Uncovering unseen fungal diversity from plant DNA banks

Introduction

Understanding biodiversity is an important goal of biology. This is particularly critical in a changing world with habitat degradation and fragmentation, population declines, and species extinctions (Vitousek et al., 1997). Once a species becomes extinct, the genetic history resulting from its evolution is lost as well (Mattick, Ablett & Edmonson, 1992). DNA banks were initially developed to collect genetic material to create a storage base for evolutionary history, biological diversity, and genomic information (Mattick, Ablett & Edmonson, 1992). Throughout the world, samples are collected and stored in these banks to document and preserve genetic diversity (Spooner & Ruess, 2014). For extinct species, DNA bank samples act as storage deposits for their genomes (Adams, 1994; Spooner & Ruess, 2014).

In addition to the importance of DNA bank repositories for archiving target organisms’ genetic information, these samples also harbor the microbial diversity associated with each accession. These samples represent well-preserved DNA at snapshots in time and space. For example, plant DNA bank samples not only preserve the genomic information of the target species, but also preserve potentially important cryptic microbial symbionts associated with the host, such as fungi known to inhabit the plant phyllosphere (Porras-Alfaro & Bayman, 2011; Vorholt, 2012).

Despite much work on patterns of plant diversity, comparatively little is known about the diversity of fungi. Fungi play crucial functions in ecosystems by acting as decomposers and nutrient cyclers, important mutualists such as mycorrhizae, and pathogens influencing host species populations (Kendrick, 2001; Lips et al., 2006). Globally, <100,000 species of fungi have been described (Blackwell, 2011), which is far less than total estimated fungal diversity, and also less than vascular plants, with <400,000 species currently described (Royal Botanic Gardens Kew, 2016). Estimates of global fungal species richness have increased almost 3–5 fold in the past 20 years, from 1.5 million (Hawksworth, 1991) to 3.5–6 million species (O’Brien et al., 2005; Taylor et al., 2014). This increase is due in part to advances in direct environmental DNA sequencing and extrapolations based on predictions of vascular plant to fungal ratios (O’Brien et al., 2005; Taylor et al., 2014). In order to obtain more accurate estimates of true fungal diversity, increased sampling using high throughput DNA sequencing of many different types of environments is needed, and DNA banks may significantly contribute to filling this knowledge gap.

Hawai‘i is a biodiversity hotspot, making it an exceptional location to study patterns of species diversity (Myers et al., 2000). However, we know very little about Hawaiian fungi, their potential rates of endemism, and patterns of biodiversity. A survey of mushrooms throughout the Hawaiian Islands conducted in the 1990’s found 310 species. The majority of these taxa were introduced, however 52 were putatively native and 46 of these taxa were considered potentially endemic (∼88%; Hemmes & Desjardin, 2002). Similar rates of endemism are found in the Hawaiian flora.

An estimated 89% of the Hawaiian vascular plant flora is endemic (Wagner, Herbst & Sohmer, 1999). The unique Hawaiian flora is threatened by habitat degradation and loss, coupled with species invasions, which have led to native species becoming endangered or extinct (Morden, Caraway & Motley, 1996). There are currently 1,175 recognized native (endemic plus indigenous) Angiosperm species in Hawai‘i (Smithsonian Institution, 2017) and 422 of these plants are currently endangered (35.9%; US Fish and Wildlife Service, 2015) with 104 taxa extinct or possibly extinct (8.8%; Sakai, Wagner & Mehrhoff, 2002). As a result of these extinctions and a strong potential for additional future losses, the Hawaiian Plant DNA Library (HPDL) was created to preserve the genetic diversity of the Hawaiian flora (Morden, Caraway & Motley, 1996).

The HPDL was started in 1992 and currently has over 10,000 accessions representing over 86% of all of the Hawaiian plant genera and over 50% of all species (Morden, 2017). Similar to other DNA banks across the world, collections for common species contain multiple individuals with their own accession number. However, the HPDL is unique due to the relatively disproportionate number of threatened and endangered native Hawaiian plants compared to other localities, and thus serves as a genetic repository for many species that are endangered or extinct in the wild (Morden, Caraway & Motley, 1996). The main goal of the HPDL is to preserve Hawaiian plant DNA and bank samples for use in future studies of biodiversity (Morden, Caraway & Motley, 1996; Randell & Morden, 1999).

