YOU’VE probably guessed this next statement. Yes, the cell walls of fungi contain chitin.
A more convincing evidence is that primitive fungal spores possess flagella. A flagellum is a thin structure that projects out of microbes, fungal spores, and sperm cells, which can move like whips, thus propelling the cell around. Thus, the spores of such fungi can actually swim around.
Most flagellated cells possess one or more anterior flagella.
And here’s the kicker.
The sperm of animals and the spores of chytrid fungi possess a single posterior flagellum.
The great biologist Thomas Cavalier-Smith noticed this, and in 1987 proposed that animals and fungi belong to the same clade, the opisthokont.
This derived (from a single ancestor) characteristic is unique to animals and fungi. Thus, animals and fungi are now grouped under the same clade, namely the opisthokont. From the Greek language, opísthios means posterior and kontós means a pole. Literally, some animals and fungi cells have a pole on their behinds (their flagella).
What are the general characteristics of opisthokonts? As mentioned above they digest food extracellularly and possess extracellular chitin. They have spores. They have cell walls that exhibit filamentous growth. They have hyphae. They share enzymes and cellular metabolic pathways peculiar to their ingroup. (Bear in mind though that some of these characteristics have been secondarily lost in many opisthokont species over long periods of time.)
Moreover, phylogenetic studies have confirmed the close relationships of the organisms under this clade, including both animals and fungi.
There are some fascinating things about this clade. For example, amoebas are regarded more closely related to opisthokonts than to plants. From within the opisthokont clade, some organisms traditionally classified as fungi have been found to be more closely related to animals than to other fungi. Some traditional protozoans and other one-celled organisms that also possess a single posterior flagellum, including the peculiar choanoflagellates, are now deemed as opisthokonts. In particular, the choanoflagellates are regarded as the sister group of sponges and all animals. (Sponges possess flagellated cells practically identical to those of one-celled choanoflagellates; remarkably sponges from one perspective can almost be seen as obligatory colonies of choanoflagellates. Sponges are probably the most ancient animals still extant. Unfortunately for them, sponges do not synthesize mucus, an important protective covering in all other animals, which results in other microbes more easily able to colonize sponges. A recent paper hypothesizes that this is probably why sponges cannot dominate water ecosystems.)
So fascinatingly, next time you eat a mushroom, you are actually eating not a plant, but something closer to animals and to you than to plants./PN
