A new study, in this week’s online edition of the Proceedings of the National Academy of Sciences , shows an incredible degree of biological diversity in a surprising location, i.e. in a single neural connection in the body wall of flies. The finding opens up a new spectrum of interesting questions regarding the importance of the nervous system structure and the evolution of neural wiring.
Geneticist Barry Ganetzky, Steenbock Professor of Biological Sciences at the University of Wisconsin-Madison declared:
“We know almost nothing about the evolution of the nervous system, although we know it has to happen – behaviors change, complexity changes, there is the addition of new neurons, formation of different synaptic connections.”
The finding proves even more astounding given that Ganetzky and his graduate student Megan Campbell discovered the unexpected diversity in a location very familiar to scientists, i.e. the neuromuscular junction 4 (NMJ4), the location where a single motor neuron contacts a particular muscle in the fly body wall to drive its activity. The synapses where neurons link to their neuronal or muscular targets have a complex structural form, looking like miniature trees decorated with tiny bulbs that are the nerve terminals (synaptic boutons).
“Synapses are where the important information transfer and integrative functions of the nervous system occur. It’s the fundamental place where information processing takes place, and there is an underlying belief that the structure of the synapse is key to understanding its function.”
Each muscle is supplied by a different motor neuron that forms an NMJ with a shape, size, and geometry characteristic for that particular NMJ. Given the consistency of the fly’s anatomy, even across different species enables researchers to identify the exact same synapse in different individual flies. The synaptic development and function of NMJ4 has been well researched, and Ganetzky has used NMJ4 for more than 20 years to identify genes with a host of biological roles from movement disorders to neurodegeneration.
The latest project derived from a simple debate of what really is “normal” for laboratory-bred fruit flies and their wild counterparts. Campbell discovered during an examination of the NMJ4 in the common lab fruit fly, Drosophila melanogaster, that the synaptic morphology between lab-bred and wild flies, as well as between strains obtained in Madison, Wis., and strains from as far away as Zimbabwe was consistent, with all flies having similar-looking arbors and boutons.
Encouraged by the discovery, they decided to investigate further. Ganetzky says: “Drosophila is a very rich genus – thousands of species with different behaviors, different food preferences, different environments, different climates, different sizes – with upward of 50 million years of divergence.” It can be compared to the evolutionary separation between mice and humans.
He adds, that regardless of these differences, the larval body plan is exactly the same across all known Drosophila species irrespective of their size, habitat, or food source, stating that:
“Cell for cell, the body wall musculature and innervation patterns are identical.”
They started investigating NMJ4 in other Drosophila species with the help of UW-Madison evolutionary biologist Sean B. Carroll’s fly collection. They anticipated finding some predictable patterns with small variations when they focused on their target synapse in 21 different species of Drosophila from around the world, however, after examining only a few species Campbell said that a different picture emerged.
Similar to Drosophila melanogaster, each species had a characteristic NMJ4 appearance, yet this appearance differed dramatically amongst species. Whilst NMJ4 in some species consisted of a few boutons arranged in a simple unbranched pattern, others had numerous boutons distributed over several long branches, or many boutons packed into dense, tightly clustered arbors.
Even though Ganetzky declares: “The results were absolutely flabbergasting – variation far beyond anything we ever anticipated,” there were more surprises.
The striking variations in complexity did not correspond to an evolutionary relationship amongst the species, i.e. the NMJs of more closely related species looked no more alike compared with those of more distantly related flies.
They even discovered obvious variations between species that were separated by less than one million years of evolution, species that otherwise appear so alike that even fly experts have difficulty in distinguishing them based on appearance. According to the researchers, such rapid evolution is astounding although its biological importance remains unclear.
One answer to the question of what could explain such extraordinary variation may be the possibility of genetic drift, a random accumulation of genetic variations that change the appearance of the NMJ but that have no impact on the organism otherwise. Basically, any NMJ that serves the purpose will suffice. Another alternative could be that each NMJ is formed by natural selection due to its particular size and structure to somehow increase survival or reproductive success for members of that species.
Assisted by fellow UW-Madison geneticists Bret Payseur and Beth Dumont, the researchers used a quantitative model to examine the different NMJ structures, as a function of the evolutionary connection amongst 11 species, whose evolutionary tree is accurately known from genome sequencing.
Ganetzky says that the results demonstrate that the variability they observed does not seem to be random, stating:
“What that suggests is that there is some driving force – natural selection – that is shaping the synapse to be a particular way.”
They hypothesized that neural function would be an obvious target of selection and measured electrical activity in the circuit, but discovered activity recordings from four species to represent the range of structural complexity revealed the same basic neural workings irrespective of the synaptic structure.
According to the researchers, there may be subtle functional differences between the different NMJ structures which remained undetected by their assay, but which could diverge into distinct biological differences. For instance, learning capacity or stress responses that would be a target for natural selection.
“We believe there’s some reason why the variation matters, but we don’t know yet what that reason is.”
Campbell and Ganetzky are currently exploring to study whether or not there are underlying genetic and molecular mechanisms and the biological importance of that naturally occurring variation.
“We think we’ve made an important discovery about nature that we think opens up all kinds of new doors. At this point, we’ve raised many, many more questions than we’ve answered… questions about the evolution of nervous systems, evolution of behavior, the relationship between neuronal and synaptic morphology and function. I hope this captures the interest of scientists in many other fields to apply their own areas of expertise.”
Written by Petra Rattue