Understanding processes governing and sustaining biological diversity is a central goal of community ecology. Unisexual community complexes, where successful reproduction relies on sperm transfer from males of one or more bisexual host species, are relatively rare and poorly understood in terms of the drivers of community diversity and structure. Unisexual salamanders of the genus Ambystoma produce mostly clonal offspring but can also produce distinct biotypes (‘genomotypes’), each with their own ecological niche. Within unisexual communities, individual hosts are often relatively rare, however, the number, identity, and relative abundances of distinct host species may govern community structure. We studied Ambystoma communities on Pelee Island, Ontario, Canada, to determine if the bisexual hosts (A. texanum, A. laterale) determine community structure of unisexual Ambystoma. From 2015-2018, we genomotyped 2027 Ambystoma tissue samples (1184 adults, 843 larvae) from 11 different sites. The relative abundance of bisexual hosts ranged from 0-18% across sites. Composition and diversity of ecological communities was highly variable and could be largely ascribed to variation in the presence and/or relative abundance of keystone host species. Additionally, the site where both hosts were present had the greatest unisexual diversity, with genomotypes reflecting contributions from both hosts, suggesting that multi-host communities generate enhanced diversity. Our results indicate that keystone hosts are pivotal in determining structure and composition of unisexual community complexes. Unisexual complexes clearly offer unique opportunities to examine the factors impacting community structure and provide a useful model for studying the role of keystone species in ecological communities.
Research Article
Diversity Among Mixed-ploidy Salamander Communities Reflects the Influence of Keystone Host Species
https://doi.org/10.21203/rs.3.rs-982855/v1
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Ambystoma
community ecology
kleptogenesis
polyploidy
species complex
A central goal of community ecology is to understand the processes governing and sustaining biological diversity. Fundamentally, ecological communities are composed of members whose niche requirements are met by prevailing abiotic conditions as well as biotic interactions within the ecosystem (Michalet et al. 2006; Hammill et al. 2018). Environmental variation undoubtedly plays an important role in determining composition and diversity of ecological communities, and more heterogenous environments may support higher species diversity because they provide a wider range of occupiable niches (e.g., MacArthur and MacArthur 1961). We know that biotic interactions among community members can also have strong impacts on community composition and diversity (Paine 1966; Stachowicz 2001), and ecologists are especially interested in species and interactions that influence a community’s diversity and the range of processes driving these patterns across spatial and temporal (Callaway and Walker 1997; Kamiya et al. 2014). While environmental conditions may set some of the broad patterns in ecological communities, the occurrence and spatial dispersion of key species across the landscape can dramatically alter the structure of local communities (Callaway and Walker 1997; Baumeister and Callaway 2006). Thus, quantifying the role of key species in driving community structure remains essential to understanding patterns of biological diversity across scales.
Broadly speaking, keystone species are recognized as those having a disproportionate role in shaping the diversity or structure of an ecological community (Cottee-Jones and Whittaker 2012). It follows that loss of a keystone species could cause a large shift in community structure, likely leading to fundamental changes in biotic interactions and local extinctions across the system (Krebs 2009). While the effects of a keystone species may be context-specific (Lindtner et al. 2019), the effects of one keystone species are generally not interchangeable with those of another within the same system (Warren et al. 2015). Consequently, sympatry of multiple keystone species can elevate species diversity through additive effects on the community (Davidson and Lightfoot 2006, 2007). In addition, interactions among two or more members of a community can produce ecosystem- or community-level effects that are greater than those of any individual species (i.e., ‘keystone interactions’, Helfield and Naiman 2006). Although diversity might also be impacted by the abundance of a keystone species, support for this relationship remains equivocal (Delibes-Mateos et al. 2007; Mackay et al. 2018), making it difficult to predict how communities will respond to variation in their relative or absolute abundance (reviewed in Cottee-Jones and Whittaker 2012). Not surprisingly, quantifying keystone species effects has been especially challenging in systems characterized by complex life histories, where the strength and direction of community-member interactions can vary across life stages (Salice et al. 2011; Piao et al. 2013).
