Clonality, local population structure and gametophyte sex ratios in cryptic species of the Sphagnum magellanicum complex

BACKGROUND AND AIMS

Sphagnum (peatmoss) comprises a moss (Bryophyta) clade with ~300-500 species. The genus has unparalleled ecological importance because Sphagnum-dominated peatlands store almost a third of the terrestrial carbon pool and peatmosses engineer the formation and microtopography of peatlands. Genomic resources for Sphagnum are being actively expanded, but many aspects of their biology are still poorly known. Among these are the degree to which Sphagnum species reproduce asexually, and the relative frequencies of male and female gametophytes in these haploid-dominant plants. We assess clonality and gametophyte sex ratios and test hypotheses about the local-scale distribution of clones and sexes in four North American species of the S. magellanicum complex. These four species are difficult to distinguish morphologically and are very closely related. We also assess microbial communities associated with Sphagnum host plant clones and sexes at two sites.

METHODS

Four hundred and five samples of the four species, representing 57 populations, were subjected to restriction site-associated DNA sequencing (RADseq). Analyses of population structure and clonality based on the molecular data utilized both phylogenetic and phenetic approaches. Multi-locus genotypes (genets) were identified using the RADseq data. Sexes of sampled ramets were determined using a molecular approach that utilized coverage of loci on the sex chromosomes after the method was validated using a sample of plants that expressed sex phenotypically. Sex ratios were estimated for each species, and populations within species. Difference in fitness between genets was estimated as the numbers of ramets each genet comprised. Degrees of clonality [numbers of genets/numbers of ramets (samples)] within species, among sites, and between gametophyte sexes were estimated. Sex ratios were estimated for each species, and populations within species. Sphagnum-associated microbial communities were assessed at two sites in relation to Sphagnum clonality and sex.

KEY RESULTS

All four species appear to engage in a mixture of sexual and asexual (clonal) reproduction. A single ramet represents most genets but two to eight ramets were dsumbers ansd text etected for some genets. Only one genet is represented by ramets in multiple populations; all other genets are restricted to a single population. Within populations ramets of individual genets are spatially clustered, suggesting limited dispersal even within peatlands. Sex ratios are male-biased in S. diabolicum but female-biased in the other three species, although significantly so only in S. divinum. Neither species nor males/females differ in levels of clonal propagation. At St Regis Lake (NY) and Franklin Bog (VT), microbial community composition is strongly differentiated between the sites, but differences between species, genets and sexes were not detected. Within S. divinum, however, female gametophytes harboured two to three times the number of microbial taxa as males.

CONCLUSIONS

These four Sphagnum species all exhibit similar reproductive patterns that result from a mixture of sexual and asexual reproduction. The spatial patterns of clonally replicated ramets of genets suggest that these species fall between the so-called phalanx patterns, where genets abut one another but do not extensively mix because of limited ramet fragmentation, and the guerrilla patterns, where extensive genet fragmentation and dispersal result in greater mixing of different genets. Although sex ratios in bryophytes are most often female-biased, both male and female biases occur in this complex of closely related species. The association of far greater microbial diversity for female gametophytes in S. divinum, which has a female-biased sex ratio, suggests additional research to determine if levels of microbial diversity are consistently correlated with differing patterns of sex ratio biases.

Short stories from Sphagnum of rare species, taxonomy, and speciation

Conserving species and their genetic variation are a global priority to safeguard evolutionary potential in a rapidly changing world. Species are fundamental units in research and nature management, but taxonomic work is increasingly undermined. Increasing knowledge on the species genetic diversity would aid in prioritizing conservation efforts. Sphagnum is a diverse, well-known bryophyte genus, which makes the genus suited to study speciation and cryptic variation. The species share specific characteristics and can be difficult to separate in the field. By combining molecular data with thorough morphological examination, new species have recently been discovered. Still, there are taxonomic uncertainties, even for species assessed on the IUCN Red List of threatened species. Here, we use molecular data to examine three rare species within the subgenus Acutifolia described based on morphological characters. All species have narrow distributions and limited dispersability. First, we confirm the genetic origin of S. skyense. Second, we show that S. venustum is a haploid species genetically distinct from morphologically similar species. Lastly, S. nitidulum was found to have a distinct haplotype, but cannot be genetically separated from other red Acutifolia species. We also found high genetic variation within red Acutifolia specimens, indicating the need of further morphological examination and possibly taxonomic revision. Until then, our results have shown that genetic data can aid in prioritizing targets of conservation efforts when taxonomy is unresolved. All three taxa should be further searched for by field biologists to increase knowledge about their distribution ranges.