The relative allocation within a plant species of asexual vs. sexual reproduction is predicted to vary according to changing environmental conditions. In bryophytes, sex expression is predicted to be reduced in unfavourable environments because sporophyte maturation is resource‒limited. The allocation between reproductive strategies in the bryophyte, Polytrichum juniperinum was examined in the low‒arctic Torngat Mountains (Labrador, Canada) in 2008/2009 along both an elevation and moisture gradient. Using genetic markers, no pattern was observed in the allocation between sexual and asexual reproductive strategies among the different environmental conditions. Although all populations showed evidence of sexual reproduction, effective sexual reproduction was depressed along both gradients. All populations showed high rates of fertilization among closely related ramets that resulted in inbreeding. This might limit the capacity to maintain high genetic diversity in local populations, which would be favoured in an environment experiencing warming.
Article
Allocation to sexual and asexual reproduction was uniform in spite of environmental gradients in case of the moss Polytrichum juniperinum in the low arctic
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population genetics
enzyme polymorphism
reproduction ecology
dry vs. wet habitats
Numerous plants reproduce through both sexual and asexual mechanisms including bryophyte species. Despite the difference in the genetic composition of the offspring, the evolution and ecological consequences of the two reproductive strategies are little understood (Templeton et al. 1995; Dorken and Eckert 2001; Eckert 2003; Yufen and Dayong 2007; Silvertown 2008; Johnson and Shaw 2016; Maruo and Imura 2020). The relative allocation to each type of reproductive strategy is thought to be shaped by evolutionary (Maynard 1978; Holsinger 2000; Ronsheim and Bever 2000), ecological (Loveless and Hamrick 1984), environmental (Lehmann 1997; Price and Marshall 1999; Fernández-Martínez et al 2021), biogeographical (Cronberg 2000; Dorken and Eckert 2001), population structural (Loehle 1987) and a mix of these and other factors (During 1979; Eckert 2002; Yufen and Dayong 2007). In theory, sexual reproduction produces high genetic variability, which is favored in changing environments, whereas populations with asexual reproduction show little genetic variability, which is favored in static environments to preserve adapted, successful genetic composition (Hoffman 1985; Widén et al. 1994; Yufen and Dayong 2007; Niklas and Cobb 2017).
Environmental gradients provide a useful framework for studying the effort and allocation between reproductive strategies because the resource requirements differ for the different reproductive strategies and thus environmental variation might limit reproductive success (Skotnicki et al. 2000; Eckert 2002; Petrů 2006; Maruo and Imura 2020). Within a plant species, variation in resource allocation between reproductive strategies is less widely studied than among plant species (Gonzalez‒Mancebo 1996; Groenendael et al. 1996; Eckert 2002 Austerheim et al. 2005). For species having mixed strategies, populations at the edge of their geographic range and at their northern limit tend to reproduce asexually (and grow clonally) more frequently than sexually (Hermanutz el al. 1989; Eckert 2002). Population structural characteristics are also known to affect allocation of resources to sexual vs. asexual reproduction and include sperm dispersal distances (Glime 2007), plant size (Sun et al. 2001), ramet density (competition; Sun et al. 2001) and successional status (Sun et al. 2001). In general, asexual reproduction may occur more frequently within species as sexual reproduction fails (Silvertown 2008). Sexual reproduction fails in bryophytes mainly because sporophyte maturation is resource‒limited (Stark et al. 2009), and sporophyte production incurs a great energy cost (Bisang and Ehrlén 2002; Maruo and Imura 2020).
For bryophyte species both population density and sex ratio can influence the allocation between sexual vs. asexual reproduction. Denser populations as compared to sparser ones, for example, have a higher allocation to sexual than to asexual reproduction (Wyatt and Derda 1997; Cronberg et al. 2006) and, populations having a 1:1 sex ratio have a higher probability for mating (sexual reproduction) and sporophyte production compared to those having an unequal sex ratio (Longton and Greene 1967; Convey and Smith 1993; Wyatt and Derda 1997; Cronberg et al. 2006; Bisang et al. 2017).