The unisexual salamander complex in the genus Ambystoma provides an interesting case for studying community dynamics and variation in community composition across space, in part because of their unique reproductive system. Specifically, these communities consist of a monophyletic lineage of all-female (unisexual) salamanders and one or more sympatric bisexual Ambystoma salamander species. Unisexual Ambystoma are obligate reproductive parasites, reliant on sperm from one of five bisexual Ambystoma species for successful reproduction (Bogart 2019a). They are also nuclear alloploid hybrids, possessing genomes derived from at least two different species, and range in ploidy from diploid (2N) to pentaploid (5N) (Bogart et al. 2007). Individuals are categorized by ‘genomotype’ based on genome counts according to the species of origin (Lowcock et al. 1987). While the majority of unisexual offspring have a genomotype identical to their mother, unisexual females also produce offspring with a variety of distinct genomotypes (Bogart 2019a; Fig. 1) that are produced by genome incorporation from sperm provided by the local host(s) (Bogart and Licht 1986). Importantly, each genomotype differs in its niche requirements (Greenwald et al. 2016; Mills et al. 2020) and morphology (Uzzell Jr. 1964; Zeyl and Lowcock 1989), reflecting both differences in ploidy and proportional genomic contributions of their host species (McElroy et al. 2017). Each genomotype can therefore be considered an ecological species within the broader unisexual complex (De Queiroz 2007; Owen and Juterbock 2013). Thus, in these complexes each sperm-donor species acts as a ‘keystone host’ promoting biological diversity by facilitating the colonization, establishment, and formation of several ecological groups of unisexual Ambystoma.
We investigated variation in community structure of the Ambystoma salamander complex on Pelee Island, in western Lake Erie Ontario, where two keystone hosts occur (i.e., A. texanum, A. laterale). Human development on the island (i.e., agriculture, irrigation, roads) has caused loss and fragmentation of natural habitat, apparently isolating remaining Ambystoma communities (G. Smith, unpubl.). We predicted that unisexual community composition would vary across the island, and that the average genome dosage of each unisexual community should be skewed toward the keystone host species available in that particular community. Accordingly, we expected to observe greater variation between sites (i.e., β-diversity) that differ in host species composition. We also expected that the average genome dosage of each unisexual community should be skewed toward the keystone host species available to that particular community. Further, we predicted that communities with two sperm donors would have the greatest diversity (i.e., α-diversity) of unisexual genomotypes. Finally, we used our unique dataset to explore the effect of host abundance on patterns of diversity and evenness in unisexual communities, to investigate the possible impact of keystone species abundance on diversity.
Study system
Our study was conducted on Pelee Island (41.79 km2) in Lake Erie, Ontario, Canada (41.6 N, -82.6 E). Amphibian populations on Pelee Island were isolated from the mainland following the island’s formation after glacial retreat, around 4000 BP (Holcombe et al. 2003). Populations of unisexual salamanders on the island are found syntopically with either (or both) Ambystoma texanum and A. laterale, with no other keystone host species being present on the island (Bogart et al. 1985; Committee on the Status of Endangered Wildlife in Canada and (COSEWIC) 2016). Note that for reference to different salamander genomotypes, we used common nomenclature (Lowcock et al. 1987; Bogart 2019a) with ‘LTT’ being a triploid (3N) with a single set of chromosomes from the A. laterale (L) genome, and two from the A. texanum (T) genome. Previous work found diploid unisexuals (LT) to be abundant on the island (Bogart and Licht 1986), which is rare compared to most sites across the range of unisexual Ambystoma (Kraus 1985; Bogart and Klemens 2008).