All naturally occurring plant tissues harbor fungi as both endophytes, living in between plant cells (Rodriguez et al., 2009) and epiphytes living on plant surfaces (Santamaría & Bayman, 2005), collectively known as phyllosphere fungi (Vacher et al., 2016). These communities form diverse assemblages with some studies showing an average of about 100 species per individual tree and ranges of about 700–4,000 species of fungi per host species (Jumpponen & Jones, 2009; Zimmerman & Vitousek, 2012). Thus, along with the HPDL’s banked plant samples it also has likely and coincidentally preserved a substantial portion of the diversity of Hawaiian fungi, acting as a repository for not only plant genetics, but their microbiomes as well.

Utilizing several plant bank samples from the HPDL we investigated the diversity of unintentionally co-sampled fungi found within banked plant samples. Phyllosphere fungal communities can be cryptic and hyperdiverse (Jumpponen & Jones, 2009; Zimmerman & Vitousek, 2012) and this poses a challenge to studies with the goal of assessing microbial diversity. Even with the adoption of next generation DNA sequencing technologies, such as high throughput amplicon sequencing from environmental samples, observed diversity is often an under representation of true diversity (Chiu & Chao, 2016). Thus, rather than superficially sequencing the fungi from all available replicate DNA extracts from a single plant species, we chose to deeply sequence ten samples of a common endemic Hawaiian plant genus, Clermontia (Campanulaceae), with species found across the Hawaiian Islands (Givnish et al., 2009). This deep sequencing was done in hopes that the vast majority of fungi from our samples would be recovered from each sample.

In this study, we utilized historical DNA bank samples to validate the use of plant bank samples as a resource for elucidating phyllosphere fungal biodiversity, while subsequently examining plant-associated fungal diversity across space. Our two main questions were do DNA bank samples store microbial diversity? And can these previously collected samples be used to uncover ecological patterns, such as changes in microbial community similarity over space? Using DNA samples of eight species of Clermontia stored in the HPDL, we sequenced the fungi found in these plants’ phylloplanes (the portion of the phyllosphere made up by leaves). We took advantage of the archipelago’s geographic spatial gradient and the previously collected samples in the HPDL to test for decreases in community similarity as the distance between communities increases, the classical ecological pattern of distance-decay of community similarity (Nekola & White, 1999).

Materials & Methods

Results

Fungal diversity and host associations

Each Clermontia DNA bank sample used in this study contained fungal DNA. In total, we found 2,944 fungal OTUs associated with the ten foliar DNA bank samples. After removing OTUs with less than ten reads and rarefying to the sample with the minimum number of reads, we removed 1,164 OTUs (39.5%) and were left with a total of 1,780 fungal OTUs. While the observed OTU accumulation curve for all ten samples combined did not reach an asymptote (Fig. S1), OTU accumulation curves by sample and by island (except in the case of C. fauriei from Kaua‘i and C. kakeana Moloka‘i) generally reached their asymptotes, indicating that overall, we successfully recovered the majority of fungi present in our samples (Figs. S2 and S3). Observed richness per sample after rarefying, ranged from 108 to 682 fungal OTUs with an average of 295 OTUs per sample (±54.69 standard error). From our ten samples combined the iNEXT extrapolation curves suggest fungal richness based on the Hill number q = 0 (Chao1 richness) will saturate around 3,947 OTUs which would require at least 50 samples. Similarly fungal diversity based on q = 1 (exponential Shannon entropy) was estimated to saturate at around 2,750, and diversity based on q = 2 (inverse Simpson concentration) was estimated to saturate at about 1,591 (Fig. 1).

We investigated patterns of fungal diversity at the phyla and ordinal levels. Overall, the majority of fungi in the subkingdom Dikarya dominated all of the phylloplane samples, with phylum Ascomycota being most abundant followed by Basidiomycota (Fig. 2). Fungi belonging to the phylum Chytridiomycota and Zygomycota were also present in lower abundances. Additional OTUs mapped to kingdom Fungi but could not be identified further (Unidentified; Fig. 2). The top ten most abundant orders were Capnodiales, Chaetothyriales, Exobasidiales, Peltigerales, Pertusariales, Pleosporales, Tremellales, Ustilaginales, and two unknown orders (Fig. 3).

