Worm puberty: a balancing act of chemistry and experience
Sexual development of an organism determines the life that the organism will have. Their nervous system, genes and other molecules within the body will result in a distinguishing behavior that is preset for them just because of their sex. Surprisingly, with the help of small worms, more specifically C. elegans, and the way they defecate, researchers Dr. Michael Hart and Dr. Oliver Hobert of Columbia University have found more insight into how two small molecules can influence sexual behavior.
Like many other animals in the animal kingdom, C. elegans have two sexes. However, due to their unique sexual development the resulting sexes are male and hermaphrodite. Hermaphrodites typically have both male and female tissue and are the most commonly seen sex within the C. elegans. As a result, hermaphrodites can produce their own sperm and eggs, reproduce on their own and lay eggs. Hermaphrodite worms do not have a drive to mate with males because of their ability to produce their own offspring; they lack sexual attraction towards males. On the other hand, males have a strong sexual attraction in order to locate and mate with hermaphrodite worms. Through a series of steps, males can locate the hermaphrodite genitalia and insert their spicules, a structure located on the end of the male tail. The focus for Hart and Hobert focused on the small spicule structures and the changes they undergo during male development.
These spicule structures are a defining feature of the male anatomy. The small structures located on the tail are essential for proper male mating, and without well timed protractions of the spicules no successful reproduction would occur.
The work published by Dr. Hart and Dr. Hobert provided a glimpse into the plasticity of neurons throughout male sexual development. By focusing on the spicules, researchers Hart and Hobert were able to demonstrate a surprising connection between defecation and spicule protraction.
Throughout the first couple of days of the male worm’s adulthood defecation behavior is linked with spicule protraction behavior. Even though these two behaviors are not linked together later in adulthood, the two actions are seen to occur simultaneously earlier in life. Although these spicule projections seem harmless at first, the constant movement can result in detrimental effects on male mating behavior later in life. To mate properly males must be able to insert their spicules quickly and efficiently. When there is constant protraction of the spicules with no proper direction the male worms will not be able to ‘learn’ how to properly mate. So then, why is this occurring in the first place?
This strange behavior then led to Hart and Hobert focusing on the sex shared neuron DVB. The DVB neuron is a motor neuron and interneuron located in the tail of all C. elegans. However, with thorough analysis the DVB neuron has been found to have a unique effect on only male C. elegans. Interestingly, the male worm’s DVB neuron displays morphological plasticity after development has finished, indicating sexually dimorphic results. The wiring pattern of the DVB neuron is also noticeably sexually dimorphic, providing another indication of the difference in behavior controlled by the DVB neuron in both male and hermaphrodite worms.
Before diving into the molecular mechanisms behind this interesting male behavior, Hart and Hobert first focused on the behavior itself. As mentioned previously, during the first couple of days of adulthood male worms will extend their spicules during defecation. However, once they reach day three, the male worms either no longer do this or the protractions are greatly reduced. After silencing DVB in males, spicule protraction during defecation remained the same rate from day one to day three, indicating that DVB is essential for the reduction of the movement. DVB most likely decreases the spicule movement during defection by inhibiting the spicule circuit components that are linked with the defecation circuit components.
Through a series of tests involving male worms that had their DVB neurons ablated, Hart and Hobert were also able to show that DVB is directly involved in the behavioral change in the male worms. DVB acts as a functional switch; starting out in an excitatory state in early adulthood to an inhibitory state in later adulthood. The excitation and inhibition directly affect the spicule protractions, indicating that DVB switches function through male adulthood.
After pinpointing the purpose of the DVB neuron in male development, the next focus was on the reason why this inhibition was occurring. It has been previously known that DVB is GABA releasing. GABA is an amino acid neurotransmitter involved in the worm's basic motor functions.
Using mutant male worms lacking GABA signaling components Hart and Hobert were able to identify that GABA contributes to the inhibition of spicule movement later in male adulthood.
Like how humans become adults, the maturity of C. elegans is also influenced by the experiences they have. To test the influence of hermaphrodite presence on the maturity of males, the researchers compared the DVB neuron growth between single males by themselves and single males housed with hermaphrodites. The result? More DVB growth was seen within the males housed with hermaphrodites, indicating that maturity is achieved faster with more experience.
The DVB neuron is necessary for inhibiting the spicule movement. As the males proceed through adulthood and gain more experience, the DVB neurons become rewired and produce more outgrowth. The result of this growth is the reduction of spicule movement and an increase in fine motor movements associated with the spicule structures.
The final layer in trying to understand the DVB neuron was focusing on the molecular mechanisms. The two small molecules, neurexin and neuroligin, work together to control the plasticity of the DVB neurons during adulthood. In a way, these two molecules could be twins with different personalities. The first couple of days of adulthood neurexin promotes the growth of projections from neurons, called neurites. The growth of the neurites is attributed to the unnecessary movement of the spicules on the male’s tail in early adulthood. Without the presence of neurexin in the male worms there was a reduction of inhibition of spicule protractions. At about day three, scientists noticed that the molecule neuroligin begins to inhibit the neurite growth which resulted in a reduction of spicule protractions. Neurexin and neuroligin have a protagonist and antagonistic relationship resulting in the perfect balance for proper male behavior.
The work of Dr. Michael Hart and Dr. Oliver Hobert provided a significant insight into how molecules can have an impact on the maturity of a sexually dimorphic neuron. The results of the experiments done show how plastic sexual behavior and neuronal growth can be throughout adulthood. The research done on the two molecules, neurexin and neuroligin, may provide new insights into many human diseases associated with these molecules.