The big-leaf mahogany (Swietenia macrophylla King) is a tree mainly found in natural wet and dry tropical forests worldwide, in a wide variety of climatic and edaphic conditions (Mayhew & Newyon, 1998; Navarro-Martínez et al., 2018). Big-leaf mahogany is a highly appreciated fine timber that is an economically important and emblematic species from the Neotropics (Langbour et al., 2011; Navarro-Martínez et al., 2020). Its natural distribution includes fragmented populations from southeastern Mexico along the Atlantic coast of Central America and northern South America, occupying a large geographical arc south of the Amazon, between Brazil, Colombia, Peru and Bolivia (Lamb, 1966; Snook, 1996). This tropical tree has been intensively exploited and subjected to international trade for over 300 years, showing, therefore, a decline in its population size and increased fragmentation in several areas of its natural distribution (Navarro & Hernández, 2004; Grogan et al., 2010). Mexico reports a loss of 76% of the tropical evergreen forest areas containing mahogany trees by the end of the 20th century (Calvo et al., 2000). The original distribution of mahogany in Peru and Bolivia decreased by 4% and 8%, respectively, while a region between Venezuela and Bolivia, underwent 58 million hectares of deforestation until 2001 (representing 20% of the original distribution) (Kometter et al., 2004). In contrast, the Yucatan Peninsula, specifically in protected areas and forest ejidos in Quintana Roo and Campeche, harbors semievergreen and semideciduous forests with abundant and conserved populations of mahogany (Navarro Martínez et al., 2018, 2020). Currently, mahogany is a preferred species for reforestation and the establishment of commercial plantations throughout tropical America (Negreros-Castillo et al., 2018).

Vascular plants host a great variety of soil fungi, being susceptible to soil-borne pathogens, but plant roots are also colonized by non-pathogenic fungi like arbuscular mycorrhizal fungi (AMF) and dark septate fungi (DSF) (Mandyam & Jumpponen, 2005). Remarkably, AMF (belonging to Glomeromycota phylum) are an important ecological and economics group of soil fungi forming symbiotic associations with the vast majority of plants (Wang & Qiu, 2006; Brundrett & Tedersoo, 2018; Chen et al., 2018), including most forest tropical species (Stürmer et al., 2018). In this association, these fungi receive their carbon sources from the plants in exchange for water and minerals (e.g., P, N). As such, they play critical roles in the biogeochemical cycle of C, N and P (van der Heijden et al., 2015). Most tropical forest species have different grades of dependence on AMF, depending on successional stages or soil fertility (Danieli-Silva et al., 2010; Schüßler et al., 2016).

The AMF symbiosis deserves more attention in tropical ecosystems, especially in degraded tropical regions where the availability of nutrients such as phosphorus is a limiting factor in plant growth. Different studies have addressed the identification of AMF spores within the rhizosphere and root colonization (i.e., vesicles and arbuscules) of seedling and mahogany trees in Neotropical natural areas (Herrera & Ferrer, 1980; Rodriguez-Morelos et al., 2014), young plantations in the Amazon region (Noldt & Bauch, 2001; Pereira et al., 2014) and agroforestry systems and tropical forests of Southeast Asia where mahogany was introduced for cultivation (Dhar & Mridha, 2006, 2012; Shi et al., 2006, 2007; Mridha & Dhar, 2007; Nandi et al., 2014). However, the taxonomic identity of the specific AMF species colonizing the roots of S. macrophylla remains unknown. An increasing number of case studies report Glomero mycota molecular diversity from ecosystems worldwide (Husband et al., 2002; Lara-Pérez et al., 2020. A unique molecular operational taxonomic unit (MOTU) nomenclature – virtual taxa (VT) –, was performed to classify AMF rRNA sequences, as implemented in a public database MaarjAM (http://www.maarjam.botany.ut.ee; Öpik et al., 2010, 2014). Then a consistently named system of small-subunit (SSU) rRNA gene sequence phylogroups can be used as a proxy for species and/or higher-level organism identification in ecological research (Öpik et al., 2014).