The degree of sexual reproduction in moss species can be studied by exploring variation among populations in the viability and germination capability of their spores. Populations with high spore viability would suggest effective recruitment via sexual production (Longton 1988). The percent viability and germination of P. juniperinum spores can be high, as has been found under laboratory conditions in which 100% of spores per capsule germinated (Paolillo and Kass 1973). However, recruitment from spores in local populations can be limited (Innes 1990; Convey and Smith 1993). Recruitment from spores has been found to be rather important in long distance (even intercontinental) dispersal in establishing new populations (Van Zanten and Pócs 1981; Longton 1988), whereas asexual reproduction is thought to play a major role in colony expansion and maintenance (Longton 2006).
The soil diaspore bank, which consists of both local and long‒distance dispersed fragments, rhizoids, protonemata and spores, also plays an important role in the realization of sexual and asexual reproduction (During 1997). There are no specialized vegetative propagules in Polytrichum species, but any part of the plant can potentially regenerate into a new plant (Crum and Anderson 1981). Fragments could play an important role in long distance dispersal due to trans‒location by water, wind, and animals, however, establishment by fragmented gametophytes is thought to be rare and appears to be restricted to only a few kilometres (Wyatt and Derda 1997).
Examining genetic polymorphism in mosses is a key to understand their reproductive behaviour (Cronberg et al. 2006; Van der Velde and Bijlsma 2000; Van der Velde et al. 2001a). Within species, reproductive behaviour and genetic diversity in mosses can be influenced by large to small‒scale ecological factors such as latitude (Cronberg 2000; Longton 1988; Smith and Convey 2002; Stark 2002), elevation (Van der Velde and Bijlsma 2003), biogeography (Van der Velde and Bijlsma 2003), habitat structure/resource availability (Stark et al. 2005), and population structure (Innes 1990; Wyatt and Derda 1997). Populations with frequent sexual reproduction show high genotypic diversity, high allele frequency, a high number of polymorphic loci and multi‒locus genotypes compared to populations established primarily through asexual reproduction (Innes 1990; Van der Velde and Bijlsma 2000; Van der Velde and Bijlsma 2001; Cronberg et al. 2006). Populations with similar observed and expected genotypic diversity and a similar observed and expected number of multi‒locus genotypes are considered to be sexually reproducing populations with a high rate of recruitment from spores (Innes 1990).
A moss population consists of ramets (clonally produced shoot) and genets (shoots originating from a single sexually produced zygote) (Frey and Kürschner 2011). Sexual reproduction can occur among different genets or through inter‒gametophytic self fertilization by mating among males and females derived from the same sporophyte (Soltis et al. 1988; Eppley 2007). The latter lowers the genotypic diversity in contrast to populations in which mating among related ramets is less frequent (Eppley et al. 2007). Within a population, many sexually related genets of limited size suggest that recruitment is more a consequence of sexual than asexual reproduction whereas, a few genets that are large and widespread in the population would imply that vegetative reproduction predominates (Cronberg et al. 2006). This can be confirmed by exploring the genetic diversity of populations: high genetic diversity within populations suggests that sexual reproduction predominates (e.g., P. formosum, Van der Velde et al. 2001b) whereas low genetic diversity suggests frequent asexual reproduction (e.g., Mannia fragrans, Hock et al. 2008a).
Little research has investigated the effect of habitat on the genetic structure of Polytrichum species (Derda and Wyatt 1999). One study showed no relationship between habitat differentiation (from mature forest to pioneer habitats) and genetic differentiation among populations of P. commune (Derda and Wyatt 1999). However, habitat fragmentation due to disturbance reduces the genetic diversity of this species (Wilson and Provan 2003). In these studies, only the level of genetic diversity was assessed with no specific discussion regarding reproductive behaviour and allocation. The relationship between habitat and reproductive behaviour has only been studied in a few moss species. In the desert moss, Syntrichia caninervis, sexual expression declined with increasing light intensity and decreasing humidity (Stark et al. 2005). The moss Bryum dunense produced more sporophytes in partially shaded than in bright microhabitats; however, regeneration by protonemata was also relatively more frequent in shaded than in bright environments (Herrnstadt and Kidron 2005). Also, sex expression has been shown to be reduced in dioicous species in dryer conditions (Stark 2002). In Pterygoneurum ovatum sporophyte maturation is resource‒limited and clonal regeneration less pervasive as plants invest in sexual reproduction (Stark 2002). Under warming conditions more sporophytes were produced compared to control sites in Antarctic mosses, altering moss population genetics and dispersal patterns (Casanova-Katny et al. 2016). Similarly, passive warming reduced stress and shifted reproductive effort (increased gametangia production) in the Antarctic moss, Polytrichastrum alpinum (Shortlidge et al. 2017). Environmental differences shaped genetic structure and level of inbreeding as Sphagnum species preferring hummocks had populations with lower genetic diversity and higher inbreeding coefficients than species inhabiting hollows (Johnson and Shaw 2015).
