Discovering new species of porcini using food, phylogenetics and fieldwork

Introduction 

Porcini (Boletus section Boletus) are among the most highly sought-after wild, edible mushrooms in the world. Porcini is the plural form of porcino, the Italian word for ‘little pig.’ (In English common use, the word porcini is used for both the plural and singular). Wherever they occur naturally, local people have discovered their edibility, but their identity is most strongly tied to Italy, where porcini have become quintessential ingredients in Italian cuisine and major export products. 

Centered on the best-known species, Boletus edulis, there are some 25 named and accepted species, only four of which are native to Europe (B. aereus, B. edulis, B. pinophilus, B. reticulatus). Although they are best known from Europe and North America, recent discoveries from food markets and fieldwork have turned up new species from all over the world, including tropical Australia, Southeast Asia, China, Central America, North America (including the Caribbean) and even Europe. These new discoveries are reshaping our understanding of the diversity, distribution, and evolution of these iconic mushrooms. 

Description 

Porcini are fleshy mushrooms with a stalk (stipe), and cap (pileus) that supports a sponge-like compact layer of spore-bearing tubes instead of gills (lamellae) underneath, a feature shared by a diverse group of fungi known as the boletes (family Boletaceae). 

They are typically large (some single mushrooms have topped 2 kg!) and conspicuous, often with a distinctive network of raised veins (reticulum) covering the top third to the entire surface of the stalk, although this feature is not unique to the group. One feature that is diagnostic for porcini (but also not restricted to them) is that the tube mouths (pores) are covered by a layer of tangled, white hyphae when they are young, a condition often referred to as 'plugged' or 'stuffed' pores. This layer of hyphae disintegrates at maturity, exposing the openings to allow the falling basidiospores to exit the tube, which is typically accompanied by the development of olive-yellow to cinnamon-brown pigmentation in the spores and hyphae composing the tubes. 

Like most boletes, porcini are thought to be obligately ectomycorrhizal (they need a plant partner, which they connect to through the root system), and they associate with all of the major and minor ectomycorrhizal plant hosts in their range, including the families Betulaceae, Cistaceae, Dipterocarpaceae, Fagaceae, Myrtaceae, Pinaceae, and Salicaceae. The scale of this interdependent relationship between fungus and plant cannot be overstated -- it has been estimated that a single B. edulis mushroom may require up to 14 million ectomycorrhizal root tips and 1800 km of hyphae in the soil to provide it with enough nitrogen!

Phylogeny and biogeography 

In a molecular analysis of 16 porcini species, four divergent groups were proposed that could be given provisional generic names: 'Boletus', 'Alloboletus', 'Inferiboletus', and 'Obtextiporus'. The former two groups were previously determined to be highly divergent from each other, which initially cast doubt on the monophyly of porcini (whether all the species have the same evolutionary origin). 

Although at the time porcini were thought to occur primarily in north temperate regions, the Inferiboletus and Obtextiporus lineages were represented by two then-undescribed, newly-discovered species from Australia and Thailand, respectively, suggesting a paleotropical origin somewhere in Australasia or Indo-Malaya approximately 40 to 50 million years ago. 

More recent additions from tropical and subtropical Asia to the porcini tree have shown a basal grade, blurring the boundaries of the four groups and lending further evidence for a tropical Asian origin. Further evidence of a paleotropical origin somewhere in the Australasian/Indo-Malayan region was provided by the recent phylogenetic placement of the New Zealand endemic B. semigastroideus (formerly Notholepiota areolata), a peculiar secotioid form that is an ancient relative of the Alloboletus clade. To date, no porcini native to sub-Saharan Africa have been reported, which suggests they arose after the disarticulation of Gondwana, consistent with the younger estimated date of origin. 

Porcini probably first evolved when the Earth was warm and humid, and the angiosperms (flowering plants) were diversifying. This is also around the time when the obligately mycorrhizal oak family (Fagaceae) originated in southeast Asia, diversifying and extending northward to Eurasia and North America. The pattern of Fagaceae origins and biogeography mirrors emerging patterns in porcini phylogeny, perhaps providing a clue to an early driving force behind porcini diversification. 

Discovery is fundamental to conservation 

Porcini have thrived for millions of years, multiplying into a panoply of species, each of which occupy essential roles in partnership with plants. Yet many of them today still remain mysterious, even in our meals. Our work in the food markets and the jungles is essential to delving deeper into their delicious and diverse world, helping us better understand where they came from and how to make sure they thrive in the future.