Gynogenesis is the primary mode of reproduction in the unisexual Ambystoma complex, but this group has three alternatives (described in Bogart and Bi 2013 and Bogart 2019a): 1) the sperm genome can be incorporated into an unreduced egg to produce a ploidy-elevated offspring (i.e., ‘kleptogenesis’); 2) genome replacement can result in offspring of the same ploidy as the mother but with a novel genotype and potentially different genomotype; and 3) diploidized unisexual offspring (e.g., LT) can arise from symmetrical tetraploid (e.g. LLTT) mothers. With a diversity of potential reproductive outcomes (see Fig. 1), multiple genomotypes can be derived in situ and can live sympatrically, but contrary to historical thought (Sessions 1982; Kraus and Miyamoto 1990), there are no records of a parental genomotype (i.e. LL or TT) being reconstituted from the unisexual lineage (Bogart 1989; Beauregard and Angers 2018).
Salamander sampling
Between September 2015 and August 2018, Ambystoma salamanders were sampled from 11 sites across Pelee Island. On average, wetland sites were separated by 5.7 km (± 3.1 SD, median: 6.0 km, range: 0.3 km – 11.3 km) and ancillary landscape resistance modelling revealed low habitat connectivity between sites (G. Smith, unpubl.). Notably, a minority of breeding sites are within estimated mean migration distances for A. texanum (Williams 1973) and A. laterale (Ryan and Calhoun 2014), and unisexual Ambystoma with elevated ploidy are thought to exhibit lower dispersal distance compared to their diploid hosts (Lowcock 1994; Denton et al. 2017). Adults were sampled using a combination of opportunistic search, coverboard surveys, and minnow trapping during the breeding season (March), and adults found in the terrestrial environment were normally assigned to sites within 300 m of the wetland (except for B8.2 and B8.3, see below). Larvae were sampled using dipnet surveys starting in May. Tail tissue samples (2-5 mm from adults, ~2-3 mm from larvae) were collected and stored in 95% ethanol for DNA extraction and genotyping (University of Guelph, Guelph, ON, Canada). Adult and terrestrial juvenile salamanders were uniquely marked with visual implant elastomer (VIE) codes for a concurrent mark-recapture study and to prevent resampling of individuals. All captured salamanders were released back to their point of capture, and all field equipment was disinfected between sites (Canadian Herpetofauna Health Working Group 2017).
Taxonomic Assessment
Individuals were given a taxonomic assignment (genomotype) based on multilocus genotypes (MLGs) derived from seven microsatellite loci with tetranucleotide motifs that are used in the unisexual Ambystoma complex (Williams and DeWoody 2004; Peterman et al. 2012). Loci Atex102 and AjeD378 were tested early on but discontinued after the first year. Though discontinued, Atex102 was necessary to determine ploidy in 14 samples and genomotypes in 24 of the 291 samples where this locus was assessed. Three loci amplify in both A. laterale and A. texanum genomes: AjeD346, which has unambiguous allele ranges (i.e., alleles derived from A. laterale and A. texanum can be distinguished), and AjeD422 and AjeD283 where alleles could not be assigned reliably to A. laterale or A. texanum. Two exclusive loci for each sexual species were used: AjeD75 and AjeD94 for A. laterale; Atex74 and Atex141 for A. texanum. Based on MLGs, individuals were given taxonomic assignments (genomotypes) that account for both ploidy and genome dosage. Ploidy for each sample was determined either based on the maximum number of unique alleles in an ambiguous locus (i.e., AjeD422 or AjeD283) or by adding allele count from the two highest species-specific loci (see Ramsden et al. 2006; Bogart et al. 2007, 2009). Samples that resulted in either low amplification (>3 failed loci, n = 23, 1.0% of total) or ambiguity in assigning genomotype (n = 202, 9.1% of total) were excluded (see S1). One additional sample was excluded due to contamination.
