Rationale for Scope

C. Rationale for the study system and scope

Syntrichia is a diverse genus of mosses containing about 79 species (Zander 1993), occurring worldwide in dryland habitats and demonstrating an unusual amount of ecological variation among close relatives. Its species occur in habitats ranging from mesic alpine meadows, to tree trunks and boulders in mid-elevation forests, and to late-successional desert biocrusts (Fig. 1). This genus is one of the most ecologically dominant groups of mosses across western and northern North America. This makes Syntrichia an excellent model system in which to investigate the role of alternative reproductive and stress tolerance strategies in shaping dimensions of biodiversity.

Figure 1. The genus Syntrichia has impressive ecological amplitude, occurring in the highest elevations (shown here in the Andes Mts, Argentina) and the lowest deserts including the Mojave Desert, NV. PHOTO: T. Clark

Syntrichia ruralis has become a model system for studying the biology of desiccation tolerance (DT), with many published studies documenting its molecular, cellular, and physiological responses to drying and rehydration (Oliver et al. 2005; Proctor et al. 2007 and references therein).

The reproductive ecology of Syntrichia caninervis has also received much attention, with a number of published studies describing sex ratios in different habitats, distribution of sex-expressing individuals and sporophytes, potential tradeoffs between sex expression and male rarity, as well as growth rates (e.g., Bowker et al. 2000; Stark et al. 2000, 2004, 2005). These two species inhabit environments with varied seasonal precipitation and temperature regimes; our work will determine if this ecological amplitude (Fig. 1) is the product of unusually plastic physiological tolerance or is maintained by the presence of physiologically differentiated genotypes that are perhaps structured geographically.

Both S. ruralis and S. caninervis are potentially dominant in soil biocrust communities of drylands, contributing to multiple ecosystem functions (Belnap & Lange 2003).

Syncan seedling
Model organism Syntrichia caninerivs shown dominating a biocrust colony in Utah. PHOTO: K. Deane-Coe

Thus, diversity in Syntrichia is likely to have repercussions throughout its associated communities. We will focus most genomic and population-level work on these two target species of Syntrichia with intensive collecting within N. America in order to capture the ecological amplitude of both. Because of ease of access, we will study elements of physiological diversity in DT using the 15 North American species of Syntrichia (Table 2), which were taxonomically revised for North America by one of the coPIs (Mishler 2007).

Table 2.

The 15 species of Syntrichia in North America. While all will be studied, boldface indicates the two species complexes to be studied in the most detail.

S. ammonsiana     S. chisosa           S. norvegica              S. princeps

S. bartramii           S. fragilis            S. obtusissima          S. ruralis

S. cainii                  S. laevipila         S. pagorum                S. sinensis

S. caninervis         S. montana        S. papillosissima

Focal Species

We will focus on both S. ruralis and S. caninervis to characterize the ecophysiology of DT as a function of the two principal ecological factors of desiccation (rate of drying and equilibrating relative humidity; Green et al. 2011), in the process deriving quantitative measures of assessment that may vary within and between populations and species. The ecophysiology of DT directly relates to, and integrates the analyses of the population genetic variation among moss patches, the transcriptomic signature of gene expression during desiccation stress, the diversity and evolution of ecological strategies of DT across Syntrichia, and the ecosystem functions of Syntrichia.

Syntrichia caninervis showing fully desiccated morphology in which leaves lay vertically appressed without spiral contortion (characteristic of S. ruralis). PHOTO: J. Brinda

We will focus on S. caninervis to study the genetic and evolutionary consequences of energetic tradeoffs involving reproduction. Sexual reproduction (indicated by the presence of sporophytes) in this species is very rare in the Mojave Desert (Stark et al. 1998; Bowker et al. 2000), and this rarity is likely a reflection of the constraints imposed by partitioning limited resources between survival, growth, and reproduction in extremely water-limited environments.

[photo of S.caninerivs with sporophytes coming soon!]

Another relevant outcome of tradeoffs related to water stress in S. caninervis is the appearance of skewed sex expression (female biased) in this dioicous plant, which has been attributed to higher reproductive costs incurred by males in this species (Stark et al. 2000). In less stressful environments typical of higher elevations, S. caninervis sex expression is less highly female biased, yet still reaches female to male ratios as high as 14:1 at some sites (Stark et al. 2001).

[Pic of Syn.can asexual gemmae coming soon!]

Furthermore, in the most heat- and drought-stressed environments of its range, S. caninervis shoots have not been observed to express sex (Stark et al. 1998, 2005), and appear to favor the energetically efficient strategy of clonal propagation from vegetative fragments or specialized gemmae. Of particular interest to our study, this energetic tradeoff between sexual and asexual strategies may have important implications for the population genetics and evolution of desert mosses like S. caninervis, because spores, the meiotic product of the sexually produced diploid sporophyte, are assumed to be responsible for occasional long-distance dispersal and gene flow (Newton & Mishler 1994; Stenøien et al. 2010).

A summary of our methodological approaches appears in Table 3, and details are outlined below.


Table 3. Proposed approaches to address the questions outlined in Table 1.

(1) Construct high quality draft nuclear genomes of S. caninervis and S. ruralis.

(2) Use next generation sequencing to develop genotypic markers for population-level genetic variation studies, signature transcriptome tools for phenotypic analyses (related to ecophysiological and ecosystem investigations), and multiple single-copy genes for phylogenetic analyses.

(3) Conduct transcriptomics experiments comparing different development stages and sexes of both species in response to desiccation stress and reproductive state.

(4) Conduct ecophysiological experiments on multiple populations of S. caninervis and S. ruralis, and all 15 species of N. American Syntrichia, to assess phenotypic plasticity in the key trait of desiccation tolerance.

(5) Characterize (a) the population structure and genetics of S. caninervis and S. ruralis in different environments, (b) the niche preferences of males and females, and (c) patterns of genetic diversity.

(6) Build a robust phylogeny for Syntrichia and use it to understand evolutionary trends and correlations among the traits under study, as well as to produce a refined classification.

(7) Examine the role of taxonomic, genetic, and functional diversity in the resilience to climate change of biocrust communities that are dominated by Syntrichia, with co-occurring mosses, lichens, and cyanobacteria in field and simulated climate change greenhouse experiments.

Syntrichia ruralis growing in shaded crevice on sandstone canyon wall, Grand Staircase Escalante National Monument, UT. PHOTO: T. Clark