Laetiporus has been shown to be a monophyletic group [11–15, 26]. Unexpectedly, despite conducting multi-locus phylogenetic analyses, our study is still unable to entirely resolve the stem relationships within Laetiporus. Nevertheless, novel phylogenetic species and certain clustering tendencies are described. Findings regarding the origin, ancestral area and diversification are also inferred.
Group I contains two sister clades, Clade D and Clade K, with disjunct distribution (Fig. 1). Phylogenetically, this group is supported by the combined dataset analyses (73% MP, 50% ML, 0.99 BPP). However, Clade D and Clade K are distant in the ITS topology. Previous studies showed that both species grow on hardwood with the common cool temperate to subtropical habitat, producing an orange pileal surface and cream pore surface (11, 13, 15). The complete gene information, as well as a similar growth habit and morphology between Clade D and Clade K, suggests that the phylogeny inferred from the analyses of the combined dataset is more reliable.
Group II consists of four North/Central/South American Laetiporus clades (Fig. 1). Within this group, Clade F is known to reside in temperate to tropical areas with a Pan-American distribution [10, 12, 27]. This distribution indicates a strong adaptive ability. The other three members of Laetiporus that behave as sister species are known to reside only in Central America [14], which is part of the Mesoamerican biodiversity hotspot [28]. Species in this group are found on hardwood and share an orange pileal surface and yellow pore surface, although the characters of Clade L and Clade M are uncertain [15]. Notably, their host plants are usually Fagaceae in North America, tropical plants such as Guarea and Dacryodes in Central America and mainly Eucalyptus in South America [11, 14, 27].
Group III contains four Laetiporus clades from East Asia and South Africa, including the novel phylogenetic species Clade P (Fig. 1). Clade P is found on Abies in cool temperate areas in the Himalayan region. It acts as a sister species with L. ailaoshanensis (Fig. 1), which has been found on Lithocarpus and Castanopsis in subtropical areas in the Hengduan Mountains [15]. Clade O is the other species collected from the Hengduan Mountains, and it grows on Quercus in temperate areas [15]. Clade H is found on Eucalyptus from South Africa, but its characters remain unclear [12]. The relationships between Clade H and the other three species are uncertain due to the low support in the topology of the combined dataset (Fig. 1). Further studies using samples from South Africa are necessary.
Group IV consists of only L. versisporus (Clade G), which has a yellow pore surface (Fig. 1). Previous studies have shown that this species is usually divided into two or three clades [13, 15]. In the current study, L. versisporus specimens grouped together with significant support from MP and ML analyses. L. versisporus covers most parts of East Asia from the Yunnan-Guizhou Plateau, Hainan to Japan and South Korea, and associate with Robinia, Castanea, Quercus, Elaeocarpus and Castanopsis [13, 15]. Infraspecific variation and infraspecific hybridization are considered to occur simultaneously [15].
Group V consists of Clade E1 and Clade E2 (Fig. 1). It is obvious that they are closely related and share similar morphology except for the pore surface [11]. Clade E1 is associated with Quercus, Eucalyptus, Salix, Acer and Fraxinus and has a disjunct temperate to subtropical areas distribution in North America, South America and Europe. Besides, it produces a yellow pore surface [10, 11]. Clade E2 is distributed in temperate areas of North America, is associated with Quercus and Fraxinus, and produces a white pore surface [10, 11].
Group VI consists of Clade C and the novel phylogenetic species Clade Q (Fig. 2). This group is only supported by the ITS phylogeny, and the phylogeny analyses do not indicate an obvious species boundary. This suggests a close relationship between Clade C and Clade Q. Laetiporus Clade C has previously been reported only from Europe [11, 13]. Our study presents the first report of Clade C in Xinjiang, China. This species usually grows on hardwoods and conifers such as Quercus, Sorbus, Populus, Castanea, Prunus, Taxus, Larix and Picea in temperate areas, producing a yellow pore surface. Clade Q is also found in temperate areas in Xinjiang, China, where it is associated with hardwoods such as Salix, Betula and Populus and produces a yellow pore surface.
The maximum crown age of Laetiporus is estimated at the early Miocene (20.17 ± 0.12 Mya) and East Asia and North America are inferred to be the most probable ancestral areas (Figs. 3 and 4). The notable finding is that three coniferous species (L. montanus, L. huroniensis and L. conifericola) in temperate areas behave as sister species in the analyses of the combined dataset (Fig. 1). Moreover, the temperate host plants are diverse, including Quercus, Salix, Populus, Picea, Larix, Abies, Tsuga, Lithocarpus, Fraxinus and Acer; in contrast, the tropical host plants are limited in variety, including Eucalyptus and Guarea [10–15]. Based on these findings, an origin in temperate East Asia and North America is proposed.