Statistical analysis
Distinct wetland sites were considered discrete units potentially supporting an independent community of salamanders, and we assessed site-level community composition separately for adult vs. larval life stages. Data were partitioned according to ploidy-level and genomotype. In our ploidy-level analysis, we differentiate between unisexual and sexual individuals, and categorize unisexual data by ploidy level. In this analysis we excluded data lacking clear ploidy level determination and site details (n = 22 samples). A finer scale genomotype-level analysis was also conducted to further explore patterns in community composition, and this was restricted to samples where genomotype could be identified (91.5% of total).
Using the genomotype-level dataset, Bray-Curtis dissimilarity indices of unisexual communities were calculated by life stage based on relative abundances for each site using the vegdist function from the R package vegan (Oksanen et al. 2019). To determine whether community composition was spatially correlated, we used the mantel.rtest from the R package ade4 (Dray and Dafour 2007) to conduct Mantel tests (with 999 permutations) using previously calculated Bray-Curtis indices and pairwise geographic distances measured to the nearest 50 m between breeding ponds. For each site the probability of failing to detect a keystone host species that was actually present was determined using a binomial distribution, assuming true abundances of 5%, 2%, 1%, and 0.5% of the total community.
Unisexual genomotype richness (i.e., number of unique genomotypes) for adult and larval life-stages was assessed for each site. Shannon-Weaver diversity (H), Stoddart and Taylor’s index (G), Simpson diversity (λ), modified Hill’s ratio (E5), and respective confidence intervals were calculated for all adult and larval unisexual communities using the bootstrap algorithm diversity_ci from the package poppr (Kamvar et al. 2018) in R (2018).
Genome bias in this study reflects the genetic input from one keystone host species, and is calculated for a community of sample size n across all i individuals by:
$$%T=\frac{1}{n}\sum {i}\frac{{T}{i}}{{N}_{i}}$$
where Ti is the number of T genomes in individual i, and Ni is the ploidy level of individual i. With all individuals having at least one L and T genome, a normalization factor of 0.5 was subtracted from %T. In this way, a community of all symmetrical hybrids (e.g., LT or LLTT) would have a value of 0, T-biased communities would be positive, and L-biased communities negative. Because of the potential influence of higher ploidy on this metric, ploidy composition in adult and larval communities were compared via Fisher exact tests for six sites (B3, B6, B8.1, D3.1, D3.2, and D7) using the ploidy-level dataset. Site D6 was omitted due to low adult sample size (n = 5). A Spearman rank correlation was used to evaluate whether unisexual genomotype diversity (H’) and evenness (E5) increase with the proportion of sexual Ambystoma present in the population.
Sampling
During 2015-18, we collected 2230 salamander tissue samples, 2027 of which were successfully genotyped (adult n = 1184, larvae n = 843). Sexual species (A. texanum, n = 88; A. laterale, n = 3) made up only 4.5% of the total (4.1% of the adult samples, and 5.1% of the larval samples); 51.1% (45/88) of the A. texanum samples and all A. laterale samples were from adults. Diploid unisexuals (LT) made up 56.4% (adult = 64.2%; larvae = 45.6%) of the total sample (Fig. 2, S1). Eight sites had A. texanum, while A. laterale was detected at only two non-neighboring sites (Fig. 2, S1). Only one site (D3.1) had both A. texanum and A. laterale. For all sites, other than B8.3, we had a >95% confidence that non-detection of a host indicated its true absence from the community. Conversely, we had >90% confidence for all sites that, if a host went undetected, relative abundance of that host was <0.5% of the community. Therefore, we have high confidence that any site where a host species was not detected, it was truly absent at the time of sampling or made up a nominal fraction of the community (ESM Table S2).
Within the unisexual complex, eight different genomotypes were observed: LT, LLT, LTT, LLLT, LLTT, LTTT, LLTTT, and LTTTT. Diploids (LT) were 58.8% of all unisexual Ambystoma at the island-level (adult = 66.8%; larvae = 47.8%). Within triploids, the LTT genomotype was more common than LLT (Adults: 232 and 38, respectively; Larvae: 218 and 51, respectively). Eleven samples were identified as triploid based on ambiguous loci and were assumed LTT given that two alleles were amplified in at least one T-specific locus. Pentaploids were especially rare (~0.6%, 14/2207; Fig. 1, Table S1).