Our goal in this study was to observe the link between spatial genetic structure and sexual vs. asexual reproductive allocation of the moss Polytrichum juniperinum (Hedw.) to identify whether sexual vs. asexual recruitment and allocation to sexual reproduction varies along an elevation and/or moisture gradient. P. juniperinum is a widely distributed and abundant species across temperate, boreal and low arctic environments (Crum and Anderson 1981; Brassard 1983). Information on how allocation between sexual and asexual reproduction in P. juniperinum varies along different environmental gradients will provide increased insight as how plant reproductive strategies are influenced by ecological factors. In addition, the survival and distribution of moss populations in an environment experiencing warming (IPCC 2007) may depend on recent population structure, genetic diversity and gene flow among populations i.e. on reproductive strategies (Pertoldi and Bach 2007).
2.1 Site description
The study area was in Torngat Mountains National Park (Canada, Labrador, near kANGIDLUASUk) (Fig. 1; N58 38.402 W63 21.150). The Park is located in the low arctic bio‒geographic region and it is characterized by coastal, mountainous tundra vegetation dominated by grasses, low‒growing ericaceous plants, bryophytes and lichens (Chan et al. 2008). The first record of bryophyte community diversities in contrasting habitats of the Torngat Mountains National Park was published in 2018 (Koncz et al. 2018).
2.2 Population structure, spores, diaspore bank sampling
Polytrichum juniperinum populations for genetic analysis were sampled from 26‒29 July 2009 in 20 × 20 cm quadrats in three 50 × 50 m sites along an elevation and a moisture gradient: low (75 m, dwarf birch/heath vegetation), mid (409 m), and high (805 m, arctic‒alpine tundra vegetation) elevation (Fig. 1). The site at the high elevation consisted of two sub‒sites due to distinct, small populations. The moisture gradient (dry, mesic and wet habitats (Fig. 1)) was only examined at the mid-elevation site because suitable moisture gradients did not exist at the low and high sites. The ‘dry’ site was dominated by mosses and lichens (Cladonia spp.) with abundant bare ground, and relatively dry soil (open tundra vegetation). The ‘mesic’ site, located between the dry and wet habitat, it was dominated by sedges (Carex spp.), ericaceous plants, P. juniperinum, and grasses (heath vegetation). In the ‘wet’ site, grasses and Sphagnum spp. dominated the water-saturated soil.
Percentage ground cover of P. juniperinum was estimated for each quadrat for male, female and vegetative shoots. Sex ratio was calculated based on the female/male surface cover. Viability of spores sampled from the sporophytes from all sites was tested with Alexander stain (Alexander 1980). A binominal logistic regression (G‒statistics) was used to compare spore viability among sites.
Germination success of spores was measured by cultivating spores from sporophytes from all sites (Paolillo and Kass 1973; Karunen and Ihantola 1977). Spores were cultivated by suspending spores in water diluted with 5ml Knopp media (Reski and Abel 1985). This was then layered on sterile perlite in transparent plastic petri dishes. The Petri dishes were then placed in a growth chamber (day‒night light condition, 1000μmol PAR m‒2 s‒1, 10 to 20◦C, 90% humidity, Versatile Environmental Chamber, SANYO, MLR‒351). Controls without spore suspension were used to detect the possible presence of air‒borne spores. After two weeks, germinating spores (protonema) was counted under a compound light microscope (400×). Germination among populations was compared using binominal logistic regression (G‒statistics).
Soil diaspore bank was investigated by cultivating six soil samples per sites in a separate growth chamber (Versatile Environmental Chamber, SANYO, MLR‒351; day‒night light condition, ca. 1000 μ mol PAR m‒2 s‒1, 10 to 20◦C, 90% humidity) until identification of species became possible (Jonsson 1993; During 1997; During 2001; Hock et al. 2004). For controls, two soil samples per population were autoclaved and placed into the same growth chamber.
