Discovering new species of porcini using food, phylogenetics and fieldwork
New species of porcini are turning up in all sorts of locations, from tropical Australia to your local food market. Kew's Head of Mycology, Bryn Dentinger, describes how these discoveries are helping to unveil the origins of porcini and their complex evolutionary history.
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 (see Dentinger et al. 2010, Dentinger, 2013 and many others listed below). These new discoveries are reshaping our understanding of the diversity, distribution, and evolution of these iconic mushrooms.
Porcini are fleshy mushrooms with a stalk (stipe), and cap (pileus - see a below) 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 (see b above), 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 (c). 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 (c).
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 (Taylor & Alexander, 2005)!
The missing diversity right under our noses
Given their prized flavor, it is perhaps not surprising that new species of porcini have been discovered recently from food markets. The distinctive new species Boletus albobrunnescens, which is initially pure white all over but turns dark brown when handled, was first known to science from specimens gathered in an Isaan market in Thailand (Arora, pers. commn.) and then later from the wild (Halling et al., 2014). We recently discovered three new species in a single commercial packet of porcini sold in a London grocer (Dentinger & Suz, 2014). The latter had been previously reported from porcini imported to Italy from China, where Chinese porcini accounts for around 50% of dried porcini on the market (Sitta & Floriani, 2008), but lacked scientific names until last December (Dentinger, 2013). These new discoveries are important to facilitating proper identification of species in food products containing porcini, but they also add important branches to the expanding porcini tree of life.
The elusive and the cryptic
Since 2000, 18 new species of porcini have been discovered, but descriptions and names have been given to only eight of them. At least one of these (B. pinetorum from northern Europe) is probably the same as B. edulis, whereas the remainder of the species appear to be distinct morphologically, geographically, and phylogenetically. However, the sampling is still vastly incomplete, as shown by Feng et al. (2012) who revealed that there are at least 12 species in Asia (Bangladesh, China, and Tibet) that have either not been documented previously or were masquerading under different names. Moreover, there are described species for which we still do not have DNA sequences, primarily because no modern material has been collected since their original description (eg Boletus phaeocephalus from tropical southeast Asia and Boletus gigas from the Himalayas).
The tropical Australasia and Indo-Malaya regions appear to harbor an exceptionally rich community of porcini and other ectomycorrhizal fungi. Recent fieldwork in Sarawak on the island of Borneo has resulted in the discovery of three more unnamed species of porcini, and ongoing DNA sequencing from preserved material from around the world has revealed both additional species not previously considered as well as cryptic species (Dentinger et al., unpubl.).
Compiling all of these sources of diversity into one as-yet-unpublished dataset shows that currently more than 50% of the known porcini diversity has yet to be named and properly documented.
And yet we have only just begun to explore the tropical forests for fungi, which are estimated to number upwards of 5-6 million species (O’Brien et al., 2005; Taylor et al., 2014), so many more species of porcini likely await discovery. More survey work in these severely underdocumented habitats is urgently needed before we lose the opportunity to document these critically important and economically valuable fungi.
Phylogeny and biogeography
Dentinger et al. (2010) were the first to apply a modern molecular analysis of porcini species, where they included 16 species of porcini in a multi-locus phylogenetic dataset. Their analysis recovered four divergent groups that they proposed 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 (Binder & Hibbett, 2006), 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 (Dentinger et al., 2010).
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 (Feng et al., 2012). 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 Boletus semigastroideus (formerly Notholepiota areolata), a peculiar secotioid form that is an ancient relative of the Alloboletus clade (Nuhn et al., 2013).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 (Manos & Stanford, 2001). 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.
- Bryn -
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