Site Diversity
Neither adult nor larval community composition was influenced by geographic proximity to neighboring communities (Mantel tests based on Bray-Curtis dissimilarity values adult: p = 0.22; larvae: p = 0.26). Site B3 differed from other sites in both adult and larval community composition (Table 1) and was the only site where A. laterale was the sole available host. Site B6 was also relatively distinct from most other sites for both adult and larval communities (Table 1). Community differentiation based on life stages was inconsistent, with larval communities being more dissimilar for some pairwise comparisons of sites, and adult communities being more dissimilar than larval communities for others (Table 1).
Table 1. Bray-Curtis dissimilarity matrix based on genomotype proportions for adults and larvae for 11 salamander populations on Pelee Island, Ontario, Canada. Adult comparisons (italics) below diagonal and larval comparisons (underlined) above diagonal. Adult comparison for D6 not provided due to low sample value (n = 5). Larval comparisons not provided for sites without larval data. Icons used to indicate level of similarity in the relative abundance of genomotypes between sites at that particular life stage (full circle - high similarity, empty circle – high dissimilarity).
As predicted, the site with both hosts (D3.1) had greater diversity across most metrics, with exception of community evenness (Table 2). Notably, D3.1 also had genomotypes requiring recent genomic input from both host species (i.e., LLTT and LLTTT) (S1). The two sites without any host species detected (B8.2 and B8.3; see S1) had the lowest diversity in all metrics and had no detected larval communities (Table 2).
|
Stage |
N |
P(LL) |
P(TT) |
P(LLTT) |
R |
H' |
G |
E5 |
|---|---|---|---|---|---|---|---|---|
|
Pelee |
||||||||
|
Adult |
1184 |
0.3% |
3.8% |
0.9% |
7 |
0.80 (±0.20) |
1.76 (±0.33) |
0.61 (±0.11) |
|
Larvae |
843 |
- |
5.1% |
3.2% |
8 |
1.25 (±0.18) |
2.72 (±0.53) |
0.69 (±0.09) |
|
B3 |
||||||||
|
Adult |
36 |
5.6% |
- |
2.8% |
4 |
0.88 (±0.14) |
2.11 (±0.27) |
0.79 (±0.09) |
|
Larvae |
96 |
- |
- |
- |
5 |
1.11 (±0.14) |
2.66 (±0.35) |
0.82 (±0.06) |
|
B6 |
||||||||
|
Adult |
349 |
- |
0.3% |
0.6% |
5 |
0.71 (±0.19) |
1.64 (±0.27) |
0.62 (±0.11) |
|
Larvae |
214 |
- |
0.9% |
0.5% |
5 |
0.88 (±0.15) |
2.03 (±0.34) |
0.72 (±0.11) |
|
B8.1 |
||||||||
|
Adult |
106 |
- |
12.3% |
0.9% |
5 |
0.75 (±0.18) |
1.69 (±0.28) |
0.61 (±0.10) |
|
Larvae |
64 |
- |
3.2% |
4.7% |
6 |
0.55 (±0.22) |
1.31 (±0.19) |
0.42 (±0.05) |
|
B8.2 |
||||||||
|
Adult |
93 |
- |
- |
- |
3 |
0.39 (±0.15) |
1.27 (±0.15) |
0.55 (±0.10) |
|
B8.3 |
||||||||
|
Adult |
52 |
- |
- |
- |
3 |
0.54 (±0.15) |
1.47 (±0.21) |
0.64 (±0.11) |
|
D1.1 |
||||||||
|
Adult |
80 |
- |
3.8% |
2.5% |
5 |
0.96 (±0.19) |
1.92 (±0.39) |
0.56 (±0.08) |
|
D1.2 |
||||||||
|
Adult |
84 |
- |
6% |
1.2% |
4 |
0.86 (±0.14) |
2.01 (±0.31) |
0.74 (±0.10) |
|
D3.1 |
||||||||
|
Adult |
161 |
0.6% |
10.7% |
0.6% |
7 |
0.99 (±0.18) |
2.28 (±0.32) |
0.75 (±0.09) |
|
Larvae |
187 |
- |
12.6% |
10.4% |
6 |
1.28 (±0.15) |
3.01 (±0.50) |
0.77 (±0.08) |
|
D3.2 |
||||||||
|
Adult |
138 |
- |
2.9% |
1.4% |
6 |
0.65 (±0.20) |
1.53 (±0.25) |
0.57 (±0.11) |
|
Larvae |
112 |
- |
1.8% |
0.9% |
5 |
0.92 (±0.18) |
1.98 (±0.37) |
0.65 (±0.10) |
|
D6 |
||||||||
|
Larvae |
51 |
- |
17.6% |
- |
3 |
0.76 (±0.10) |
2.02 (±0.18) |
0.89 (±0.08) |