2.3 Population genetic analysis
The allozyme variability of the different populations was investigated by examining their enzyme polymorphisms (Innes 1990). At each site and at each quadrat, live samples of female, male and vegetative shoots were taken along with a soil core (5×5×5 cm). To do this, moss and soil samples were placed in plastic containers, transported to St. John’s, NL and maintained in growth chambers (day‒night light and temperature condition, 5 to 15oC, ca. 1000 μ mol PAR m‒2 s‒1, 90% humidity). Allozyme variability was investigated using cellulose acetate gel‒electrophoresis for five enzyme loci (Got; glutamate‒oxoacetate tramsaminase, Gpi‒1; glucophosphate isomerise‒1, Gpi‒2; glucophosphate isomerise‒2, Pgm‒2; phosphor‒gluco‒mutase‒2, Pgd; phosphor‒gluconate dehydro‒genase) based on Innes (1990), and Hebert and Beaton (1989). These enzymes have been widely used in Polytrichum species and have been observed to be polymorphic (Innes 1990; Derda and Wyatt 1999; Bijlsma et al. 2000; Van der Velde and Bijlsma 2003). Staining methods followed Hebert and Beaton (1989). Within sample quadrat at least six female and six male gametophytes and six sporophytes were analyzed and alleles scored. During the analyses, sex (gender) was also treated as a genetic marker with two alleles (female, F; male, M).
Genetic diversity within populations was explored by quantifying the number of polymorphic loci as determined by electrophoresis. Allele frequency and Nei’s genetic identity and distance (Nei 1972) were calculated using Popgene 1.32 (https://sites.ualberta.ca/~fyeh/popgene_download.html). The observed genotypic diversity (G) was based on the frequency of the ith multi locus genotype (gi) for the k multi‒locus genotypes observed in the population (Stoddard 1983):
The number of gametophyte genotypes were quantified by the genetically distinct individuals within a population based on their multiple genotype and sex (Innes 1990). The observed number of multi‒locus genotypes (N) was calculated using the number of the unique genotypes at each population. The expected genotypic diversity and expected number of multi‒locus genotypes were based on random simulations assuming a random sexually reproducing population with the same allele frequency and number of individuals (Innes 1990). The expected and observed number of multi‒locus genotypes and genotypic diversity were compared with a one sample t‒test to determine whether populations were randomly sexually reproducing.
The number of sporophyte genotypes within populations was quantified and the most frequent sporophyte genotypes identified. Deviation from Hardy‒Weinberg equilibrium for the diploid sporophyte populations were investigated using Genepop 4.0 (Rousset 2008). The frequencies of mating between males and females with identical observed genotypes were estimated at each population and compared among populations. Distances were measured from population maps developed for each population if the fathering male was not revealed within the quadrat but was revealed within the population. From the genotype of the sporophytes, the expected number of gametophyte genotypes was deduced. These gametophyte genotypes were expected to be represented in the population if the spores from local populations of sporophytes are recruited into the population. In a population with effective spore recruitment, all the gametophyte genotypes, based on sporophyte genotypes, are predicted to be present. We tested whether the number of missing gametophyte genotypes, based on sporophyte genotypes, was significantly lower than the expected number of gametophyte genotypes using a paired t‒test. If all the expected gametophyte genotypes are present in the population, then local populations are recruiting from spores.
3.1 Population structure, viability and germination ability of spores in the diaspore bank
Coverage by P. juniperinum populations was highest in the low elevation but did not differ between the remainder of the populations (Table 1). The sex ratio increased with decreasing elevation and moisture gradient (Table 1). Spores from all populations of the elevation and moisture gradient were viable and germinated on perlite in the growth chambers with no significant differences observed among populations (Table 1). This suggests a high potential for recruitment via sexual reproduction. However, no spores from any population germinated following inoculation on original soil, suggesting inhibition by either biotic or abiotic factors (e.g., due to microorganisms and/or soil conditions). Sporelings or shoots also failed to emerge from the diaspore bank from any population, which suggests that both sexual (via spores) and asexual (via fragments) recruitment was highly limited.
