Fungi are old

Earlier this week, I wrote that fungi are weird. Fungi are also old. Probably.

Recently discovered fossils from Arctic Canada have been interpreted as fungi, an important finding since the sediments in which they were found date to 0.89-1.01 billion years ago, around half a billion years older than the next oldest unambiguously fungal fossils. Corentin Loron and colleagues have described microfossils from the Grassy Bay Formation and presented several lines of evidence that they represent ancient fungi.

Loron et al. 2019 Fig. 1

Figure 1 a-g from Loron et al. 2019. Microphotographs of Ourasphaira giraldae specimens. a, Sketch of O. giraldae, displaying the main features of the microfossil. b–g, Unornamented terminal sphere (spore). Transmitted light microscopy images show specimens with secondary branching at a right angle (b, d–g), with terminal spheres connected together (c), with a bulbous connection (e) and with tertiary branching (d, f, g). Arrows show septate connections.

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Fungi are weird

I think about the evolution of multicellularity a lot, and I talk about it with colleagues. One of the things we talk about is what general principles we can infer from the many independent origins of multicellularity, for example in land plants, animals, red algae, brown algae, green algae, and fungi. Those are the groups that have evolved what we might call complex multicellularity, and one of the things we notice is that they all develop clonally; that is, they start out as a single cell, and when that cell divides, the daughter cells stick together. We notice that complex multicellularity has never evolved in species with aggregative development, when free-living cells come together to form a multicellular body, as they do in cellular slime molds and myxobacteria. Some aggregative developers have evolved a couple of different cell types, but all of the groups that have reached higher degrees of complexity develop by cell division and the products of cell division staying together. All, that is, except for fungi. Fungi are weird.

Fungi don’t really develop clonally in the way I’ve described, but they don’t really not develop clonally either. That’s because their cells don’t divide in the way we’re used to thinking about, through repeated rounds of mitosis. In mitosis, duplication of the genome is coupled to cell division: the chromosomes duplicate, they move to either end of the cell, then the cell divides. The chromosomes double, then they halve, so the daughter cells end up with the same number as the mother cell. That’s not how it works in fungi. Instead, they form filaments called hyphae (singular hypha) that grow at the tip. In some cases, partitions called septa (singular septum) form behind the growing tip, dividing the hyphae into individual cells. In some cases, no septa form, and each hypha is effectively one long, skinny cell with lots of nuclei (this is called a coenocyte).

So fungi don’t really develop by repeated rounds of cell division in the same sense that animals, plants, etc. do. Hyphae just grow, and they are divided into cells as sort of an afterthought, if they are divided into cells at all. Fungi with coenocytic (or aseptate) hyphae aren’t really even multicellular in the same sense as plants and animals are. Different people have different qualifications for what counts as multicellular, but it’s a stretch to call something multicellular that doesn’t have multiple cells. Fungi are weird.

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