|
D7 |
||||||||
|
Adult |
80 |
- |
2.5% |
1.3% |
6 |
0.81 (±0.18) |
1.87 (±0.29) |
0.69 (±0.12) |
|
Larvae |
119 |
- |
4.2% |
2.5% |
6 |
1.19 (±0.12) |
2.95 (±0.36) |
0.85 (±0.07) |
Host Influence
Consistent with our prediction, both adult and larval communities at sites where A. texanum was present had unisexual communities skewed towards genomotypes with T-genome bias (Fig. 3). Comparably, both adult and larval communities skewed towards L-biased genomotypes were only at site B3, where A. laterale was the sole known host (Fig. 3). Though site D3.1 has both potential hosts, adults and larvae had genomic bias for A. texanum, which, based on our capture success, is the more prevalent host at that site (Fig. 1, Table S1). Larval communities had a stronger genome-bias than their associated adult communities, except for B8.1. Ploidy composition of adult and larval unisexual communities differed significantly at four sites (Fisher exact tests; B6: p < 0.001; B8.1: p = 0.048 D3.1: p < 0.001; D7: p < 0.001). Only at B8.1 was the larval community more biased towards lower ploidy than the adult community. At two sites we did not detect a significant difference in ploidy composition across life stages (D3.2: p = 0.064; B3: p = 0.111), although the pattern observed at D3.2 was consistent with sites B6, D3.1, and D7.
Interestingly, adult community diversity (H’) was positively correlated with the relative abundance of the host species (ρ = 0.679, p = 0.022), whereas evenness (E5) was not (ρ = 0.41, p = 0.21) (Fig. 4A). Neither community diversity nor evenness were correlated with host species abundance in larval communities (H’: ρ = 0.071, p = 0.91; E5: ρ = -0.321, p = 0.50) (Fig. 4B).
Our study examined patterns of diversity in unisexual salamander communities across an island that has been isolated from the mainland for ~4000 years. These communities were surprisingly diverse relative to analogous mainland communities (Noël et al. 2011; Charney et al. 2014; Bogart et al. 2017) and varied substantially among one another in community composition. Historically, Pelee Island had been divided by a large central swamp into 3 primary regions. Irrigation and human settlement have since fragmented the landscape leaving these sites strongly isolated (G. Smith, unpubl.). Despite historic landscape structure, community composition and diversity were not spatially autocorrelated in either adult or larval life stages, and instead were associated with variation in the host community. Larval communities generally had greater diversity and higher ploidy than the corresponding adult population, with some exceptions. As expected, genomic bias in the unisexual community at each site corresponded to local host species, a pattern that was exaggerated in the larval stage likely due to the higher average ploidy level in larval communities. Additionally, unisexual diversity peaked where both sexual hosts were present and was lowest where no host was detected. This supports the critical influence that multi-host availability plays in generating and maintaining diversity in unisexual communities. Interestingly, community diversity, but not evenness, was related to the relative abundance of host species, though only in the adult life stage. Overall, our work provides new insights into the role of keystone hosts in generating and maintaining biological diversity across local and regional scales.
