Many molluscs, like Limax maximus here (same genus, different species than used in this research) have big, honkin’ neurons. This is what has made some slugs, particularly Aplysia californica, valuable animals to people who want to record the electrical activity from neurons. Somewhere along the way, someone measured the amount of DNA in Aplysia, it seemed suspiciously high.
There are many ways that you could end up with large amounts of DNA in a cell. The cell might form from several smaller cells that have been fused together (this happend in some giant axons in squid). An alternative hypothesis is that the neurons have extra DNA because the DNA has replicated without the cells dividing.
Yamagishi and colleagues tries to test this using slug. Reasoning that the effects they were seeing were related to regular growth, they fed one group of slugs a lot of food, the controls slightly less, and starved a third group for a few weeks. At the end of this, they successfully grew groups of slugs that differed in the size of:
- Their bodies;
- Their brains;
- Specific regions within the brain, and;
- Individual giant neurons in the brain.
That bigger bodies mean bigger brains is not unusual; this is a well-known scaling effect. It’s also not surprising that the main cause of the size increase is in the size of existing neurons, rather than increased numbers of neurons, because most invertebrates seem to have fairly rigid numbers of identified neurons across a wide range of sizes.
The authors are most interested in whether these extra giant neurons in the well fed animals also have additional DNA (they do) and how it gets there. To test this, they added a dye called BrdU that will show up in cells that are actively making DNA. Usually, this means in cells that are actively dividing.
This is one of the critical pieces of evidence, showing BrdU-labelled neurons in the subesophageal ganglion, with the starved animals on the left and the overfed animals on the right:
It’s also a bit puzzling as to why the normally fed control group isn’t in many of these comparisons.
This all suggests that the neurons are undergoing mitosis (genetic duplication) without cytokinesis (physical separation of the cells). This might be happening because neurons are metabolically very active. They have to synthesize and transport all their proteins quite long distances from the nucleus. And because the number of cells doesn’t change much in invertebrates, growth could put significant demand on a cell’s ability to deliver enough material.
That said, it isn’t a simple relationship. You would expect that because muscles vary tremendously with body size, motor neurons would show this tendency to have more DNA more than, say, interneurons. But that turned out not to be the case.
It’s not clear from these experiments if the neurons are duplicating the entire genome in the cell, or whether they are being more selective, and only duplicating key components – particular chromosomes, say.
It’s also not clear how the signals of more food and more growth gets picked up by the neurons and turned into a signal to make more DNA.
That this was all tipped off by an observation in one species and confirmed in another suggests that this is widespread among molluscs. But would it be expected in other species, too?
I prediction that this loading up large neurons with DNA will be found in other invertebrate, but will be rarer in vertebrates. Arthropods and some worms also have giant neurons and stereotyped numbers of neurons, so they would seem to be facing the same problem as molluscs. Vertebrates seem to have more variable numbers of smaller neurons, so I would expect it to be more rare there.
Yamagishi M, Ito E, Matsuo R. 2011. DNA endoreplication in the brain neurons during body growth of an adult slug. the Journal of Neuroscience 31(15): 5596-5604. DOI: 10.1523/JNEUROSCI.0179-11.2011
Picture by rhonddawildlifediary on Flickr; used under a Creative Commons license.