5.2 Gametophyte allozyme variability along the elevation gradient
Along the elevation gradient, a total of 1619 shoots were used to explore population and reproductive structure of P. juniperinum. Out of the five putative enzyme loci, five were polymorphic in the mid and high elevation and four were polymorphic in the low elevation population (Table 2). The mean allele frequency, the genetic distance (Table 2), the observed number of multi‒locus gametophyte genotypes (Figure 2a), and the genotypic diversity (Figure 2c) all increased from the low to the high elevation population. This suggests that the low elevation population consisted of large clones whereas the high elevation population consisted of a greater diversity of clones within the same unit area.
Assuming random reproduction among male and female gametophytes within populations along the elevation gradient, simulation results showed that the observed number of multi‒locus genotypes (Figure 2a) and genotypic diversity (Figure 2c) were significantly lower at all populations than that expected for a random mating population. This suggests that the random reproduction was reduced at all elevation. The highest expected multi‒locus genotypes and genotypic diversity was predicted in the mid elevation population.
5.3 Sporophyte allozyme variability along the elevation gradient
The number of sporophyte genotypes (Figure 3a) and sporophyte genotypic diversity (Figure 3c) was lowest in the low elevation population. The high and the mid elevation population had similar sporophyte genotypic diversity (Figure 3c) but the mid elevation population had higher number of multi‒locus sporophytes (Figure 3a). Despite the different number of sporophyte genotypes and sporophyte genotypic diversity among the populations, they all deviated from the Hardy‒Weinberg equilibrium (p<0.05). Deficiency of heterozygotes suggested depressed sexual reproduction in all populations.
The observed and expected number of gametophyte genotypes based on sporophyte genotypes was compared to determinate the possible rate of recruitment from spores. Recruitment of spores, based on sporophyte genotypes, was not limited in either population, as a significant number of the expected genotypes were observed in the population (Table 3). In the low elevation population all the expected gametophyte genotypes, based on sporophyte genotypes, were observed suggesting a high rate of recruitment from spores produced by inter‒gametophytic inbreeding. Even though sexual reproduction occurred, it was realized among closely related ramets (i.e. mixing of individuals with low genetic variability will not result in high genetic variability, unless mutation occurs). Most missing genotypes based on sporophytes genotypes were found at the high elevation population suggesting a low rate of sexual reproduction as some males were not involved in mating and not represented in sporophyte genotype.
5.4 Gametophyte allozyme variability along the moisture gradient
Out of the five enzymes examined, five were polymorphic in the mesic and dry populations, and two were polymorphic in the wet population (Table 2). The mean allele frequency, the genetic distance (Table 2), the observed number of multi‒locus gametophyte genotypes (Figure 2b), and the genotypic diversity (Figure 2d) was lowest in the wet, intermediate at the dry and highest at the mesic populations. This suggests that the wet population consisted of large clones whereas the mesic population consisted of a greater diversity of clones within the same unit area. There were many unique gametophyte genotypes found in the mesic population resulting in the highest number of multi‒locus gametophyte genotypes.
Assuming random reproduction among male and female gametophytes within populations along the moisture gradient, simulations suggest that the observed number of multi‒locus genotypes (Figure 2b) and observed genotypic diversity (Figure 2d) were significantly depressed compared to a random mating population. Similarly, to what was found along the elevation gradient this suggests that the random reproduction was reduced at all sites along the moisture gradient.
5.5 Sporophyte allozyme variability along the moisture gradient
The number of sporophyte genotypes (Figure 3b) and sporophyte genotypic diversity (Figure 3d) was lowest in the dry, intermediate in the wet and highest in the mesic population along the moisture gradient. The most common sporophytes in the mesic, wet and dry populations produced genetically identical offspring according to the four polymorphic loci examined, indicating inter‒gametophytic inbreeding. This is also suggested as all populations deviated from the Hardy‒Weinberg equilibrium (p<0.05), signifying a limited number of heterozygous and a high number of homozygous sporophytes.
Recruitment of individuals by spores, based on sporophyte genotypes, was not limited in any population along the moisture gradient as a significant number of the expected genotypes were observed in each population (Table 3). The highest rate of recruitment was observed in the wet population as all the expected numbers of gametophyte genotypes, based on sporophyte genotypes, were observed in this population (Table 3). Sexual reproduction occurred but among closely related ramets, thus it resulted in identical offspring. Lowest recruitment i.e., the most undetected genotypes were observed in the mesic population (Table 3). Even though there was evidence for recruitment of gametophytes by sexual reproduction, some males were not involved in mating (detected as male genotypes observed in the population but not detected in sporophytes).