The high diversity in unisexual Ambystoma communities studied here contrasts with the traditional expectation that clonal and hemiclonal complexes should have constrained niches (Case and Taper 1986; Vrijenhoek 1998) and limited diversity (Muller 1964; Lynch et al. 1993). Most unisexual communities of Ambystoma are dependent on a single host, are triploid-dominated, and generally have only one or two genomotypes (e.g., Bogart and Klemens 1997, 2008). Comparatively, the Ambystoma communities we examined were dominated by diploid unisexuals and had up to seven distinct genomotypes. Ambystoma communities dominated by diploid unisexuals are rare and have been poorly sampled prior to this work (Bogart and Klemens 2008; Noël et al. 2011). The high relative abundance of diploid unisexuals on Pelee Island may indicate that diploid unisexuals generally have high relative fitness compared to ploidy-elevated conspecifics. This is consistent with previous research showing that higher-ploidy unisexuals have lower survival across all life stages (Teltser and Greenwald 2015). While reduced survival of higher-ploidy animals across life stages helps explain why larval samples exhibit greater α-diversity and stronger genome bias, future work is required to implicate specific mechanisms that may generate this pattern (for potential explanations see Lowcock et al. 1991; Lowcock 1994; Greenwald et al. 2016).
On Pelee Island the production of several genomotypes (e.g., LLTT, LLTTT) requires that unisexuals have access to sperm from both A. texanum and A. laterale (Committee on the Status of Endangered Wildlife in Canada and (COSEWIC) 2016; Hossie 2018), which is consistent with our detection of these genomotypes on the island and the fact that the site with the highest proportion of larvae with such genomotypes was the only site currently occupied by both host species (i.e., D3.1). Correspondingly, this site had the greatest unisexual community diversity for both life stages, including two pentaploid genomotypes not previously recorded (i.e., LLTTT and LTTTT; Bogart 2019a), and the greatest genomotype richness in adults. Although the level of community diversity reported here is unprecedented in unisexual Ambystoma research outside of Pelee Island (Bogart et al. 1987; Kraus et al. 1991), it is a direct prediction of kleptogenetic processes (Bogart and Bi 2013; Hossie 2018). The rarity of diploid unisexual Ambystoma elsewhere likely reflects inability of these lineages to persist long-term in single-host systems due to proliferation of ploidy-elevated offspring, and inability to produce ploidy-reduced offspring. While other studies have reported various effects between keystone species on local community structure (Fahnestock and Detling 2002; Davidson and Lightfoot 2007), our study provides evidence of a positive effect of keystone hosts on local adult salamander community diversity.
In addition to the high community diversity on Pelee Island, there is also notable variation among local communities that is clearly associated with host availability. Specifically, each Ambystoma host species was associated with a distinct unisexual community, which is a consequence of ploidy elevation in unisexual offspring through incorporation of host sperm genomes (i.e., kleptogenesis, Bogart 2019d). Data from analogous complexes indicates that unisexual communities generally maintain genetic bias favoring local host genomes (e.g. Cobitis: Janko et al. 2007, 2012; Squalis: Cunha et al. 2011). More broadly, this is consistent with previous work showing that the presence of distinct keystone hosts can alter the composition of their communities in different directions, even within analogous ecosystems composed of species with similar niches (e.g., Davidson and Lightfoot 2006, 2007). Thus, our work highlights that variation in the keystone host present within local communities can cause divergence in the structure of adjacent communities, thereby functionally increasing beta and gamma diversity.
