Transgenerational effects of mycorrhiza are stronger in sexual than in clonal offspring of Fragaria vesca and are partly adaptive

Abstract

1 INTRODUCTION

Plants possess the remarkable capability to alter their phenotype to suit the changing environmental conditions without the need to change their underlying genetic code, which is commonly referred to as phenotypic plasticity. Although phenotypic plasticity is typically regarded as an intra-generational phenomenon, it has also a transgenerational overlap (Germain & Gilbert, 2014). Transgenerational plasticity, also known as transgenerational, maternal, or parental effects, occurs when the offspring's phenotype is influenced by the ecological interactions of their parents. These effects can be a powerful driver of plant adaptation to local environmental conditions, particularly in situations when the offspring environment is predictable (Bonduriansky et al., 2012; Latzel et al., 2014; Yin et al., 2019).

Transgenerational effects (TGE) can be mediated by two mutually non-exclusive mechanisms: storage and hormones in propagules (Roach & Wulff, 1987) and/or epigenetic mechanisms via alteration of gene expression through heritable variation in cytosine methylation or histone modifications (Boyko et al., 2010; Richards et al., 2017; Rossiter, 1996; Sammarco et al., 2024). Since most plant species reproduce both sexually and asexually (clonally) but clonal and sexual reproduction represent fundamentally different strategies with distinct ecological and evolutionary implications, it is striking that their comparative roles in TGE remain insufficiently explored (González et al., 2018; Latzel & Klimešová, 2010; McKey et al., 2010). For example, whilst meiosis resets most environmentally induced epigenetic changes in sexual reproduction (Dziegielewski & Ziolkowski, 2021), this is not the case in clonal reproduction. Moreover, clonal offspring may receive different levels of storage and hormones compared with sexual offspring from the same parent. Finally, whilst clonal reproduction allows for efficient local spread and persistence, sexual reproduction plays particularly a critical role in long-distance dispersal (Winkler & Fischer, 2002). In other words, the environment for clonal offspring could be more predictable than for sexual offspring suggesting stronger evolutionary potential of TGE in clonal reproduction. All these specific aspects of TGE between clonal and sexual offspring could further influence the ecological and evolutionary significance of TGE between the two reproductive strategies.

The majority of evidence on the ecological and evolutionary importance of TGE in plants comes from studies on sexual species (reviewed by Yin et al., 2019). However, pioneering studies have already shown that TGE strongly influences also asexual generations. Verhoeven and van Gurp (2012) found that exposure to various stresses can cause transgenerational phenotypic effects in apomictic, that is, clonal dandelions. In the clonal plant white clover (Trifolium repens), TGE is common, genotype-specific, and potentially under epigenetic control (González et al., 2018). Dong et al. (2019) discovered that TGE can be transmitted via vegetative reproduction in plants, and the effects can be more pronounced under high resource availability, depending on the organ from which the offspring are produced. Overall, these and other studies suggest that clonal TGE plays a crucial role in clonal plants and can vary depending on environmental conditions. Additionally, clonal TGE can be adaptive, meaning that clonal offspring may perform better if they experience their parental than non-parental environment. For instance, González et al. (2017) provided clear evidence that clonal offspring of white clover are better able to cope with water shortage if their parental clone also experienced drought. Despite the prevalence of clonal species in many ecosystems (up to 70% in some cases; Klimešová & Herben, 2024; Klimešová & Klimeš, 2006), there has been no direct comparison of the ecological and evolutionary implications of TGE between clonal and sexual reproductive strategies thus far.

Most studies on TGE have focused on plant interactions with abiotic environmental factors, such as nutrients, water, and temperature, and have shown that these effects can be adaptive and thus evolutionarily significant (Latzel et al., 2014; Yin et al., 2019). However, the role of biotic interactions, which are key to species coexistence, biodiversity, and ecosystem functioning (Kraft et al., 2015; Valladares et al., 2015; van der Putten et al., 2013), has been largely overlooked in TGE studies. Biotic interactions present a layer of complexity not typically encountered with abiotic factors. Mycorrhizal fungi, which associate with over 80% of plant species (Smith & Read, 2008), form intricate relationships with their host plants, significantly influencing nutrient uptake and stress tolerance (Johnson et al., 1997; Smith et al., 2010). These interactions are dynamic and can vary widely depending on environmental conditions, plant species, and the specific fungal species involved. For example, plants benefit more from mycorrhiza when phosphorus (P) or water is limited compared with when these resources are readily available (Martínez-García et al., 2012; Voříšková et al., 2019). The few studies on TGE published so far have shown that mycorrhizal parents can produce offspring with better performance or fitness compared with non-mycorrhizal parents, particularly when essential resources are limited (Heppell et al., 1998; Koide, 2010; Puy et al., 2022; Varga et al., 2013). However, they did not analyse whether TGE triggered by mycorrhiza can be adaptive.

The extent of TGE due to mycorrhiza between clonal and sexual offspring might be further influenced by the potential differences in symbiont dependence between these reproductive groups. This could be because clonal plants, by virtue of their growth form, often have the ability to spread horizontally and access resources directly through interconnected ramets, reducing their reliance on fungal symbionts for nutrient acquisition (Hempel et al., 2013; Onipchenko & Zobel, 2000). Furthermore, clonal individuals can compensate for the service of the fungal symbiont by sharing resources between ramets in patchy conditions (Alpert & Stuefer, 1997). In contrast, sexual plants, which do not have this interconnected growth habit, may depend more heavily on mycorrhizal fungi to enhance nutrient uptake and stress tolerance (Dominiak-Świgoń et al., 2021; Fu et al., 2010). This differential reliance on mycorrhizal associations implies that the benefits conferred through TGE from mycorrhizal parents could be more critical for the fitness and performance of sexual offspring. In clonal plants, the reduced dependency on mycorrhizal symbiosis might result in a weaker expression of TGE related to mycorrhizal interactions.

To sum up, it is evident that some aspects of TGE, such as the higher fidelity of epigenetic inheritance in clonal than sexual reproduction, may promote stronger TGE in clonal offspring compared with sexual offspring. However, other factors, like potentially distinct relationships with mycorrhizal symbionts amongst individuals of different reproductive origins suggest that TGE could play a stronger ecological role in sexual than in clonal offspring. It is intriguing that we still lack a rudimentary understanding of whether these contrasting aspects of TGE translate into functional differences between different reproductive strategies in plants. In this study, we investigated TGE of mycorrhiza on the performance of sexual and clonal offspring of the clonal herb Fragaria vesca, comparing conditions when mycorrhiza highly benefits the host plant (low phosphorus (P) availability) and when it provides no benefits in terms of P nutrition or growth (high P availability). We hypothesized that the ecological and evolutionary significance of TGE varies between clonal and sexual offspring based on the specific environmental conditions experienced by the parent. Specifically, we propose that parental mycorrhizal associations will have a more prominent impact on TGE in sexual offspring, whereas abiotic conditions such as P availability will trigger stronger TGE in clonal offspring. Furthermore, we hypothesized that TGE due to mycorrhiza would be modulated by P availability to parents and offspring, as it influences the intensity of resource exchange and strength of mutualism between the partners. We also generally expected adaptability of TGE as previously reported for TGE due to abiotic conditions (González et al., 2017; Yin et al., 2019), that is, that offspring would perform better if they experience their parental environment as compared with non-parental conditions. To test these hypotheses, we conducted a controlled experiment, where we exposed parental plants to different combinations of mycorrhiza and P availability and assessed the resulting TGE in their (a)sexual offspring, which experienced both the parental and non-parental environments.