Simple to complex: the importance of sexual maturation
Imagine a train station connecting the entire world. From one platform you can take a train from New York, New York to Shanghai, China while from another you can take one from Sydney, Australia to Reykjavik, Iceland. Through this one train station the world can be easily connected to each other and would create a hub of commerce and easy communication. This imaginary train station resembles the hub sensory neuron found in the male C. elegans worm. A hub neuron is an essential player in relaying information from the core nervous system of an organism to other neuronal networks throughout the body. In the case of the C. elegans, the hub neuron connects to sex specific networks.
To understand the importance of the hub neuron in the male worm there must be a full understanding of the neurons that innervate this hub and the impact these neurons have on sexual differentiation.
Similar to other organisms throughout the animal kingdom, the C. elegans worm undergoes sexual differentiation to determine the sex of the new offspring. The process by which this occurs is not fully understood but a basic understanding has been found and is used as a foundation. The process by which worms become a certain sex is regulated mainly by the master sex regulator called tra-1. Tra-1 is a transcription factor found in the sex determination pathway that essentially determines the sex of the worm.
However, tra-1 can only function properly when it receives the proper signals. These signals are passed on based on the ratio between the sex chromosomes and the autosomal or body chromosomes. For males this ratio would be one to two or 0.5. This ratio is because male worms only have one X chromosome. On the other hand, hermaphrodite worms have two X chromosomes, resulting in a ratio of two to two or 1. The resulting ratio for each sex signals to the tra-1 transcription factor to be turned on or off which in turn determines the sexual development of the worm. Although tra-1 is an essential factor in sex determination, many other transcription factors, genes and neurons play an essential role as well.
The insightful work published by Dr. Esther Serrano-Saiz and colleagues from Columbia University in the journal Current Biology in 2017 provided significant understanding on the development of sexual dimorphism. The focus on the sex-shared neuron, PHC, allowed for the Dr. Serrano-Saiz to pinpoint an important facet of male sexual development and behavior.
In male worms, a pair of neurons called the PHC neuron has a significant impact on the determination of the worm’s male mating behavior. PHC neurons are neurons found in both the male and hermaphrodite worms. Although this neuron is found in both sexes the function differs drastically between the two. In hermaphrodites the PHC neuron functions like a simple pain sensory neuron without much impact on the sex specific behaviors that hermaphrodites portray. On the other hand, the male form of the PHC neuron differentiates into a densely connected hub sensory neuron. The hub neuron connects to multiple male specific and sex shared circuitry lending to the unique function in the male worm.
The PHC neuron in the male worm differentiates into a mating function later in development. The male worm’s ability to locate the vulva of the hermaphrodite during mating is the behavior the PHC neuron modulates. Locating the vulva is essential for the male to properly mate.
The anatomical and physiological differences between the male and hermaphrodite correlates with the behavior changes between the two sexes. The main anatomical difference between the male and the hermaphrodite worms are the presence of a longer axon length found in the tail of the male. In the beginning stages of development, the axon length of the PHC neuron is the same for both sexes. Once males reach a later stage of development the axon length becomes longer and differentiates into a hub neuron. The physiological and chemical dimorphisms between the two sexes can be seen in the gene flp-11 and the transporter eat-4/VGLUT. The flp-11 gene is expressed exclusively in the male PHC neuron and modulates the activity of other neurons. The Saiz lab also determined the eat-4/VGLUT transporter is responsible for increasing the PHC neuron presence within the male worm, while repressing this scaling within the hermaphrodite worm. These differences indicate strong sexual dimorphisms within the PHC neuron.
The PHC neuron has a rare characteristic that lends to its importance in the male worm. Unlike many other sex shared neurons, PHC is a hub neuron, meaning they connect sex specific networks to the core nervous system of the worm. The PHC neuron receives multiple male specific signals from sensory neurons throughout the body which then translates to specific behavior. However, in the beginning stages of male development the PHC neuron is like the hermaphrodite neuron. At this point both sexes use the PHC neuron as a generic sensory neuron without significant impact. Later in development the PHC neuron in the male worm becomes differentiated into a hub neuron while in the hermaphrodite the neuron’s function remains the same. As a result of the tra-1 master regulator and a gene called dmd-3, the function of the neuron changes from one of insignificance to an essential player in the male’s mating behavior.
The presence of the tra-1 transcription factor takes on dramatically different roles between the male and hermaphrodite worms. Once the Saiz lab was able to determine the sexual dimorphisms between the two sexes, they then focused on whether the sex of the PHC neuron is essential to the role the neuron has. To test their hypothesis, they masculinized the PHC neuron within hermaphrodites and feminized the neuron within the males. The neuronal sex reversal involved degrading the tra-1 regulator within the hermaphrodites and promoting the regulator in the males. As a result, the hermaphrodite showed an increase in PHC neuron axon growth and the male showed a lack of axon growth. These results indicate that tra-1 represses male morphological features and is required and enough to control male features of the PHC neuron. Through the sex reversals the Saiz lab discovered that the sexual identity of the PHC neuron is essential to behavior.
Along with tra-1, the gene dmd-3 plays an essential role in controlling the PHC neuron. The gene dmd-3 can only be found in the male worm because it is not repressed by tra-1 as it is in hermaphrodites. Dmd-3 is a gene that belongs to the family Doublesex. This family of genes are highly conserved across many different species, meaning this gene is identical or similar to the genes in other species. To determine the role dmd-3 plays in the male worm the Saiz lab formed dmd-3 lacking mutants. These mutants no longer showed the PHC axon growth typically seen in normal male worms and lacked the other physiological components seen in male PHC neurons. Dmd-3 works to sufficiently control the male specific identity of the PHC neuron.
The work done by Dr. Esther Serrano-Saiz and colleagues improved the understanding of male sexual differentiation and development. Through the thorough analysis of the PHC neuron and the gene dmd-3 the Saiz lab was able to determine the importance these two components play in C. elegans male mating behavior.