According to Rodriguez et al. (2009), the DSF (Class 4 endophytes) are distinguished as a functional group based on the presence of darkly melanized septa, and their restriction to plant roots, primarily ascomycetous fungi that are conidial or sterile and that form melanized structures such as inter- and intracellular hyphae and microsclerotia in the roots. DSF are found worldwide and coexist often with different mycorrhizal fungi. They have been reported from 600 plant species including plants that have been considered non-mycorrhizal (Jumpponen & Trappe, 1998). However, studies of endophytic fungi carried out in tropical forests are limited to fungal species that colonize the above ground part of the plant (Arnold et al., 2000; Cannon & Simmons, 2002; Silva et al., 2018). Lately, 55 endophytic fungi (Class 2 endophytes) were isolated from the roots of mahogany monoculture and identified by their rDNA ITS1 region (Rodriguez et al., 2009; Maulana et al., 2018). A close relationship between DSF and AMF with P availability and uptake in plants was suggested. Whereas DSF increases the pool of P in the rhizosphere, AMF are responsible for P transfer to the host, with co-colonization of plants by dual fungal colonization suggesting a synergistic outcome (García et al., 2012; Della Monica et al., 2015).

In the present study, the specific objectives were to (i) describe the diversity of AMF in the roots of S. macrophylla, based on rDNA sequences and (ii) evaluate the dual colonization by AMF and DSF, showing the types of fungal colonization patterns.

The great majority of the VT detected in this study belonged to the Glomeraceae, which is the most widespread and largest family within the phylum Glomeromycota that includes 16 genera (Wijayawardene et al., 2022). Nowadays, comParisons between AMF diversity studies are difficult due to the scarcity of studies in natural tropical ecosystems, and the different methods used (e.g., spore identification, PCR-cloning, Terminal Restriction Fragment Length Polymorphism, pyrosequencing) (Rodríguez-Echeverría et al., 2017). A high diversity of AMF colonizing the roots of woody species from tropical forests has been reported (Husband et al., 2002). However, morphological and molecular studies on colonized roots by AMF in neotropical rain forests are still limited.

In general, a high predominance of Glomeraceae has been observed by spore identification within the rhizosphere of mahogany mature trees, like in agroforestry systems established in Bangladesh (Dhar & Mridha, 2006, 2012; Mridha & Dhar, 2007), tropical evergreen forest (plantation) in China (Shi et al., 2006, 2007), or young plantations in the Atlantic Forest in Brazil (Pereira et al., 2014). However, Rodríguez-Morelos et al. (2014) observed a weaker predominance of Glomeraceae in mature trees of tropical rain forests in Mexico; they reported 21 AMF spore morphotypes, primarily of Glomeraceae (52.3%) and Acaulosporaceae (38%). Indeed, in general co-dominance by Glomeraceae and Acaulosporaceae in the tropical forest has been suggested (Leal et al., 2013). Our study showed a single VT belonged to the Diversisporaceae family. Likewise, an AMF spore morphotype (e.g. Diversispora aurantium) was identified whatever the phenological stage of mahogany (Rodríguez-Morelos et al., 2014). We recorded a VT (Glomus macrocarpum), previously reported as the dominant AMF in the mahogany plantations (Pereira et al., 2014). Members of Acaulosporaceae, Ambisporaceae, Gigasporaceae and Paraglomeraceae previously reported in mahogany rhizosphere were not found colonizing roots (Rodríguez-Morelos et al., 2014). These groups are characterized by a limited ability to colonize the roots (Hart & Reader, 2002; de Souza et al., 2005). Conversely, the Glomeraceae family allocates energy to high intraradical colonization (e.g., arbuscules, vesicles, coils, and unspecialized hyphae) (de Souza et al., 2005). Root colonization levels for individual fungi strongly depend on the host tree species and the colonization strength does not correlate with plant growth promotion (Schüßler et al., 2016).

Results from studies based on the isolation of glomerospores from the rhizosphere of mahogany and the molecular approach in this study indicate that the predominant AMF species belong to the genus Glomus. Certain species in this family have been shown to be easily propagated in trap cultures, which can be used to obtain inoculum for commercial forest plants (Schüßler et al., 2016). The interaction between these species and mahogany seedlings has been demonstrated to improve their relative growth rate and water potential, both of which are key factors in increasing seedling survival in commercial plantations and restoration programs (Rajan et al., 2020). The first step in this process is to isolate the AMF from seed tree stands in conserved and natural populations of mahogany, and then to test the different effects of single AMF species or consortia in controlled environments (Holste & Kobe, 2017). An alternative method is to collect roots from mahogany to generate trap cultures and obtain the species associated with the target species, although this approach is less successful than using soil to increase inoculum. The percentage of root length colonization by DSF is high, which could be significant for nutrient acquisition by established adult plants and seedlings. It is noteworthy that some species of the DSF can be isolated and cultured successfully using basic techniques (Maulana et al., 2018).