Based on our observed data, total average fungal OTU richness by island was 507.6 (±128.46 standard error). O‘ahu had the highest total observed richness with 1,045 OTUs, followed successively by Hawai‘i (926 OTUs), Maui (685 OTUs), Moloka‘i (362 OTUs), and Kaua‘i again had the lowest richness (108 OTUs), though unlike the other islands we did not saturate our OTU richness by sequencing effort curve for C. fauriei from Kaua‘i and C. kakeana from Moloka‘i indicating that these are an underrepresentation of fungal OTU richness (Fig. S3). Overall about twenty OTUs were found on all of the five islands (Fig. 4).

Physical distance decay

Clermontia DNA bank extracts used in this study spanned across the main Hawaiian Islands. The nearest samples were collected less than one kilometer apart from a single site in Kohala, Hawai‘i Island, and the furthest distance was 524.78 km from Kohala, Hawai‘i Island to the Alaka‘i Swamp, Kaua‘i. Over this spatial range, while taking into account time (number of days) between sample collections, the fungal phylloplane communities exhibit a significant decrease in community similarity across increasing geographic distance (Fig. 5, Partial Mantel test: r = 0.423, p = 0.005). Time was also significantly correlated with physical distance between sample sites (Fig. S4, Mantel test: r = 0.455, p = 0.048). Time between sampling and community dissimilarity was marginally significantly correlated (Fig. S5, Mantel test: r = 0.619, p = 0.051). Despite the significant relationships with sampling time the Partial Mantel between distance and community dissimilarity while accounting for time, was significant (Fig. 5).

Discussion and Conclusion

In this study we investigated the diversity of phylloplane fungi associated with Clermontia spp. that were collected across the Hawaiian Islands and stored as DNA bank samples. While sample collection methods were not developed with the intention of preserving or analyzing phyllosphere fungi, they coincide with common practices for these purposes (Jumpponen & Jones, 2009; Zimmerman & Vitousek, 2012).

We found that these specimens harbored a considerable diversity of fungi. After quality control, we found 1,780 fungal OTUs from just ten samples, representing 20 Clermontia individuals and eight species. Fungal richness ranged from 108 to 682 OTUs per plant sample. Incredibly, this diversity was recovered from a total of just 20 grams of leaf tissue from which DNA was extracted and preserved. Despite our high sequencing depth, the observed species accumulation curves for all samples and islands did not saturate, indicating our sequencing efforts likely underestimated true Clermontia phylloplane fungal diversity. However, this novel use of DNA bank samples revealed substantial undiscovered fungal biodiversity stored in plant samples. These results provide further evidence of microbes making up the “unseen majority” of biodiversity (Whitman, Coleman & Wiebe, 1998), where a single macroorganism associates with a multitude of microorganisms both within and on their surfaces (Turner, James & Poole, 2013).

This study highlights a new and underutilized function of biological collections, as well as gives insights into regional fungal diversity patterns. Previous estimates of total regional fungal richness have been based off of plant to fungi ratios ranging from 1:6 (Hawksworth, 1991) to 1:17 (Taylor et al., 2014). Our data supplement these studies using environmental NGS data. If we assume that the diversity of phylloplane fungi associated with Clermontia species is representative of the native Hawaiian flora, we would estimate based on Chao1 richness (q = 0) extrapolations (determined by species; Fig. S6) that the entire Hawaiian flora (c. 1,000 species) harbors about 4,000 fungi. This results in an approximate 1:4 plant to fungi species ratio. However, just considering phylloplane fungi associated with a single genus is likely an underestimate of total fungal biodiversity due to potential host-fungi specificity (Hoffman & Arnold, 2008). Supplementary to host specificity, only taking into consideration phylloplane fungi likely underestimates total regional fungal richness due to habitat niche partitioning among fungal species and guilds (Hibbett, Gilbert & Donoghue, 2000).

