Nettie and the Sex Chromosomes

In my last year as an undergraduate I had the good fortune to work on a very interesting project where I helped to map a novel sex chromosome: that of Fragaria chiloensis, a species of strawberry native to the west coast of the Americas.  F. chiloensis is one of the forebears of cultivated strawberry, along with another American species called Fragaria virginiana.  The history of cultivated strawberry is pretty fascinating (mistaken identities!  French spies!) and probably deserves a space of its own here.

Fragaria chiloensis on the beach: Photo by Claus Holzapfel

While working on this project I became interested in the history of our understanding of sex chromosomes.  I was excited to learn from a visiting lecturer of a scientist working during the turn of the century whose contributions were very important to that understanding.  In fact, some scholars cite her as the unsung discoverer of sex chromosomes whose untimely death likely robbed her of the recognition that she deserves.  Her name was Nettie Stevens.

Nettie Stevens, 1904.  Photo from the Carnegie Institution of Washington.

Nettie Stevens was not your typical academic scientist.  She was born in 1861 to a carpenter in Cavendish, Vermont and was educated at the Westford Academy in Massachusetts, which still exists today as a public prep school outside of Boston.  The Westford Academy at that time was open to people of any nationality, age or sex.  That disclaimer has since grown considerably and is displayed at the bottom of the school's website: "Westford Public Schools does not discriminate on the basis of race, color, sex, religion, national origin, sexual orientation, disability or homelessness."  After graduating from Westford, Nettie spent the next fifteen years alternating between work as a teacher and librarian and continuing her studies, all the while saving up to pursue bachelor's and master's degrees across the country at Stanford.

After four years in California and at the age of 39, Nettie returned to the east coast in 1900 to complete her doctoral work at Bryn Mawr College outside of Philadelphia.  Around that time, Bryn Mawr, which had been established some 15 years prior, supported an enviable biology faculty despite being an institution for the education of women.  Successively chairing the biology department were Edmund B. Wilson and Thomas Hunt Morgan, the so-called father of modern genetics, whose work with fruit flies would later cement the chromosomal theory of inheritance and earn him the name of a chromosomal unit of measure- the centiMorgan.  Morgan mentored Nettie during her doctoral studies and eventually would write her a glowing recommendation when she was applying for a fellowship through the Carnegie Institute in order to continue her research at Bryn Mawr after graduating.  He wrote:

"Of the graduate students that I have had during the last twelve years I have had no one that was as capable and independent in research work as Miss Stevens and now that she has her degree she is devoting all of her time to research...  Miss Stevens has not only the training but she has also the natural talent that is I believe much harder to find. She has an independent and original mind and does thoroughly whatever she undertakes. I fear to say more lest it may appear that I am overstating her case."

Morgan had good reason to talk up his protegé; in his recommendation he goes on to describe the collaborative work they would undertake in characterizing sex determination upon her successful receipt of the fellowship- they would study the "accessory chromosome" that C. E. McClung had in 1901 proposed to be linked to sex determination.  He concludes his plea: "It is also of the greatest importance to me to have someone working with me on this problem and I know of no one who is so well suited to carry out work of this sort as Miss Stevens."

Stevens and Morgan did not end up publishing their work together; Morgan was still clinging to the idea that traits were passed from one generation to the next by the mixing of parental cytoplasm, the fluid inside of cells, upon the union of sperm and egg at fertilization.  To Edmund B. Wilson, a colleague of Stevens and Morgan, the purpose of the "accessory chromosome" was, at best, to carry hereditary information important for, but not determinant of, sexual traits.

Stevens' work suggested otherwise.  While studying the lowly mealworm, she found that cells from males of the species had 19 large and one small chromosome, while cells from females had 20 large chromosomes.  Moreover, during the creation of sperm cells, half of the sperm were endowed with 10 large chromosomes, while the other half received 9 large and one small chromosome.  In this observation lay a tantalizing explanation for sex determination that fit Mendelian expectations- the small chromosome was in fact a male determinant.  These findings were published in 1905 in her book, Studies in Spermatogenesis, alongside hundreds of painstakingly hand-drawn images of her observations under the microscope.

Mealworm sperm progenitor cell with 19 large
and one small chromosome (highlighted)

Mealworm egg progenitor cell
with 20 large chromosomes

Four mealworm sperm cells:
two with 9 large and one small chromosome (above)
and two with 10 large chromosomes (below)

Nettie clearly recognized the significance of this finding.  She summarizes it in the text of her book:

"Since the somatic cells of the female contain 20 large chromosomes, while those of the male contain 19 large ones and 1 small one, this seems to be a clear case of sex-determination... the spermatozoa which contain the small chromosome determining the male sex, while those that contain 10 chromosomes of equal size determine the female sex."

