November 20, 2013 | 1
In many fungi, the DNA storage compartments called nuclei are not prisoners of the cells they reside in, the way they are in animals and plants. Instead, fungal nuclei are free to move about the cabin. They flow through the joined, tube-shaped cells of fungi like busy commuters, and experience many of the same dynamics. Have a look:
Why might fungi do this? As you saw in the video, a fungus is a collection of pipe-like filaments called hyphae (HIGH-fee). In many, but not all, fungi, each hypha is divided into cells by walls called septa (You can see such a wall in the video from 1:26-1:36. Look in the center of the image. A pore permitting the nuclei to pass is invisible from our viewpoint). The hyphae often branch and rejoin to form a complex network, collectively called a mycelium (my-SEE-lee-um).
A fungus like this one is essentially one giant cell. This is because each septum is pierced by a giant hole that allows the cytoplasm — the goodies inside a cell — to flow freely within the mycelium. The flow of cytoplasm is important in fungi because they can only make new cells at the tips of hyphae — not in all directions as animal and plant cells do. These fungi need to be able to push cytoplasm to feed and fill that growing tip.
This situation of continuous cytoplasm and shared nuclei — a state shared by the fascinating forest-dwelling giant protists plasmodial slime molds and deep-sea giant protists xenophyophores as well as the deep-sea animals glass sponges (both forest floors and deep sea sediment seem to promote this lifestyle for some reason) — is far different from most other organisms, where cell membranes and/or walls segregate the cytoplasm within each cell.
Aside from nourishing growing hyphae, what might be the advantage to such a communal cell biology? Fungi also tolerate the presence of extraordinarily dissimilar nuclei in their bodies. Most often, these are mutated versions of their own genomes. But they also have a propensity for occasionally asexually hybridizing with different individuals they encounter. These individuals may be members of the same species, of different species, or perhaps even different genera, forming what biologists call a “chimera”.
Such a chimera unites the powers of two individuals in one organism. This union can result in an improved ability to infect and cause disease in a host, or perhaps the ability to eat food that one or both fungi could not digest alone. Because the constraints on chimera formation are more relaxed than those on mate selection, chimeras may also allow distantly related fungi to swap or donate genes or whole pieces of DNA called chromosomes to fungi that would be far too distantly related to mate with (a process called horizontal gene transfer), which could be an under-appreciated driver of fungal diversity and evolution.
But without continued mixing, the nuclei of chimeras tend to re-segregate in different sections of the fungus’s filamentous body, effectively de-chimerizing. This is because in the growing tips of hyphae, one type of nucleus will tend to dominate numerically over time by pure chance. As you saw in the video, actively mixing the contents of the shared cytoplasm is one solution to this problem, and preserves the benefits of chimerism throughout the fungus. The nuclear flow seems to be powered by the same “gentle pressure gradients that drive colony growth,” the authors of this video report in a paper in PNAS highlighting their explorations of those dynamics.
Cross walls, it turns out — even those with big holes — provide structural support to the tubular hypha just like the partitioning walls of a ship. And like the watertight doors in bulkheads on submarines, many fungi have plugs (large proteinaceous crystals or Woronin bodies) located next to their pores that can block the septum should part of the mycelium become damaged or old and in need of sealing. Finally, for some purposes, like forming a spore, it’s necessary to create a special biochemical environment. Having a closeable door means you can create a closed compartment for this purpose.
For yet another fantastic video that vividly illustrates both the flow of cytoplasm through a septal pore and the Woronin bodies nearby, go here. The dynamics of these fungi are further proof that the basic models of cell biology we all learned in high school have radical exceptions, once you start looking around — perhaps no further than your own back yard.
Roper M., Simonin A., Hickey P.C., Leeder A. & Glass N.L. (2013). Nuclear dynamics in a fungal chimera, Proceedings of the National Academy of Sciences, 110 (32) 12875-12880. DOI: 10.1073/pnas.1220842110
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