dominant haploid state of bryophytes limits their ability to store recessive alleles, but autopolyploidy, somatic mutations, vegetative reproduction, and independent assortment at meiosis contribute to diversity.  Despite their clonal nature, bryophytes still exhibit considerable genetic variation.  This may be explained in part by the Red Queen hypothesis, a hypothesis that also might explain the persistence of evolution to a dioicous condition despite the difficulty of accomplishing sexual reproduction.  Inbreeding depression may occur in monoicous bryophytes, but very limited data suggest that may be to a limited degree compared to that of tracheophytes.

Bryophytes may lack the morphological diversity expressed by sporophytes in higher plants, but there is evidence that haploid plants and their diaspores can contain as much diversity as tracheophytes, often expressed in their biochemistry as a variety of secondary compounds rather than in morphology.  They have life strategies that have survived since the beginning of land plants.