Closely related species have a tendency to resemble one another as a consequence of their shared evolutionary history (Darwin, 1859; Blomberg and Garland, 2002). Thus, related lineages may occupy similar ecological niches and overlap in their climatic tolerance. However, the extent to which lineages resemble one another may vary both by trait and across clades as a result of variable evolutionary processes. As climate change has, and is predicted to, disrupt ecosystems and force ranges to shift, it is important to understand which lineages may be especially vulnerable. This vulnerability, is in part, a product of climatic preference. Lineages with slow rates of climatic niche evolution may have a more limited ability to adapt to and persist through climatic perturbations than those with a legacy of evolutionary lability (Holt, 1990; Wiens et al., 2010). Indeed, estimates of predicted climate change often vastly outpace past rates of climatic niche evolution in diverse lineages, suggesting these species may not be able to adapt to the rapid climate change that is underway (Quintero and Wiens, 2013; Jezkova and Wiens, 2016). Thus, it is important to understand how the climatic niche of a lineage has evolved through time in order to identify its potential sensitivity or ability to adapt to shifting climates.

Symbioses are expected to be especially vulnerable to climate change (Dunn et al., 2009; Colwell et al., 2012; Kikuchi et al., 2016; Sampayo et al., 2016; Baker et al., 2018; Rolshausen et al., 2018). Lichens—symbiotic associations between heterotrophic fungi and photosynthetic microbes (eukaryotic microalgae and/or cyanobacteria)—make important and diverse ecosystem contributions (Seaward, 2008; Elbert et al., 2012; Porada et al., 2014, 2018; Rutherford et al., 2017), and are well-known bioindicators (Nimis et al., 2002; Giordani and Incerti, 2008; Will-Wolf et al., 2015; Loppi, 2019; Will-Wolf and Jovan, 2019) with certain lichens predicted to exhibit strong responses to changing climates (Allen and Lendemer, 2016; Ellis, 2019; Nelsen and Lumbsch, 2020). Thus, understanding the evolutionary legacy of climatic niche evolution in these symbionts is paramount to predicting their extinction vulnerability and potential to adapt to changing climates.

The eukaryotic algal genus Trebouxia is the most common lichen photobiont (Tschermak-Woess, 1988; Rambold et al., 1998; Voytsekhovich et al., 2011; Miadlikowska et al., 2014; Nelsen et al., 2021). It occurs in nearly all terrestrial ecosystems, but is especially frequent in more temperate-boreal/alpine ecosystems (Honegger, 1998, 2009; Sanders, 2001; Pérez-Ortega et al., 2012; Ruprecht et al., 2012; Wagner et al., 2020; Nelsen et al., 2021). Global efforts to characterize the environmental preferences of Asterochloris, a Trebouxia relative, have revealed the differential climatic tolerance of individual species-level groups, while suggesting closely related accessions shared similar environmental preferences and exhibited strong phylogenetic conservatism (Peksa and Škaloud, 2011; Vančurová et al., 2018; Kosecka et al., 2021). Previous multivariate efforts in Trebouxia have revealed coarse-scaled trends such that members of a specific clade (Trebouxia clade “C”) tended to exhibit a more limited climatic preference than others, and a joint analysis of two climatic variables suggested rates of climatic niche diversification across Trebouxia have remained constant through time (Nelsen et al., 2021). However, these two variables (mean annual temperature and precipitation) exhibited similar responses in a phylogenetic principal components analysis, and it remains unclear whether preference toward other climatic variables exhibit similar trends and were shaped by similar evolutionary processes. Here we extend previous work (Nelsen et al., 2021) and ask whether climatic niche evolution varies across climatic variables. These past estimates are then used to make predictions about projected responses to climate change.

We used the K-statistic to evaluate the extent to which climatic preferences of related OTUs resembled one another, relative to those drawn at random. The K-values obtained typically ranged from 0.3 to 1 (Figure 1) and were significant in all cases (except for some BIO3 analyses), indicating that climatic preferences were not independent of phylogeny, and those of closely related taxa tended to be more similar to one another than to those of distantly related taxa. Within climatic variable groups, values were broadly consistent. K-values for traits from climatic variable Groups 1 (BIO1, BIO12), 2 (BIO5, BIO8, BIO10, BIO13, BIO16, BIO18), and 3 (BIO6, BIO9, BIO11, BIO14, BIO17, BIO19) were all consistent with expectations under BM (K = 1) or when trait values for closely related lineages are more similar than expected under a BM model (K > 1). In contrast, K-values for climatic variable Groups 4 (BIO4, BIO7), 5 (BIO15), 6 (BIO3), and BIO2 were lower than expected under pure BM (K = 1).

