The slippery jack circled the world with pines — its genomes say from where

The passenger nobody listed

Plantation forestry keeps records of its trees. The fungi that arrived with the seedlings kept none: ectomycorrhizal partners travel attached to roots and to the soil around them, and no shipping manifest has ever named one. Suillus luteus has been reported under introduced pines in Australia, New Zealand, South America and Africa for more than 150 years, on hosts it never had in Eurasia — chiefly Pinus radiata, P. patula, P. ponderosa and P. contorta — and nobody wrote down where the fungus itself had come from.

A study by Yi-Hong Ke, Rytas Vilgalys and twenty co-authors supplies the missing half of that record. It appeared online on 25 June 2026 and in the September issue of New Phytologist, volume 251, issue 5, pages 2964 to 2979. The team sequenced 208 whole genomes of S. luteus, the slippery jack, alongside five of its sister species S. brunnescens, drawn from the native range in Europe and Asia and from North America, South America, Africa, New Zealand and Australia. The paper is open access.

Restricted to S. luteus alone, the dataset held 2,617,891 single-nucleotide polymorphisms, or 6.54 per cent of the non-repeated portion of the reference genome. The tree built from them broke into three deeply divergent clades: one Asian, one northern European holding every Norwegian collection, and one central European. Every introduced population sampled, from Bariloche in Argentina to Western Australia, sits inside the third.

Dating a founding event from allele frequencies

Genetic distance was measured as Fst. Against the central European population the smallest value was North America's, 0.0042, followed by New Zealand at 0.0203, South America at 0.0246 and Australia at 0.0261. These describe how variation is distributed, not how many mushrooms stand in a wood; they invite that misreading because they look like a tally.

Demographic models converged on a divergence from the European population, represented by Belgian collections, 72.5 plus or minus 2.5 generations ago. The figure rests on two assumptions the authors state openly: a generation time of ten years, on the grounds that most S. luteus reproduce on trees five to fifteen years old, and a mutation rate of ten to the minus seventh per site per year, borrowed from another introduced basidiomycete, Serpula lacrymans. It dates no particular shipment; it sets an order of magnitude consistent with the documented timing of pine planting. The negative result is firmer: a model forcing dispersal before human activity, more than a thousand generations ago, scored over 200 AIC units worse and was rejected.

The European effective population size came out at 3,300 plus or minus 50. Outside North America the introduced populations began far smaller, with founder effective sizes from 5.5 in Western Australia to 456 in New Zealand. Some then grew sharply — Western Australia 524-fold, Bariloche 35-fold — while New Zealand and the rest of Australia expanded less than tenfold.

North America refuses the pattern

One region declines to fit the bottleneck story, and it is the most interesting thing in the paper. In North America the constriction was negligible: initial and current effective sizes are the same number, 2,786, with a growth rate of zero and diversity comparable to Europe's. Admixture analysis could not separate the two up to K = 8.

The authors connect this to what the tree is for. Scots pine, on which S. luteus is dominant in North America, is used there mainly for horticulture — Christmas tree farms, roadside plantings, windbreaks, shelter belts, erosion control — rather than by the industrial plantation forestry that carried pines across the southern hemisphere. The fungus itself has been reported from North America since 1887, on the authority of Charles Peck.

Space behaves differently too. In the native central European population genetic distance rises with geographic distance, a Mantel test giving P below 0.05 — textbook isolation by distance. In New Zealand and North America, where the same test was run, it does not: subpopulation structure is present and clear, a G-test returning P = 0.001, but it does not follow the map.

Where the argument stops

In Poland, and anywhere in Europe, the slippery jack is native. The introduction story belongs entirely to its range overseas. Nor does the paper document deliberate inoculation of seedlings; it infers transport accompanying pine planting, which is not the same as intent.

Sampling was uneven: Africa is represented by a single collection, and the demographic models leaned on three representative demes, Belgium, Bariloche and Massachusetts. The generalisation is hedged too — the patterns are, in the authors' wording, likely also applicable to other pine co-introduced ectomycorrhizal fungi, an expectation rather than a demonstration.

The clades need the most care. They are not three described species; none carries a name. Neither are they merely populations within S. luteus: the authors write that the differentiation strongly implies sympatric reproductive isolation as cryptic species, and that the Asian clade probably represents a species distinct from the central European one, with any morphological differences still awaiting observation. Separately, three Japanese collections labelled S. luteus fell in with S. brunnescens, and raising a new species from them would, the authors say, require additional study.

What it looks like in the litter

The reversal is what lingers. A mushroom known by its glutinous cap and a membranous ring banded in purple-brown is entirely ordinary in a European pine wood, and that ordinariness is why its European genomes could serve as the reference against which four other continents were measured. The narrow start of the overseas populations remains legible in their descendants, not as a count of fruit bodies but as variation that never arrived. The publisher's version and the underlying data are at the journal page and in the PubMed record.