The independent sister species in Group I indicate an East Asian–eastern North American dispersal route before the estimated divergence time (4.64 Mya) in the early Pliocene (Fig. 4). This divergence time is close to the break time of the Bering Land Bridge (BLB) at approximately 5.4–5.5 Mya [5]. We speculate that their ancestor covered East Asia and North America via the BLB route and that regional speciation after the vicariance emerged due to the disconnection of the BLB and the severe climate change at that time [29–31]. This route is also present in the dispersal of other organisms, especially the common host plant Quercus [1]. There may be a strong dispersal and vicariance correlation between Laetiporus spp. and their host plants.
Four Laetiporus species in Group II with Pan-American distribution exhibit a North American–Central American–South American dispersal route. This group first diverged at approximately 9.88 Mya. North and Central America are inferred to be the most probable ancestral areas. Clade J, Clade L and Clade M are from Central America and the estimated crown age is approximately 5.38 Mya, which coincides with the paleo-elevations that occurred during the late Miocene and early Pliocene [32]. The second intercontinental distribution between North America and South America is exhibited in Group V (Fig. 4). This route has been confirmed by biogeographical research on plants and animals [1, 33–36]. We speculate that the severe climate change that has occurred since 15 Mya [29] drove the migration from North and Central America to South America and the adaptation to tropical host plants such as Eucalyptus, Guarea and Dacryodes. The vicariance due to tectonic activity is thought to be responsible for the endemism of Laetiporus in Central America.
In Group III, four Laetiporus species from East Asia and South Africa are closely related (Fig. 1). The estimated divergence time of this group is 6.35 Mya. The DEC analysis inferred East Asia as the most probable ancestral area. However, it is notable that Clade H does not form a robust sister relationship with Clade O (Fig. 1). We speculate that there is incomplete sampling from the Indian Subcontinent to Africa because suitable host plants, such as Eucalyptus, are abundant in these areas [12, 37]. Although the estimated divergence time is potentially inaccurate, the dispersal route between East Asia and South Africa is proposed.
The species in Group V also exhibit a continuous distribution in Europe and eastern North America (Fig. 4). The DEC analysis inferred a North American origin for this group, with an estimated divergence time of 2.89 Mya. Clade E1 is found in the eastern North America and Europe with low host-plant specificity. The short-lived North Atlantic Land Bridge acted as a dispersal route until the low Oligocene [6, 36]. Migration to Europe seems unlikely, so the reasonable interpretation is that the human activity introduced this species into new habitats as proposed by Feng et al. [3]. The wind and ocean current could be another driving force and reasonable explanation for the dispersal of fungal basidiospores between Europe and eastern North America [8].
The species in Group VI and Clade A2 have an East Asian-European dispersal route. This route is probable because an exchange of species occurs for Laetiporus and its most common host plants such as Quercus, Salix, Populus, Picea, Abies and Larix [1, 22–25, 38]. It is reasonable to accept this route because the Eurasian Plate is continuous.
Group IV consists of three different types of L. versisporus that are endemic in East Asia (Figs. 1 and 4). The infraspecific variation is obvious in these three types, but gene exchange and recombination still exist according to the clonal research of Ota et al. [13]. This finding indicates that vicariance is important for regional speciation.
In addition, the migration of Clade I to Hawaii is surprising and worth exploring. We speculate that this example results from an incomplete sampling of molecular data. However, there are many standalone islands in the South Pacific indirectly connecting Hawaii, Australia and Malay Archipelago. The frequent strong winds and continuous ocean currents are potentially responsible for the dispersal of basidiospores between islands. The humid climate and abundant host plants such as Quercus, Castanea and Eucalyptus from the Malay Archipelago to Australia [39, 40] are suitable for Laetiporus. A dispersal route of East Asia–Malay Archipelago–Australia–Hawaii seems unlikely. Interestingly, Eucalyptus, the host plant of Clade I has been proven to colonize Hawaii via this route [2].
In our study, the samples of Laetiporus are scanty in some areas around the world, such as South America, Indian Subcontinent, South Africa and Australia. The taxonomic situation is still unclear, and the evolutionary history of Laetiporus remains incompletely understood. A wider range of sampling and further morphological studies, incompatibility tests, and more information of host range and distribution are needed.