Even the father of modern biology Charles Darwin could not explain some of the evolutionary patterns he saw. Referred to as his “abominable mystery”, Darwin could not understand how rapidly angiosperms (flowering plants) diversified. Once plants engaged in mutualisms with flying insects by offering a sugar reward in exchange for pollen transfer, angiosperms permeated to almost every ecosystem on Earth. Before plants “figured this out”, reproduction ensued after pollen was transported to female organs via wind, or by flagellated sperm finding female plant organs-like in bryophytes and ferns. These strategies have been in use since land plants evolved, so I don’t want you to think these plants are inferior. Rather, let’s realize the flowering adaptation as an extraordinary mode of reproduction for sessile organisms. In fact, wind pollinated plants make up most of our diet (wheat, rice, corn, rye, barley, and oats). In expansive monoculture stands, wind pollination is extremely successful, but most plant species occur at more patchy distributions, which is why insects helped plants through a period of rapid adaptive radiation.
Charles Darwin
Instead of relying on the stochastic process of wind pollination, by pairing with insect’s, plants became better suited at living in diverse ecosystems. Their partner actively searched for more nectar in a complex three-dimensional world. With so many insect species already on Earth, selection favored plants that adapted different morphological structures and scents to attract a specific mutualist. Remember, competition reduces the fitness of both parties, so it’s common for a plant to be a specialist. Still, these ecological forces don’t explain the speed at which angiosperm dominated Earth. Recently, using phylogenomic analyses of sequenced plant genomes, scientists realized that land plants went through a series of whole-genome duplication events. These events are central for adaptive processes across the plant, animal and fungal kingdoms. Plants after these whole-genome duplication events that followed this pollinator trajectory went on an evolutionary tangent that is still going on today.
There is no doubt that pairing with insect’s aids organisms in diversification. For this reason, I think one family of fungus is going through a period of adaptive radiation now. The stinkhorns which make up the Phallaceae are a group of closely related fungi that have differentiated from their ancestral condition of wind spore dispersal. Instead, the stinkhorns produce a sticky, smelly, spore filled mucus called gleba, that attracts a diverse assemblage of insects. Gleba contains carbohydrates that insects can metabolize. After consuming some of the gleba, flying/crawling insects become covered in fungal spores. Like insects, fungi have a similar requirement for temperature and water, which makes them perfect vectors for spore dispersal.
Phallus indusiatus
It’s hard to not see this analogous condition of pollination/spore dispersal used by angiosperms and stinkhorns alike. Similar to flowers, stinkhorn fruiting bodies are amongst the showiest fungal structures to have evolved. However, instead of producing sweet fruity smelling flowers like many angiosperms, the gleba produced by these fungi smells exceptionally terrible. This attracts a wide array of carrion (dead flesh eating) insects. Attracting carrion feeding insects is a strategy found in both fungal and plant kingdoms, but fungi are limited in only engaging in mutualisms with generalist insects like these. Fungi and plants that use generalist mutualists will experience a more stagnant evolutionary trajectory. Given enough time, I believe the species in this family will become more specialized. When this transition from a generalized to a more specialized spore dispersal occurs, I think this family can become the angiosperms of the mycological world. The forest floor will become even more dynamic, as some fungi will even compete for flowering plant pollinators.
Mutinus caninus with a beetle mutualist
After about five years of examining the forest floor, I have only found one stinkhorn species. If I were to time travel 20,000 years into the future, (given that humans don’t annihilate the natural world past the point of ecological rebound) I predict stinkhorns to be as abundant as they are diverse. Like early angiosperms, the stinkhorns are on a path of ecological dominance. Once they attract specialized insects, their partner will select for certain traits, ultimately enhancing this family’s diversification. Is the Phallaceae another “abominable mystery” in the making? A few duplication event might be required but I certainly think so.
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