Allocation between the sexual vs. asexual reproduction for the moss P. juniperinum did not differ along elevational and moisture gradients in the Torngat Mountains low arctic area.
Effective sexual reproduction was depressed in all populations. All sporophyte populations deviated from the Hardy‒Weinberg equilibrium suggesting that all populations deviated from a randomly mating sexually reproducing population. This was also supported by the observation that the genotypes of the sporophytes were predominantly homozygotes. Therefore, effective sexual reproduction (production of new gametophyte genotypes) was low. Populations appeared to consist of large clones in which sexual reproduction occurred among closely related ramets. Sporophytes were most often a result of inter‒gametophyte self fertilization. Reproduction among closely related ramets by cross‒fertilization of haploid gametophytes produced by a single diploid sporophyte resulted in genetically similar offspring to the parents. In our case the closely related ramets were also identical therefore, the offspring too (even if there was recombination it would not change the genetic composition of the offspring compare to the parents). This inherently means that the observed high rate of recruitment is due to intensive production of identical offspring by homozygote sporophytes (based on scored enzymes).
The degree of observed inbreeding in our study is similar to the intra‒gametophyte mating of the monoicous species in which spores are actually asexual propagules (Mishler 1988). Sexual reproduction among ramets (identical genotypes) lowers the potential for recombination in contrast to interbreeding among genets (where genotypes differ from each other) (Gunnarsson et al. 2007). For example, Hock et al. (2008a) found in the liverwort, Mannia fragrans that the mating of closely related haplotypes (haploid genotypes) did not lead to the formation of new haplotypes. Mating among closely related haploid siblings (genets) can be a result of the close vicinity of the female and male gametophytes and the leptokurtic distribution of spores; consequently a female shoot may be surrounded by its haploid progeny (Szövényi et al. 2009). Such a situation was observed in populations of P. juniperinum and P. strictum in which only a few genotypes occurred within populations and the frequency of inter‒gametophyte self‒fertilization was shown to be high (Derda and Wyatt 2003). Others showed that haploid selfing was common in bryophytes with bisexual gametophytes, which resulted in homozygous sporophytes (Haig 2016).
Our results are similar to this but we also showed that haploid selfing is common between closely related individuals (ramets) and environmental gradients (stress) did not change the allocation between reproduction strategies. A similar study however found also no differences in genetic and clonal diversity between higher‒ and lower‒stress populations but in that case the levels of sex expression and sporophyte production differed (Paasch et al. 2015), suggesting that there is no general pattern in the reproduction structure and strategies along environmental/stress gradients.
The generality of the present results should be tested because no other studies have investigated the number of multi‒locus genotypes and genotypic diversity along elevational or moisture gradients in dioecious bryophytes.
Population structure, viability and germination ability of spores, diaspore bank
Mosses in general, and especially Polytrichum species often express a female biased sex ratio, the cause of which is not understood (Stark 2002; Bisang et al. 2006; Stark et al. 2010). The female‒biased sex ratio is most likely due to phylogenetic history i.e. genetic based rather than as a consequence of actual environmental influences (Bisand and Hedenäs 2005). male-biased sex ratio in gametophyte was also found, where the effect of inbreeding on the fitness at the sporophyte was studied in case of an aquatic peat moss Sphagnum macrophyllum (Johnson and Shaw 2016). Hypothetically, if the sex ratio is skewed towards either males or females then the dominant gender has a higher capacity for clonal expansion (Stark 2002). Therefore, based on observed sex ratios in this study, the high elevation population and the dry population along the moisture gradient should have had the highest allocation to sexual reproduction, which was not the case. Also, in contrast to our predictions, there were no significant elevation or moisture gradient effects on the viability and germination capability spores. Spores from all populations germinated well under the same laboratory conditions, but showed not in the diaspore bank, which suggests a low rate of recruitment via spores in semi‒natural conditions. This is in agreement with the general observation that recruitment from spores in natural populations is limited in Polytrichum species (Longton 1988; Innes 1990; Miles and Longton 1990; Derda and Wyatt 1999; Van der Velde et al. 2001). The soil microbial or invertebrate community may inhibit spore viability either/or by the consumption of spores or protonemata (Miles and Longton 1990). Innes (1990) suggested that a limited number of suitable micro‒sites may also prevent the recruitment of spores.
