Apalone spinifera is a species native to North America of freshwater turtles. It was the turtle species that was involved in this study. The researchers claim that their findings shed light onto the evolutionary role of sex-chromosome dosage compensation for many species. Credit: Nicole Valenzuela
An Iowa State University scientist has led a new study that sheds light on the evolution of organisms to correct imbalances in sexchromosomes.
This study focuses on a softshell turtle species. However, the results could be used to help illuminate important evolutionary processes in other species, according to Nicole Valenzuela (professor of ecology, evolution, and organismal biology) and the lead author.
Many organisms decide their sex using a pair specialized chromosomes. These chromosomes can be found in almost every cell of the organism. One sex is a matched pair of genes, and a mismatched one results in another. In humans, and in many other species, the sex chromosomes can be referred to as X or Y. Usually, two X chromosomes will result in a male and XY chromosomes will result in a woman. These chromosomes contain the genetic codes to produce essential proteins. The disproportion of chromosomes in individuals with XY chromosomes, which have only one X for each pair of non-sexchromosomes (called Autosomes), can cause an imbalance in protein production. This study sheds light into how organisms have evolved in order to correct such imbalances using a process called sex-chromosome dosage compensation (or SCDC).
The study was focused on Apalone spinifera, a softshell turtle species that is among the largest in freshwater turtles. It inhabits large parts of North America including Iowa. The research could also help scientists to understand how the process works in other organisms. This study could also help scientists understand how diseases can occur if the SCDC process isn't working correctly.
Valenzuela stated that understanding the variety of SCDC mechanisms found in nature and how they evolve helps to understand how animals and people compensate for gene imbalances and why failure to compensate leads to diseases.
The peer-reviewed scientific journal Philosophical Transactions of the Royal Society published the study this week.
What is the sex chromosome dosage compens?
Individuals with mismatched sexchromosomes can get compensation through the Sex Chromome Dosage Program. The study included softshell turtles. In this case, the sexchromosomes are called Z and W. It's the females who have mismatched, or ZW chromosomes. They lack a second copy Z chromosome. This is in contrast to their male counterparts, who have two Z.
Instructions for some proteins are contained in the Z chromosomes. Having only one copy of a particular chromosome can lead to a decrease in the amount of proteins that cells produce. This is because gene copies often affect protein production. More copies equals more protein production. Uneven distribution of genes can cause developmental, physiological, or other disorders. SCDC mechanisms increase or decrease the amount of protein produced by genes on the single Z (orX) chromosome. A proper balance is crucial for diseases such as Turner syndrome or Klinefelter syndrome, which are caused by an abnormal number of sexchromosomes. Valenzuela stated that these processes also have evolutionary and health consequences in other organisms.
Valenzuela, along with her co-authors, collected softshell turtles from various stages of their development. They also analyzed different tissues to identify which genes were active. The researchers then compared gene activity from sexchromosomes with autosomes, which are broken down by male turtles and female turtles.
This study is the first to examine sex chromosome compensation in turtles. It also shows that temperature seems to have a significant impact on the SCDC process in turtles. Valenzuela has studied temperature-dependent sex determination (TSD), or the way environmental temperatures influence whether a turtle embryo develops into a male or female in species that lack sex chromosomes, in previous research. She said that the thermal sensitivity of the SCDC was surprising because softshell turtles have lost their ancestral TSD system. The way softshell turtles perform SCDC is complex and also quite unusual.
Study results showed that softshell turtles have two times the activity of Zs during early embryonic development. This fixes the imbalance in ZW females' expression (twice Z expression now matches the autosomal expression). This same response causes an imbalance in the males, as Z expression now doubles that of autosomal expression. According to the study, Z expression in males decreases at later stages of embryonic development. This effect is stronger at lower temperatures than at higher incubation temperatures. Valenzuela stated that the new study is the first to demonstrate that temperature has an impact on SCDC in turtles or animals. However, it is also possible that this effect can be seen in eukaryotes (organs in which genetic material is contained within a cell nucleus). Eukaryotic species encompass a wide range of organisms including plants and animals.
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