Our genus-wide disparity through time analyses for individual climatic variables further demonstrated that disparity in these traits was not disproportionately concentrated early in the phylogeny. Mean disparity index (MDI) values for the MCC (Figure 2 and Supplementary Figures 2, 3) and the set of trees derived from the posterior (Supplementary Table 1) were not significantly lower than expected under BM, and therefore, inconsistent with an EB model. In contrast, climatic variable Groups 5 (BIO15), 6 (BIO3), BIO2, and a member of Group 4 (BIO4) regularly yielded MDI values significantly more positive (1—p-value, since test is one-sided) than expected under BM, and instead consistent with an OU model of evolution. The MDI values and significance levels achieved for the other variable in climatic variable Group 4 (BIO7) were also consistent with an OU model of evolution, but significance and MDI values were not as extreme as those for BIO4.

Our subsequent model-fitting procedure confirmed that a BM model of evolution best characterized the evolution of traits in climatic variable Groups 1 (BIO1, BIO12), 2 (BIO5, BIO8, BIO10, BIO13, BIO16, BIO18), and 3 (BIO6, BIO9, BIO11, BIO14, BIO17, BIO19) (Figure 3). In contrast The OU model was favored for climatic variable Groups 4 (BIO4, BIO7), 5 (BIO15), and 6 (BIO3), while support for an OU model was equal to, or slightly higher than that of BM for climatic variable BIO2. Diffusion rate estimates (σ²) for targeted thermal (BIO5, BIO6) and moisture (BIO16, BIO17) variables derived from the best-fit models of evolution (Figure 3) generally suggest that thermal maxima (BIO5) evolved at a slower rate than thermal minima (BIO6), and moisture maxima (BIO16) largely evolved at a faster rate than moisture minima (BIO17) (Figure 4). While there is some overlap among rate estimates within each variable class, distributions appear largely segregated. Finally, our absolute rate estimates of climatic niche evolution varied among OTUs and across variables, but were generally less than 1°C/Myr, regardless of thermal values and approaches employed (Figure 5 and Supplementary Tables 2–4).

Significant phylogenetic signal at the genus level was detected across nearly all variables; however, the magnitude of this signal varied substantially across climatic variables. All variables linked to PC1 (BIO1, BIO12, BIO5, BIO8, BIO10, BIO13, BIO16, BIO18, BIO6, BIO9, BIO11, BIO14, BIO17, BIO19) in previous work (Nelsen et al., 2021) yielded K-values consistent with a BM expectation (K = 1), DTT plots and MDI values consistent with BM, and had a BM model of evolution overwhelmingly favored in our model-fitting analyses. These climatic variables reflect averages and extreme thermal and specific humidity values, and suggest that at this phylogenetic scale, Trebouxia algae have evolved thermal and moisture tolerances under a neutral evolutionary process and at a relatively constant rate.

In contrast, lower (K < 1) or non-significant levels of signal were detected at the genus level for a limited number of climatic variables—all (BIO4, BIO7, BIO15, BIO2, BIO3) of which were linked to PC2 and PC3 in previous work (Nelsen et al., 2021), and reflected thermal or moisture variability (seasonally, diurnally). These variables also yielded DTT plots inconsistent with BM, and MDI values consistent with a non-BM process. Moreover, our model-fitting analyses suggested an OU model was almost always overwhelmingly favored for these variables. Together, this may indicate that, at this phylogenetic scale, the evolution of thermal and moisture variability tolerance may be under selection and evolve toward an adaptive optimum. However, it may instead suggest the rate of trait evolution is exponentially increasing through time (AC model, Blomberg et al., 2003), as these two models are equivalent on an ultrametric tree lacking fossil taxa (Slater et al., 2012; Uyeda et al., 2015); thus, we are unable to distinguish among these two patterns. Regardless, it does reveal a departure from Brownian motion and, together with our findings for other climatic variables, indicates variable evolutionary processes underlie climatic niche evolution in this clade of keystone symbionts.

Our DTT plots, MDI values and model-fitting analyses also demonstrate that at this phylogenetic scale, climatic preference under an exponentially decreasing rate of evolution (EB) consistent with that of an adaptive radiation is highly unlikely. Thus, clades have not evolved exceptionally distinct climatic preferences at deep phylogenetic scales, and instead exhibit overlapping climatic tolerances. This is consistent with previous analyses demonstrating the presence of multiple clades in the same geographic locality (Doering and Piercey-Normore, 2009; Werth, 2012; Leavitt et al., 2013, 2015), and observations that climatic niche divergence of annual mean temperature (BIO1) and specific humidity (BIO12) did not jointly evolve under an EB process (Nelsen et al., 2021). Extending our analyses to include numerous individual climatic variables further demonstrates that at the genus level, early bursts in climatic niche tolerance appear rare.