As temperature, moisture, humidity, and UV radiation varies along elevation gradients, allocation to sexual reproduction changes in many vascular plant populations (Gugerli 1998, Körner 1999). Similarly, in bryophytes, sex expression is also reduced in dioicous moss species towards the poles, at high elevations, and in arid environments (Stark 2002). In bryophytes, sex expression is reduced mainly because sporophyte maturation is resource‒limited (Stark et al. 2009) and thus, allocation to asexual reproduction is increased in habitats where resources are relatively limited. In this study, no pattern was observed in the relative allocation between sexual and asexual reproduction in P. juniperinum along either an elevation or a moisture gradient. The low rate of effective sexual reproduction (ability to produce new genotypes) could lead to a low potential to adapt to changing environmental conditions. P. juniperinum is a pioneer species that occurs in dry, sandy habitats, has a world‒wide distribution and is especially common in the arctic (Crum and Anderson 1981), thus at a global scale, the species clearly has the ability to thrive under a wide range of environmental conditions.
In this study the lack of genetic variation within populations of P. juniperinum and the limited expression of effective sexual reproduction in all populations could be explained by the local severe climate, which may have limited sexual reproduction among genetically different genets. As the climate of Labrador continues to warm (IPCC 2007), it would be important to know whether local genotypes of P. juniperinum have the phenotypic plasticity to be successful under these changing conditions. The ability of this species to thrive in low arctic environments is important because mosses, like P. juniperinum together with lichens play important roles in producing and maintaining the soil (Turetsky 2003, Cornelissen et al. 2007). To renew genetic diversity, sexual reproduction would be desirable among genets with distinctly different genotypes in contrast to the observed situation in which sexual reproduction appears to occur among closely related genets.
6. Acknowledgment
We are thankful for the support of the Torngat Mountains National Park Base Camp (kANGIDLUASUk) and Parks Canada. Special thanks to Angus Simpson and Judy Rowell and the bear monitors Eli Merkuratsuk, Rodney Gear, Robert Harris, John Merkuratsuk, and the camp setup assistances Jacko Merkuratsuk, John Anderson and Eli Merk. We are grateful for the aid and the great working environment of the members of the Labrador Highland Research Group, especially for Luise Hermanutz, David Innes and for the members of David Innes’s and Tom Chapman’s laboratory.
We thank Alain Cuerrier for his help during the sample collections. We would like to express our thanks for the financial support of the CiCAT (Climate change Impacts on Canadian Arctic Tundra ecosystems) and the IPY (International Polar Year) projects. The study was conducted when Peter Koncz was an MSc student at the Memorial University of Newfoundland. The manuscript was finalized when Peter Koncz worked at the MTA‒SZIE Plant Ecology Research Group, at the Duna‒Ipoly National Park Directorate, Hungary and at the Eco-Scale Association, Hungary.
7. Author contribution statement
Peter Koncz performed the sampling, lab analysis, calculations and writing. Paul Marino was responsible to plan the study design, and to check the manuscript.
8. Conflict of Interest
The authors declare no competing financial interests.
9. Data archiving
https://www.polardata.ca/pdcinput/datafile/list?ccinRefNumber=13344
10. Research Ethics Statement
The authors declare no involvement of human subjects, animals.
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Table 1. Polytrichum juniperinum population ground cover (%), sex ratio (female/male shoots), spore viability (%), and spore germinatibility (%) along the elevation (low, mid, high) and moisture (wet, mesic, dry) gradients (sampling numbers are also indicated)
|
Population |
Low |
Mid |
High |
|||
|
|
|
Dry |
Mesic |
Wet |
Mean |
|
|
Cover (%) |
42.6±39.3* |
6.35±15.8 |
20.8±15.5 |
16.14±20.6 |
14.43±7.37 |
17.02±21.3 |
|
Sex ratio |
3.44 |
0.43 |
1.71 |
2.2 |
1.44±0.91 |
0.75 |
|
Number of quadrates |
15 |
18 |
21 |
20 |
|
21 |
|
Spore viability (%) |
94.3±4.7 |
95.5±2.1 |
95.8±13 |
95.7±5.9 |
95.66±0.15 |
93.5±0.7 |
|
Sampled spores |
300 |
200 |
500 |
700 |
|
200 |
|
Spore germination ability (%) |
93.0±4.4 |
89.5±2.2 |
89.4±138 |
91.8±7.1 |
90.23±1.35 |
88.0±1.4 |
|
Sampled sporelings |
300 |
200 |
500 |
700 |
|
200 |
|
*indicates if a population was significantly different from the rest of the populations (p>0.05), ± indicates standard deviation |
||||||
Table 2. Allele frequencies of the loci, genetic identity and distance (Nei 1973) and number of gametophyte genotypes (Innes 1990) in the populations of Polytrichum juniperinum along an elevation (low, mid, high) and a moisture (wet, mesic, dry) gradient. Codes for alleles: VF=very fast, F= fast, M=medium, S=slow.