The genome composition of adult unisexual Ambystoma salamanders within a local community corresponded with the available host but was not correlated with relative host abundance. It remains unknown whether this pattern simply arises as a natural consequence of host availability, or if individuals with genetic makeup favoring a less abundant or non-local host are also selected against (e.g., via sexual or natural selection). It is conceivable, for example, that the fitness of T-biased genomotypes (e.g., LTT) may be greater than L-biased genomotypes (e.g., LLT) in habitats optimal for A. texanum if their niche is influenced by genome dosage (McElroy et al. 2017). Consistent with this, unisexual Ambystoma in Ohio had greater niche overlap with the host they were most genetically similar to, than with unisexual Ambystoma genomotypes that had more genetic similarity to an alternative host (Greenwald et al. 2016). The distribution of unisexual Ambystoma across Ontario similarly indicates that the environmental niche of unisexuals is governed primarily by their hybrid nuclear genome (Mills et al. 2020). Quantifying genomotype-specific patterns in survival and reproductive output across habitats with different environmental conditions is a clear next step towards understanding factors driving unisexual community composition.
Finally, relative abundance of the host species was positively related to adult community diversity, which indicates that keystone hosts can support greater diversity as they become more abundant. In combination with the absence of a relationship between evenness and relative abundance of the host, this suggests that rare genomotypes are more likely to arise or be sustained when host availability is high. This could be due to selective pressures and niche similarities between unisexual genomotypes and their hosts (Greenwald et al. 2016; McElroy et al. 2017), but the complexity of the system provides for many other unique explanations. For example, this pattern might arise as a natural consequence of a greater proportion of unisexual eggs being fertilized, probabilistically increasing the chances that rare genomotypes are formed via ploidy elevation or genome swapping. Greater host availability might provide the late arrivals, which tend to be of higher ploidy (Lowcock 1994), with more opportunities for fertilization thereby expanding the potential sperm recipient diversity. Alternatively, greater host availability may provide a greater diversity of new parental genomes and increase the probability that newly incorporated male DNA in ploidy-elevated offspring functions well with the unisexual mother’s existing genome. More generally, we might expect increases in keystone species abundance to have diminishing returns on community diversity, as observed here. Additional work should evaluate whether analogous patterns are also observed in other systems.
In sum, we report high levels of salamander community diversity in an island system, which are higher than corresponding mainland systems. The substantial variation we found in the composition of adjacent ecological communities can be largely ascribed to variation in the presence and/or abundance of keystone host species, which dramatically influenced beta and gamma diversity. In addition, alpha diversity increased as a function of syntopic presence of two keystone hosts through a unique multi-generational interaction characteristic of unisexual Ambystoma. We also found that the keystone host’s relative abundance impacted community composition and diversity, but only in the adult communities. We suspect that these general patterns are not restricted to unisexual Ambystoma, although these complexes provide a valuable model for studying the role of keystone species in community ecology.
Funding: The Ontario Species At Risk Stewardship Fund (SAR-00109, SARSF_23_18_Dmurr2)
Conflicts of Interest: Not applicable
Ethics approval: Animals were handled in accordance with Animal Care protocols approved by Trent University and Ontario Ministry of Natural Resources (Protocols: 23906, 25301, 25344). Land access permits, Wildlife Scientific Collector’s Authorizations (1079527, 1082275, 1085623, 1088782), and a Notice of Activity under the Ontario Endangered Species Act (Confirmation ID: M-102-3802796883, M-102-9225853169, M-102-1254262277) were secured to conduct this work.
Consent to participate: Not applicable
Consent for publication: Not applicable
Availability of data and materials: The datasets used and analyzed during the current study are available from the corresponding author upon request.
Acknowledgements:
The Ontario Species At Risk Stewardship Fund provided financial support for this work (SAR-00109, SARSF_23_18_Dmurr2). Nature Conservancy of Canada, Essex Region Conservation Authority, Ontario Parks, as well as J. Ambrose, J. DeMarco, and D. Kraus provided authorization to sample on their properties.
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