An Iowa State University researcher led a study that provided insight into how organisms have adapted to resolve racial chromosomal abnormalities. The study, which looked at a species of soft-shell turtle, was published in the journal Philosophical Transactions of the Royal Society B, but Nicole Valenzuela, professor of ecology, evolution and organismal biology and lead author of the study, said the findings may shed light on an important evolutionary process in many species.
Many organisms determine their sex by a pair of specialized chromosomes that occur in almost every cell in an organism’s body. A matched pair of chromosomes results in one sex, while a mismatched pair results in another sex. For example, in humans and many other species, the sex chromosomes are referred to as X and Y. Typically, two X chromosomes result in a female while XY chromosomes result in males. These chromosomes also contain the genetic codes for making essential proteins, and the disparity in chromosomes in XY individuals caused by them carrying only one X for each pair of non-sex chromosomes (called autosomes) can lead to an imbalance in protein production. The study sheds light on how organisms have evolved to deal with such imbalances through a process called sex chromosome dosage compensation, or SCDC.
The study focused on a species of softshell turtle known as Apalone spinifera, which is among the largest freshwater turtles and inhabits much of North America, including Iowa. But the research could help scientists understand the process in other organisms as well. The study could also create a better understanding of how disease can occur if the SCDC process does not work properly. “Understanding the diversity of SCDC mechanisms in nature, how they occur and evolve, informs more broadly how animals and humans compensate for gene dosage imbalance and why failure to properly compensate for these differences leads to disease states,” said Valenzuela.
The study was published this week in the journal Philosophical Transactions of the Royal Society B. Race chromosome dosage compensation comes into play for people who have mismatched race chromosomes. In the case of the softshell turtles included in the study, the reproductive chromosomes are referred to as Z and W, and it is the females of the species that have mismatched, or ZW, chromosomes. This mismatch means they are missing a second copy of the Z chromosome, unlike their male counterparts who have two Z chromosomes.
Z chromosomes contain instructions for some of the proteins that normal functioning cells must make, and having only one copy of a chromosome can result in a reduced amount of proteins being produced, because protein production is often affected by the number of gene copies. More copies mean more protein production. Thus, nonuniformity in the copy number of cooperating genes can lead to developmental, physiological, or other disorders. But SCDC mechanisms work to up-regulate or increase the level of protein production from genes on individual Z (or X) chromosomes. The importance of maintaining a proper balance is made evident by diseases caused by abnormal numbers of race chromosomes, including Klinefelter syndrome and Turner syndrome in humans, and Valenzuela said these processes have evolutionary and health implications for many others. organizations.
Valenzuela and her co-authors took samples of soft-shell turtles at various stages of development, including embryos, young hatchlings and adults, and analyzed various tissues to determine which genes were activated. The researchers then compared the activity of genes from race chromosomes and from autosomes analyzed from male and female turtles. The study represents not only the first such study to analyze dosage compensation of sex chromosomes in turtles, but the findings also show that remarkably, temperature appears to affect the SCDC process in turtles. Valenzuela has studied temperature-dependent sex determination (TSD), or how environmental temperatures affect whether a turtle embryo develops into a male or female in species that lack sex chromosomes, in previous research. But because softshell turtles lost this ancestral TSD system, this thermal sensitivity in SCDC came as a surprise, he said. And the way soft-shelled turtles carry out SCDC is also unusual and complex.
The study found that both sexes of softshell turtles double the activity of Zs in early embryonic development, which corrects the expression imbalance in ZW females (twice-Z expression now matches autosomal expression). But this same response creates an imbalance in males (Z expression now doubles autosomal expression). At later embryonic stages, male Z expression declines, and this effect is more pronounced at cooler than at warmer incubation temperatures, according to the study. Valenzuela said the new study is likely the first to show that temperature can affect SCDC not just in turtles or animals, but as widely as eukaryotes, or organisms in which genetic material is contained in a cell nucleus. Eukaryotic species include a huge range of organisms, including animals, plants and fungi. (A I)
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