In addition to the study of microbial diversity, questions regarding microbial biogeography, host specificity, and the effects of global change on microbial communities could be addressed with DNA banks. For example, we were able to confirm the distance decay of microbial community similarity from DNA bank samples collected across the Hawaiian Islands. This finding is similar to other microbial systems where significant distance decay patterns were found in foliar endophytic (Vaz et al., 2014) and ectomyorrhizal fungal communities (Bahram et al., 2013), as well as bacteria and archaea (Barreto et al., 2014). We were also able to identify a temporal partitioning of phylloplane fungal communities (Fig. S5), indicating that DNA bank collections may be used to study changes in microbial communities over time. However, in addition to geography and temporal factors, taking into account host genotype, age, specificity, and differences in environment, such as light exposure and humidity, may potentially explain additional variation in fungal communities (Hoffman & Arnold, 2008).

In agreement with other phyllosphere studies, the majority of fungal taxa were identified as belonging to the subkingdom Dikarya, with the majority in phylum Ascomycota followed by Basidiomycota (Rodriguez et al., 2009). It is not surprising that we found so many unknown fungal taxa (45.16% of total OTUs at the family level) including 28 OTUs we were unable to place at the phylum level. The plant samples from this study represent an endemic Hawaiian genus whose microbial associates are previously unstudied, and possibly associate with undescribed fungi endemic to Hawai‘i. However, this degree of unassigned fungal OTUs is not unique to our system and highlights our limited current knowledge of fungal diversity (Nilsson et al., 2016). For example, recent discoveries using environmental DNA sequencing have reshaped the fungal tree of life by uncovering a new fungal Phylum, the Cryptomycota (Jones et al., 2011). This stresses the need for further investigations of fungal biodiversity, their cryptic nature and diverse functions make for intriguing new discoveries that have the potential to change evolutionary and ecological theories based primarily on macroorganisms.

With the recent advent of next generation sequencing (NGS) techniques genomic investigations of non-model organisms have become readily accessible (Da Fonseca et al., 2016). However, there are important caveats to consider when using these methods and analyses. For example, working with environmental samples poses the challenging prospect of encountering hyperdiverse microbial communities such as the fungi found here, and other studies of plant phyllosphere fungi (Arnold, 2007; Arnold & Lutzoni, 2007). As seen in this NGS study, thousands of fungi can be associated with a small number of plant leaf samples. While uncovering this diversity is a goal of some microbial ecologists, for researchers using NGS techniques focused on the host organism (in this case plants), microbial symbionts may interfere with downstream analyses and results. Microbial taxa associated with macroorganismns should be taken into account when using NGS methods such as RAD seq, RNA seq, targeted sequencing, among other techniques (Da Fonseca et al., 2016). Additionally, future research into the microbiomes of hosts preserved in DNA banks should take into consideration how sample processing and storage may affect microbes. When initially collecting samples for these purposes, care should be taken to avoid microbial contamination from non-target hosts or environments.

Most DNA bank samples likely harbor unintended microbial communities associated with each target individual from a specific location at distinct snapshots in time. While DNA banks are a common genetic biodiversity repository (Seberg et al., 2016), to the best of our knowledge this is the first study where they were used to investigate genetic material other than that of the target organism. By using these archived samples we were able to rapidly recover previously undocumented microbial diversity. The abundance of DNA bank samples stored throughout the world represent a large proportion of the globes extant and extinct biological diversity. This storage provides the opportunity for microbes associated with these organisms to be easily investigated without the associated costs of sample collection. This may be important for conservation efforts, giving insight into potentially important symbionts (Van der Heijden, Bardgett & Straalen, 2008; Busby, Ridout & Newcombe, 2016). For those species that go extinct, their genomes are preserved in DNA banks along with their corresponding microbial symbionts. For extant organisms, DNA bank samples could be used to better understand the ecology of symbiosis and possibly identify coevolutionary patterns. Overall, this study highlights the potential use of DNA bank samples for the study of global biodiversity. This study also demonstrates the benefits of in-depth sample sequencing to uncover the majority of fungal diversity found in each plant bank sample. With DNA bank samples stored throughout the world, already collected, processed, and extracted, they harbor the potential for new and exciting investigations.

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