Our analyses of the evolution of thermal and moisture climatic niche limits suggest an asymmetry such that upper thermal limits of Trebouxia algae evolve at a slower rate relative to lower thermal limits, and that upper moisture limits evolve at a more rapid rate than lower thermal limits. Both of these findings are consistent with previous work. For instance, a greater range of lower thermal limits relative to upper limits was detected in animals (Araújo et al., 2013), and subsequent work integrating evolutionary the age and relationships of a diverse range of animals and plants has further revealed phylogenetic constraints on thermal limits, with lower limits evolving faster than upper limits (Muñoz et al., 2014; Culumber and Tobler, 2018; Liu et al., 2020; Bennett et al., 2021). We emphasize that these limits may represent hard physiological boundaries that may restrict the potential for lineages to adapt in situ to changing climates. Together, these findings suggest the limited potential for several lineages to evolve elevated upper thermal limits in response to increasing global temperatures (Colwell et al., 2008; Liu et al., 2020; Bennett et al., 2021). Similarly, previous work on a range of plants and animals has demonstrated that lower moisture limits evolve at a slower rate than upper limits, suggesting that it may be more difficult to adapt to increased aridity than to increased moisture (Liu et al., 2020). Consistent with other systems, our analyses of the evolutionary legacies of Trebouxia algae suggest a limited capacity to adapt in situ to increased temperature and decreased moisture (relative to decreased temperature and increased moisture). However, we note that lower thermal limits frequently decrease with increasing latitude, while upper thermal limits remain constant across latitudes (Addo-Bediako et al., 2000); this trend could in turn underlie the observed asymmetric rates of thermal niche evolution, as thermal minima may exhibit greater global variation relative to thermal maxima (Liu et al., 2020). We thus encourage further work to disentangle the roles intrinsic and extrinsic variables have played in structuring these discordant rates of evolution.

Our estimates of absolute rates of climatic niche evolution suggested Trebouxia OTUs have evolved at a relatively slow pace, on order with some of the slower rates previously estimated from a diverse range of animals and plants (Quintero and Wiens, 2013; Jezkova and Wiens, 2016). Given that the global heating we are currently experiencing is expected to increase temperatures by 1–4°C (relative to 1986–2005) by the year 2100 (IPCC, 2014)—less than 80 years from now, these algae with their comparatively slow rates of thermal niche evolution [mean rate estimates of 0.13–0.62°C/Myr (BIO1), 0.07–0.71°C/Myr (BIO5), 0.1–1.08°C/Myr (BIO6)] will likely undergo range modification. This suggests that in certain portions of an OTU’s range, individuals are unlikely to adapt in situ to increased temperatures experienced at such a rapid rate, and will instead be required to disperse to more habitable locations, thereby altering their distribution. Previous work has emphasized species migration as a means to persist in the face of changing climates; for instance, the Quaternary record illustrates how species ranges have changed in response to shifting climates (Ackerly, 2003). However, this migration is likely to also affect lichen fungi that depend on these algae for nutrition. Such disruptions of biotic interactions are expected to be one of the major drivers of climate change-mediated local extinction (Cahill et al., 2013), with symbioses in particular anticipated to be especially vulnerable to the effects of climate change (Dunn et al., 2009; Colwell et al., 2012; Kikuchi et al., 2016; Sampayo et al., 2016; Baker et al., 2018; Rolshausen et al., 2018). Thus, algal range shifts may further disrupt interaction webs and force fungi to also disperse or acquire new algal symbionts. We emphasize that our climatic niche variables are not direct estimates of the physiological limits of species, but are instead regarded as proxies. Similarly, our characterization of their niches only encompassed their realized niche, rather than their fundamental niche. We therefore stress the need for detailed studies to identify the physiological limits of these algae, both in culture and in the lichenized state. Despite this, our analyses of past rates of climatic niche evolution paint a troubling picture by suggesting a limited evolutionary capacity for Trebouxia algae to adapt in situ to rapidly increasing temperatures.

Publicly available datasets were analyzed in this study. Data and code used here are provided at: https://github.com/mpnelsen/Trebouxia_Niche_Divergence.

MN, SL, KH, LM, and HL designed research and wrote the manuscript. MN performed research. MN and KH collected, analyzed, and interpreted data. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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