This is an important point because in the same year, Edmund B. Wilson published a paper making a similar inference about a type of leaf-footed bug in which he had found males to have one less chromosome than females.  Wilson, however, was less willing to call a spade a spade than was Stevens, relegating his comment in support of a chromosomal hypothesis of sex determination to a footnote.  Morgan at this time was still convinced of the primacy of cytoplasm in inheritance and in his private correspondence expressed that his colleague Wilson had gone "wild over chromosomes."

Morgan eventually came around a few years after this work was published.  Not only did he concede that chromosomes are the vehicles of hereditary information, he also went on to conduct truly brilliant work in fruit flies that showed that genes for specific traits were strung like beads along the lengths of chromosomes- physically attached to one another in a phenomenon called linkage.  The closer two genes are to one another on a chromosome, the more likely they are to be inherited together.  This fact can be used to map the distance (now measured in centiMorgans) between genes on a chromosome, and this is exactly what Morgan did, generating the first genetic maps in his lab.  This work established the fruit fly species Drosophila melanogaster as the most important model organism of the 20th century- thousands of researchers study Drosophila to this day.

Nettie Stevens did not fare so well.  Her career was off to a solid start at Bryn Mawr, where she continued her research on sex chromosomes throughout the first decade of the 20th century.  In 1912, Bryn Mawr offered Nettie a professorship, but she was unable to accept the position.  She had fallen ill with breast cancer and moved from Philadelphia to Baltimore to receive state-of-the-art treatment at Johns Hopkins Hospital.  William S. Halsted, the father of modern surgery and pioneer of many surgical practices including radical mastectomy, likely treated her there.  In a summary of Bryn Mawr president M. Carey Thomas' personal papers in the College archive is the following clue: "Writing of Nettie Stevens's illness in May 1912, [Halsted] asserted that he would never accept a fee from a college professor."  Nettie died on May 4, 1912.

Nettie was remembered warmly at Bryn Mawr, eulogized by one of her students as a caring mentor, "taking great personal interest in her students... always eager to help them, especially in making a start with any research work."  In the same eulogy is an anecdote in which Nettie reassures an inquisitive student, "How could you think your questions would bother me?  They never will, so long as I keep my enthusiasm for biology; and that, I hope, will be as long as I live."

Morgan's assessment of Stevens' work was not as favorable.  He certainly acknowledged the importance of her work in characterizing the chromosomes of 59 species of flies and beetles, stating in a summary of her career published in Science:

"Such an extensive study will not seem superfluous when the reception of this important discovery in regard to sex is remembered, for the profound significance of the results were by no means generally appreciated, and it is not going too far to say that many cytologists assumed a sceptical or even antagonistic attitude for several years towards the new discovery."

However, Morgan goes on to criticize her careful thoroughness, writing that her "work is characterized by its precision, and by a caution that seldom ventures far from the immediate observation.  Her contributions are models of brevity- a brevity amounting at times almost to meagerness."  Stevens was perhaps not outwardly romantic about her love of science since Morgan felt that "She was a trained expert in the modern sense- in the sense which biology has ceased to be a playground for the amateur and a plaything for the mystic."  But perhaps the most scathing remark is the combination of these two perceived faults: "...she was careful to a degree that makes her work appear at times wanting in that sort of inspiration that utilizes the plain fact of discovery for wider vision."

This harsh judgment seems unfair.  I cannot doubt that Stevens sat daily before her microscope in awe of what lay before her, for she was so singularly devoted to her work- a career for which she worked so hard even just to begin.  She paid her way through school with money she had spent 15 years saving and fought for funding to continue her research once she had finished her training.  I am inclined to believe that her caution was necessary- Stevens did not have the reputation to help her weather a false step.  If a wild extrapolation from her data proved incorrect, her hard-won career would be over.

There is some scholarly work out there about Nettie Stevens (in addition to her own body of work) on which I relied heavily in researching this post. There are two biographical sketches- one by Stephen G. Brush and another by Ogilvie and Choquette- and I went to their sources as well as additional ones for some of the information on Morgan and Wilson.  However I feel there is more to this story.  Ogilvie and Choquette concede that personal biography is difficult because so few sources exist regarding Stevens' life, but I am intrigued by that little nugget in the M. Carey Thomas papers at Bryn Mawr about Halsted treating Nettie.  Perhaps a research trip to Philadelphia and Baltimore is in order?