Student Education Repository (மாணவர்கள் கல்விக் களஞ்சியம்): Today (May 4, 1912) is the Memorial Day of Nettie Stevens for American geneticist credited with the discovery of sex chromosomes.

Tuesday, May 4, 2021

Today (May 4, 1912) is the Memorial Day of Nettie Stevens for American geneticist credited with the discovery of sex chromosomes.

Today (May 4, 1912) is the Memorial Day of Nettie Stevens an American geneticist credited with the discovery of sex chromosomes.

Nettie Maria Stevens was born on July 7, 1861, in Cavendish, Vermont, to Julia and Ephraim Stevens. In 1863, after the death of her mother, her father remarried and the family moved to Westford, Massachusetts. Her father worked as a carpenter and earned enough money to provide Nettie and her sister, Emma, with a strong education through high school. During her education, Stevens was near the top of her class. She and her sister Emma were 2 of the 3 women to graduate from Westford Academy between 1872-and 1883. After graduating in 1880, Stevens moved to Lebanon, New Hampshire to teach high school zoology, physiology, mathematics, English, and Latin. After three years, she returned to Vermont to continue her studies.

Stevens continued her education at Westfield Normal School (now Westfield State University) She completed the four-year course in two years and graduated with the highest scores in her class. Seeking additional training in sciences, in 1896, Stevens enrolled in the newly established Stanford University, where she received her B.A. in 1899 and her M.A. in biology in 1900. She became increasingly focused on histology after completing one year of graduate work in physiology under Professor Oliver Peebles Jenkins and his former student, and assistant professor, Frank Mace Macfarland.

After studying physiology and histology at Stanford, Stevens enrolled in Bryn Mawr College to pursue her PhD in cytology. She focused her doctoral studies on topics such as regeneration in primitive multicellular organisms, the structure of single-celled organisms, the development of sperm and eggs, germ cells of insects, and cell division in sea urchins and worms. During her graduate studies at Bryn Mawr, Stevens was named a President's European Fellow and spent a year (1901–02) at the Zoological Station in Naples, Italy, where she worked with marine organisms, and at the Zoological Institute of the University of Würzburg, Germany.

Returning to the United States, her PhD advisor was the celebrated geneticist Thomas Hunt Morgan, who later moved to Columbia University. In addition, Stevens' experiments were influenced by the work of the previous head of the biology department, Edmund Beecher Wilson, who had moved to Columbia University in 1891. Stevens received her PhD from Bryn Mawr in 1903 and remained at the college as a research fellow in biology for a year. She continued there as a reader in experimental morphology for another year and worked at Bryn Mawr as an associate in experimental morphology from 1905 until her death. She was offered the position she had long sought, as a research professor at Bryn Mawr College, just before cancer took her life, but she was unable to accept it due to her ill health.

Nettie Stevens. The american #geneticist who discovered… | by Sci-Illustrate | Sci-Illustrate Stories | Medium

After receiving her PhD from Bryn Mawr, Stevens was awarded a research assistantship at the Carnegie Institute of Washington from 1904–to 1905. Stevens' post-doctoral year of work at the Carnegie Institution required fellowship support, and both Wilson and Morgan wrote recommendations on her behalf. She applied for funding for research on heredity related to Mendel's laws, specifically sex determination. After receiving the grant, she used germ cells of aphids to examine possible variations in chromosome sets between the two sexes. One paper, written in 1905, won Stevens an award of $1,000 for the best scientific paper written by a woman. Her major sex determination work was published by the Carnegie Institution of Washington in the two-part monograph, "Studies in Spermatogenesis," which highlighted her increasingly promising focus on sex-determination studies and chromosomal inheritance.

In 1908, Stevens received the Alice Freeman Palmer Fellowship from the Association of Collegiate Alumnae, now the American Association of University Women. During that fellowship year, Stevens again conducted research at the Naples Zoological Station and the University of Würzburg, in addition to visiting laboratories throughout Europe. Stevens was one of the first American women to be recognized for her contribution to science. Most of her research was completed at Bryn Mawr College. The highest rank she attained was Associate in Experimental Morphology (1905–1912). At Bryn Mawr, she expanded the fields of genetics, cytology, and embryology.

Although Stevens did not have a university position, she made a career for herself by conducting research at leading marine stations and laboratories. Her record of 38 publications includes several major contributions that further the emerging concepts of chromosomal heredity. By experimenting on germ cells, Stevens interpreted her data to conclude that chromosomes have a role in sex determination during development. As a result of her research, Stevens provided critical evidence for Mendelian and chromosomal theories of inheritance.

Using observations of insect chromosomes, Stevens discovered that, in some species, chromosomes are different between the sexes and when chromosome segregation occurs in sperm formation, this difference leads to outcomes of female versus male progeny. Her discovery was the first time that observable differences in chromosomes could be linked to an observable difference in phenotype or physical attributes (i.e., whether an individual is male or female). This work was published in 1905. Her continuing experiments used a range of insects. She identified the small chromosome currently known as the Y chromosome in the mealworm Tenebrio. She deduced that the chromosomal basis of sex depended on the smaller Y chromosome carried by the male.

An egg fertilized by a sperm that carries the small chromosome becomes a male while an egg fertilized by a sperm with a larger chromosome becomes a female. Studying egg tissue and the fertilization process in aphids, mealworms, beetles, and flies, Stevens saw that there were chromosomes that existed in small-large pairs (now known as XY chromosome pairs) and she also saw unpaired chromosomes, XO. Hermann Henking had studied firebug chromosomes earlier and noticed the chromosome now called X but didn't find the small chromosome now called Y. Stevens realized that the previous idea of Clarence Erwin McClung, that the X chromosome determines sex, was wrong and that sex determination is, in fact, due to the presence or absence of the small (Y) chromosome.

Stevens did not name the chromosomes X or Y. Their current names came later. Edmund Wilson worked on spermatogenesis preparations simultaneously with Stevens' studies. He performed cytological examination only on the testes, that is he didn't examine the female germ cells (eggs) but only the male germ cells (sperm) in his studies. His paper claimed that eggs were too fatty for his staining procedures. After reading the papers describing Stevens' discoveries, Wilson reissued his original paper and in a footnote acknowledged Stevens for the finding of sex chromosomes.

At Bryn Mawr, following her 1905-6 publications, Stevens bred and studied Drosophila melanogaster in the laboratory. She worked with these fruit flies as subjects of her research for some years before Morgan adopted them as his model organism. At 50 years old, and only 9 years after completing her PhD, Stevens died of breast cancer on May 4, 1912, in Baltimore, Maryland. Her career span was short, but she published approximately 40 papers. She was buried in the Westford, Massachusetts cemetery alongside the graves of her father, Ephraim, and her sister, Emma.

Source By: Wikipedia

Information: P.Ramesh, Asst prof of Physics, Nehru Memorial College, Puthanampatti, Trichy.