Earthstar

Indulge me by letting me start with a short quote from The Forest Unseen:

August 8th — Earthstar. Summer’s heat has coaxed another flush of fungi from the mandala’s core. Orange confetti covers twigs and litter. Striated bracket fungi jut from downed branches. A jellylike orange waxy cap and three types of brown gilled mushroom poke from crevices in the leaf litter. The most arresting member of this death bouquet is the earthstar lodged between rafts of leaves. Its leathery outer coat has peeled back in six segments, each segment folded out like a flower’s petal. At the center of this brown star sits a partly deflated ball with a black orifice at its peak.”

And, several years later, right on cue in early August, here are the earthstars in Shakerag Hollow. They must be the most gorgeous fungi ever. The one pictured above is Geastrum saccatum. It is about the size of a quarter.

My essay in The Forest Unseen rambles off in the direction of golf balls. Here I’ll keep my eye on the fungus.

Earthstars belong to the Gasteromycetes (“stomach fungi”), a motley collection of mushrooms that hold their spores in a stomach-like sac. Other members of the group of puffballs, stinkhorns, and bird’s nest fungi. Unlike the gilled mushrooms, bracket fungi, and others that forcibly eject spores using microscopic catapults, Gasteromycetes take an entirely passive approach to dispersing their spores. They hope for a raindrop, or the step of a beetle, or a prod from a falling twig to puff their spores into the air.

The evolution of these Gasteromycete fungi reveals some interesting evolutionary processes. Once the “stomach-like” form evolves, there is no turning back. Although “normal” gilled mushrooms have evolved earthstar or puffball-like structures at least four times, there are no known evolutionary transitions in the other direction. Why? The catapult mechanism is so complicated that it is very unlikely to re-evolve once it has been lost. After the catapult genes have been discarded, only a very long stretch of time and some lucky mutations could bring them back.

The loss of forcibly discharged spores has unexpected consequences. It turns out that having a passive spore dispersal mechanism makes a species more likely to split into new species. Exactly why this should be so is not clear. Perhaps their spores do not travel as far, so allow populations to become isolated from each other, leading to reduced gene exchange and then speciation? Regardless of the mechanism, the Gasteromycete fungi have been speciating more rapidly than their gilled relatives.

So the future belongs to the earthstars. Especially those that can figure out how to eat golf balls.

____

For those interested in digging deeper, the relevant articles in the scientific literature are listed below.

Hibbett, D. S., E. M. Pine, E. Langer, G. Langer, and M. J. Donoghue. 1997. Evolution of gilled mushrooms and puffballs inferred from ribosomal DNA sequences. Proceedings of the National Academy of Sciences, USA 94: 12002–6.

Wilson, A. W., Binder, M. and Hibbett, D. S. (2011), Effects of gasteroid fruiting body morphology on diversification rates in three independent clades of fungi estimated using binary state speciation and extinction analysis. Evolution, 65: 1305–1322. doi: 10.1111/j.1558-5646.2010.01214.x

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