By Joe Luna

Now here is a paper title from 1905 – “Studies in Spermatogenesis with Especial Reference to the Accessory Chromosome” published in the 36th publication from the Carnegie Institute of Washington. For all the male readers out there, your father may have given you your Y chromosome, but it was a woman who first discovered it. And her name, the sole author of the above paper, was Nettie Maria Stevens (born July 7, 1861).

Nettie Maria Stevens – Image Courtesy of Carnegie Institution of Washington

As you might imagine, academic science was a tough shell to crack for a woman growing up after the Civil War. Stevens was lucky enough to attend the Westford Academy for high school, but further educational options for unmarried young women were limited. To make ends meet, Stevens became a school-teacher and that’s where the story would’ve ended for most women. But not for Nettie Stevens. After saving for more than 15 years, she had enough money to enroll in the four-year program at the Westford Normal School; she finished it in two. Her strong academic record earned her at age 35 enrollment at a relatively new university in Palo Alto, California. Four short years later, Stevens earned her BA (1899) and MA (1900), both from Stanford University. Still, she aimed even higher.

In 1900, Stevens went to Bryn Mawr for graduate school, where she would receive her PhD in 1903. At the time, Bryn Mawr was an incubator for early American geneticists: Thomas Hunt Morgan (remembered for pioneering drosophila research) was a faculty member, having replaced the noted cytologist Edmund Beecher Wilson (more on him later) who accepted a position at Columbia some years earlier. Genetics was very much still in its infancy at the turn of the 20th century, primarily for lack of experimental model systems, but not for lack of microscopes, which were aplenty. And so, with a slue of staining reagents in hand, early geneticists (cytologists in actuality) did what they could do: look at every cell they could get their hands on.

In 1904, Morgan left for Columbia, while Stevens stayed at Bryn Mawr in somewhat of her own research position, winning fellowships to study in Europe and securing funds from the Carnegie Institute of Washington (Andrew Carnegie’s version of the Rockefeller Institute, except without a central campus). That year, she set about describing spermatogenesis in a variety of insects and stumbled upon something extraordinary.

At the time, Morgan, Stevens and Wilson were interested on the chromosomal basis of heredity. They and others had observed that when cells divided, cells went through an orderly cycle that basically involved the condensation and separation of a certain number of chromosomes. But what were these chromosomes? What sort of information did they contain that made their orderly separation so critical? What governed their number? Could they determine the sex of an organism? This last question was of particular interest, but also the most enigmatic, because one could conclude just about anything depending on the organism under study. Still one thing was clear, males (of many species) typically had a weird shrunken poor-excuse-of-a-chromosome (isn’t that always the case?), while females did not. Wilson had argued that this male “accessory” chromosome determined sex. Still, many had their doubts, and contended that this “accessory” chromosome had nothing to do with sex but was an instead an intermediate stage of chromosome number in the evolution of a species.

Stevens tackled this problem by looking at gamete formation in many insects: from termites to sand crickets to aphids and mealworms. In this last species, Tenebrio molitor, she noticed something odd in the chromosome number of fully formed spermatids: half of them had 9 large chromosomes and 1 small one, while the other half had 10 large chromosomes. Somatic cells from the male always had a total of 19 large chromosomes and always 1 small “accessory” chromosome. Turning to female mealworms, she noticed in their ovaries that eggs had 10 large chromosomes with no exception, while somatic cells always contained 20 large chromosomes. Putting 19 and 1 together, she correctly concluded that the accessory chromosome had to determine the male sex and more importantly, she realized that this accessory chromosome passed from father to son. Heredity demonstrated in a microscope!

It is worth mentioning that in 1905, Wilson independently came to the same conclusion in fruit flies. But instead of discovering the Y chromosome as the arbiter of male-ness, he noted that the presence of two X chromosomes is what defines female-ness. In many ways it’s the complementary finding to Steven’s work, and it remains just as ironic that a man discovered it. Wilson and Stevens cited each other in their respective 1905 manuscripts, and as a result we can say that both man and woman discovered the chromosomal basis for sex.

By 1912, it was clear to the faculty of Bryn Mawr that a professorship should be created for Steven’s to continue her independent research, finally free of teaching. Following her mealworm studies, Stevens was renowned as an accomplished scientist in a male dominated profession. She continued to study and describe the chromosomal behavior of a number of insect genera. But as a professorship was created for her, she fell ill with breast cancer and was never able to fill her academic appointment. She died that year at age 51.

Professor or not, Nettie Stevens was a self-made scientific pioneer. And at a time when women could not yet vote, she demonstrated that in American science, unlike the voting booth at the time, a Y-chromosome is not required.

1) Can you believe it? Google books is good for something! Nettie Steven’s 1905 and 1906 papers on spermatogenesis can be read in full with figures. They are fantastic reads.

2) I was considering writing a GOTW of Edmund Wilson, whose story is as compelling as Steven’s, but to be honest, I’m completely out done by Prof. Qais Al-Awqati at Columbia whose article “Edmund Beecher Wilson: America’s First Cell Biologist” is just plain awesome. Read it! It succinctly frames Stevens, Morgan, and Wilson during the birth of modern genetics, development and cell biology.

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