Blocking programmed cell death, leads to the development of additional functionally active neurons in the olfactory system of Drosophila melanogaster. Such neurons can carry different receptors and axonal projection than the cells that develop naturally, and therefore, this mechanism may be the basis for the emergence of new signaling pathways and the evolution of sensory systems. The researchers showed that the surviving nerve cells have received new functions in some species of Drosophila, and mosquitoes. Article published in the journal Science Advances.
Evolution of the nervous system often requires the involvement of new neurons: thus it is possible to increase the sensitivity of the sensory system, improve cognitive abilities such as memory capacity or the possibility of parallel processing of multiple streams of information. To increase the number of neurons in a variety of ways. The most obvious — even during development to create more cells-precursors of neurons or make them more to share.
There is another possibility to increase the number of working nerve cells. The fact that in the process of development, many neurons pass through programmed cell death. Accordingly, if you stop this mechanism, a greater number of cells can integrate into the nervous system. Indeed, if genetically to block cell death in mice or Drosophila, develops enlarged but deformed, brain. A similar experiment conducted on worms Caenorhabditis elegans, and many of the surviving cells differentiated into neurons and even compensated function remote nerve cells.
In fruit flies, the main olfactory organ, the antenna, covered with sensitive hairs (sensillae). Each sensilla is formed from a single cell precursor that gives rise to four cells-satellites and four sensory neurons. However, only a few hairs have four olfactory nerve cells, in the other there is only one or two, and the others pass through cell death.
Lucia Prieto-Godino (Lucia Prieto-Godino) from the University of Lausanne and her colleagues from Switzerland, Germany and the UK explored the possibilities of the development of new neural connections in the olfactory system of Drosophila melanogaster by blocking programmed cell death.
Scientists have blocked the genes that are required for programmed cell death, transgenic animals or locally using RNA interference. Then studied the electrophysiological characteristics of neurons evolved, including their activity in response to olfactory stimuli. Also evaluated the expression of the receptors in the cells of the hairs.
Neurons marked by adding a fluorescent tag to one of the proapoptotic genes: if cage was doomed, this gene was expressed, and with it the glowing protein. So scientists were able to trace axonal projections surviving in blocking programmed cell death of neurons.
The researchers suggested that the survival of neurons during the development of the antennas could serve as the basis of evolutionary changes in a phylogenetic range of insects. To test this, I compared the number of nerve cells in the hairs on the antennae of fruit flies to 26 different types.
In addition, scientists have noticed that mosquitoes have an additional neuron hairs on segments maksill (jaws) compared to fruit flies. This neuron is sensitive to carbon dioxide, whereas the remaining cells of this type senzill are other receptors. In fruit flies, the cells that recognize carbon dioxide, there is only the antennas, but not on jaws. To find out, could this new property to appear in the result settings programmed cell death, the researchers blocked it in maksill Drosophila and analyzed the composition of the evolved receptors on neurons.
In the blocking programmed cell death, the number of neurons in the antennae increased by 200-300 pieces, while scientists estimate the number dying in the process of development of cells ranges from 300 to 400. In the antennae increased spontaneous electrical activity and arousal in response to olfactory stimuli, the number of receptors in the hairs was increased compared with control (and the distribution across types of receptors exist) — so the surviving neurons are functional.
Axons of the surviving neurons came into the olfactory lobe of Drosophila and formed there, glomerular synapses, and cells that develop naturally. However, cells with the same receptors formed contacts with more than one ball, as in the control animals, and different. The authors conclude that the surviving neurons can serve as a basis for the new projections and the evolution of signaling pathways.
In most species of Drosophila, the number of neurons in the hairs of one type were identical, however, in sensillar type at1 nine species of flies electrophysiological active were two cells instead of one. One of the two cells did not respond to typical of these hairs olfactory stimulus, probably, it are the additional receptors, which have new touch features.
By blocking cell death in maksill Drosophila melanogaster, some of the neurons found receptors to carbon dioxide, and the projection of these cells was similar to those of mosquitoes. So, regulation of programmed cell death could lead to evolutionary differences in the olfactory system of these insects.
Despite its apparent simplicity, the olfactory system of Drosophila is effectively analyses information. The device even has inspired scientists to create a machine learning model that allows for similarities of objects.