In this study, a high AMF colonization (80%) in mahogany roots was observed. Meanwhile, colonization between 30% and 69.3% was reported from plantations of introduced populations of mahogany in Southeast Asia (Shi et al., 2006; Mridha & Dhar, 2007; Dhar & Mridha, 2012; Nandi et al., 2014) and 53.2% was noticed on mahogany plants after two years in Costa Rica (Holste & Kobe, 2017). A pot experiment found a percentage of root colonization between 27.4% and 44.9% according to AMF inoculation after 180 days (Rajan et al., 2020). Our findings show different percentages of root colonization in mahogany compared to previous works, where they have studied various environments such as natural and introduced populations (Shi et al., 2006; Mridha & Dhar, 2007; Rajan et al., 2020). However, all works showed that mahogany is consistently colonized by AMF.Our results showed mainly Paris-type colonization in roots. Similarly, a predominant presence of Paris type colonization was observed in Meliaceae trees of natural forest (Smith & Smith, 1997; Shi et al., 2006). Noldt & Bauch (2001) recorded in roots of mahogany seedlings, under plantation, structures of the Arum-type, with appressoria penetration and coiled hyphae with a high frequency of vesicle and arbuscules. Arum-type colonization is more commonly found in crop plants while Paris-type is more common in plants from natural ecosystems (Matekwor Ahulu et al., 2005), although several studies (Dickson, 2004; Yamato, 2004) have found that AMF morphological structures appear to be dependent on individual plant species, the fungal species involved, and environmental conditions (e.g. salinity, drought). Additionally, these fungal symbionts have been reported to be functionally distinct (Jumpponen, 2001). In our study, the simultaneous occurrence of DSF and AMF were observed which is consistent with the findings of Muthukumar et al. (2006) and Zhao et al. (2016), in tropical ecosystems. Also, Rodríguez-Morelos et al. (2014) recorded a percentage of mahogany root length colonization by DSF between 6.09% (trees) and 5.5% (seedlings). However, despite the mix colonization, we do not know the functions in Meliaceae. Further investigations need to be done towards the identity of the fungi and to carry out an experimental assay to test their functions, and to consider implementation in mahogany seedling production. More recently, 55 endophytic fungi (Class 2 endophytes according to Rodriguez et al., 2009) were isolated from a S. macrophylla plantation and identified by the rDNA ITS1 region (Maulana et al., 2018) elucidating a high DSF diversity associated with mahogany roots.

Remarkably, inoculation of tropical tree seedlings with AMF can improve tree growth and viability, but efficiency may depend on plant and AMF genotype (Schüßler et al., 2016). Particularly, a differential effect of AMF inoculation on mahogany was noticed lately (Holste & Kobe, 2017; Rajan et al., 2020). Furthermore, dual colonization by AMF and DSF may aid plants in surviving in highly stressed environments (Della Monica et al., 2015; Zhao et al., 2016).

The roots of S. macrophylla display a mixed colonization pattern, with both AMF and DSF. Percentage of root length colonization was significant higher in AMF than DSF. Among the AMF, there was a predominance of the Paris-type colonization in the roots, while the presence of septate hyphae characterized the DSF; AMF and DSF were colonized. Through metabarcoding, 14 virtual taxa (VT) belonging mainly to the genus Glomus and Diversispora were obtained, VTX00186 being the most abundant. However, information on the diversity and the effect of the dual colonization by AMF and DSF on tropical trees remains unknown. Studies of dual colonization by AMF and DSF would deserve more attention due to the little knowledge about the diversity and potential of these fungi in association with tropical plants. Studies related to the production of fungal inoculum for the production of tropical plant species are necessary. Despite its paramount importance, currently, there is only limited use of these fungi in reforestation programs on a large scale.