|
|
|
|
Population |
|||||
|
|
|
|
Low |
|
Mid |
|
High 1 |
High 2 |
|
Locus |
Allele |
|
|
Wet |
Mesic |
Dry |
|
|
|
Glutamate‒oxoacetate tramsaminase |
VF |
|
0.06 |
|
0.06 |
0.03 |
0.03 |
0.11 |
|
|
F |
|
0.83 |
1 |
0.88 |
0.97 |
0.97 |
0.78 |
|
|
M |
|
0.11 |
|
0.06 |
|
|
0.11 |
|
Glucophosphate isomerise‒1 |
F |
|
|
|
|
|
0.03 |
0.06 |
|
|
M |
|
0.94 |
0.18 |
0.52 |
0.62 |
0.38 |
0.17 |
|
|
S |
|
0.06 |
0.82 |
0.48 |
0.38 |
0.59 |
0.78 |
|
Glucophosphate isomerise‒2 |
F |
|
0.06 |
1 |
0.44 |
0.29 |
0.55 |
0.78 |
|
|
M |
|
|
|
0.14 |
0.03 |
0.24 |
|
|
|
S |
|
0.94 |
|
0.42 |
0.68 |
0.21 |
0.22 |
|
Phosphor‒gluco‒mutase‒2 |
F |
|
|
|
0.02 |
|
|
|
|
|
M |
|
0.91 |
|
0.42 |
0.32 |
0.38 |
0.11 |
|
|
S |
|
0.09 |
1 |
0.56 |
0.68 |
0.62 |
0.89 |
|
Phosphor‒gluconate dehydro‒genase |
F |
|
|
|
0.06 |
0.03 |
|
|
|
|
M |
|
1 |
0.42 |
0.44 |
0.09 |
0.52 |
0.22 |
|
|
S |
|
|
0.58 |
0.5 |
0.82 |
0.48 |
0.78 |
|
|
VS |
|
|
|
|
0.06 |
|
|
|
Genetic identity |
|
0.85 |
0.71 |
0.59 |
0.65 |
0.64 |
0.64 |
|
|
Genetic distance |
|
0.15 |
0.29 |
0.41 |
0.35 |
0.36 |
0.36 |
|
|
Number of gametophyte genotypes |
|
35 |
38 |
47 |
34 |
28 |
16 |
|
Table 3. Observed, expected and predicted but missing number of gametophyte genotypes per quadrates of P. juniperinum along an elevation (low, mid, high) and a moisture (wet, mesic, dry) gradient. The expected number of gametophyte genotypes were based on sporophyte genotypes observed at each quadrates.
|
|
|
Population |
|||||||||
|
|
|
Low |
|
|
Mid |
|
|
|
High-1 |
High-2 |
|
|
|
|
|
|
Wet |
Mesic |
Dry |
mean |
|
|
|
mean |
|
Observed |
1.60±0.5 |
|
2.4±0.7 |
2.6±1.3 |
1.9±0.5 |
2.3±0.36 |
|
2.4±1.6 |
1.9±0.9 |
2.15±0.35 |
|
|
Expected |
2±1 |
|
3.3±1.7 |
5.1±4.3 |
2.7±1.6 |
3.7±1.29 |
|
3.8±2.6 |
2.9±1.7 |
3.35±0.6 |
|
|
Missing |
0* |
|
0* |
2±3.4* |
0.4±0.8* |
0.8±1 |
|
1.1±1.1* |
0.8±1.2* |
0.95±0.21 |
|
|
Comparisons between expected genotypes at each quadrat vs. genotypes not observed within each population*= p <0.05 (±Stdv.) |
|||||||||||
There is no duality of interest