Islands are unique in that they’re comprised of a subset of species from the mainland. The distance of the island from the mainland helps explain the larger scale ecology, as well as the small scale genetic processes. Islands closer to the mainland receive more gene flow, as genetic material readily makes its way into the genomes of the island populations. Species living on islands closer to the mainland resemble those original mainland populations more so than populations living on more isolated islands. When a distant pine population existing on an isolated island receives no pollen, or seeds from the mainland, over millennia, a process called speciation occurs. If isolated for long enough, many times the separate populations cannot reproduce, and the island organism becomes a new species. A pine native to the islands of Southern Japan from the Kagoshima Prefecture followed this trend, and like many isolated island species is endangered.
Previous to this study, the mutualisms Pinus amamiana engages in was largely unknown. So, researchers Masao Murata, Seiichi Kanetani and Kazuhide Nara set out to study the endangered plant from a mycorrhizal point of view. Before, we only really knew P. amamiana like other pines joined forces beneath the forest floor with ectomycorrhizal fungi (ECM). These scientists wanted to change that, and learn about a fundamental component of the plants fitness. To do this, they collected soil samples from island forests that supported P. amamiana populations. They identified the soil fungi and conducted bioassays, growing different species of ECM trees, including P. amamiana, with some of the fungi they isolated. Their results attracted ecologists and mycologists alike, and have left an impactful impression on the biological conservation scene.
Within their soil samples, these researchers isolated a new species of Rhizopogon. The fruiting bodies of the species within this genus are hypogeous, forming subterraneous, globular structures. These spore-filled fruit bodies are analogous to truffles, and entice mammals to vector their dispersal through mycophagy. One can see the sense it makes in dubbing this genus as the false truffles. After isolating and sequencing their genes, these scientists couldn’t match it with any other Rhizopogon genome deposited in the international sequence database. With this, a new species was discovered, but disappointingly, in this study they just refer to it as Rhizopogon sp. 1. Why not give it a cool name!? I guess they wanted to be 100% sure the species was previously undescribed.
When they grew the fungal isolates with the four different species of ECM trees, (P. amamiana, P. parviflora, P. densiflora and Castanopsis sieboldii) they found that Rhizopogon sp. 1 didn’t lose its ability to form mutualisms with the mainland species, but did so to a lesser degree. This empirical test really nails home the processes island speciation. Other mushrooms isolated from the soil on these islands utilize wind dispersed spores, so mainland genes readily make their way into these island ecosystems. Mainland, aerial spore forming fungi maintain a more general ecology, forming mycorrhizae with a wide range of tree species. However, this truffle like species is en route to becoming more specialized, as mycophagous mammals with ingested Rhizopogon spores have an exceedingly lower chance of making it to these isolated islands. These processes take millions of years, so we are witnessing an intermediate stage at which Rhizopogon sp. 1 hasn’t fully lost its ability to pair with trees its ancestors had, but has a preference for forming mutualisms with the Pinus amamiana trees it has co-evolved with on these Japanese islands.
This study highlights the importance of taking a mycological approach in the field of biological conservation. To aid efforts in maintaining these endangered tree species, tree nurseries should inoculate Pinus amamiana with the newly discovered false truffle-Rhizopogon sp. 1. Just like its plant counterpart, living from the forest floor on isolated islands is taking effect, and an intermediate stage in the fungi’s speciation can be observed. Without much gene flow from the mainland, because of its evolutionary dependence on mammalian dispersers which, let’s face it, can’t swim well, Rhizopogon sp. 1 is slowly specializing in forming mutualisms with the tree species that inhabit the same islands from the Kagoshima Prefecture. With this in mind, I thereby propose the species name, Rhizopogon kagoshima, to mirror its island confines. What do you think?