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Boveri subcylindrica depictions (Long et al. 2006)
Ciliate Species Identification (1900-1901)
Ciliate Species Identification (1900-1901)
Dr. Stevens studied one-celled parasitic ciliate infusoria, discovering and describing two new species; Licnophora macfarlandi and Boveri subcylindrica and describing their life cycle which she used for her doctoral dissertation. During these studies she explored many characteristics of their microanatomy; specifically, noting the four distinct chromosomes in the Boveri subcylindrica micronucleus (Ogilvie and Choquette 1981).
Planaria Regeneration (1901)
Planaria Regeneration (1901)
Initially observing regeneration of new ciliate species, Dr. Stevens began to study regeneration of planarians and hydroids, her main focus being the Planaria lugubris (Ogilvie and Choquette 1981). Drawing on previous comparative studies by T. H. Morgan on Planaria lugubris and Planaria maculata she continued to do observational and experimental studies on the Planaria lugubris, a species commonly found in the streams near Bryn Mawr. The main aim of her study was to analyze whether the regenerated tissue at the cut surface arose due to cell multiplication at the surface or migration of cells from the old tissue. Upon cutting various planes in the Planaria lugubris, she described detailed accounts of the gradual regeneration of the pharynx, digestive tract, excretory system, nervous system, eyes, and auricular sense organs. From this study, she concluded that the regenerated regions are sourced primarily from the old ectodermal cells on the surface of the cut and the new embryonic regions originate primarily from two sources; (1) migration from older cells, and (2) multiplication of embryonic cells at the surface (Stevens 1901).
Reductional stage of spermatogenesis. Arrow pointing to the accessory chromosome (Gilgenkrantz 2008)
Sex Determination (1905)
Sex Determination (1905)
Upon receiving a research grant to the Carnegie Institution of Washington, Dr. Stevens began her research studying Tenebrio molitor (common mealworm), with a goal of resolving the function of the accessory chromosome in sex determination. By synthesizing previous discoveries of Clarence Erwin McClung and the concepts of Gregor Mendel with her own observations of sperm types in male mealworms, she hypothesized that female offspring are produced when the sperm with the large chromosome fertilizes the egg and male offspring are produced when the sperm with the small chromosome fertilizes the egg (Ogilvie and Choquette 1981; Morgan 1912). This was a breakthrough discovery and led to Dr. Stevens and Dr. E. B. Wilson being cocredited with the independent discovery of the role of X and Y chromosomes in genetic sex determination.
Parthenogenetic Reproduction (1906)
Parthenogenetic Reproduction (1906)
When Dr. Stevens moved on to working with aphids, she observed a problem in her first study of interpreting sex determination. Aphids had parthenogenetic reproductive characteristics, a process where females undergo asexual reproduction. This was an issue within the Mendelian framework because it did not align with the concepts of dominance and segregation. Dr. Stevens rejected the environmental explanation in favor of Mendelian mechanisms, and published Studies on the Germ Cells of Aphids in 1906, proposing two possible alternative mechanisms for this mode of reproduction (Ogilvie and Choquette 1981; Stevens 1906). For her paper entitled, "A Study of the Germ Cells of Aphis rosae and Aphis oenotherae", Stevens received the Ellen Richards Prize of $1000 from the American Women's Table, given to promote scientific research in women (Ogilvie and Choquette 1981). The reason for this specific paper being chosen is not explicitly mentioned; however, due to the lack of knowledge on spermatogenesis in aphids, one can infer that Stevens paper expanded knowledge on development and sex determination, paving the way for future study on these organisms (Stevens 1905).
Parthenogenetic Reproduction in Aphids (Sophisticated Edge)
Drosophila melanogaster (McKie 2017)
Drosophila Melanogaster (1908)
Drosophila Melanogaster (1908)
After publishing many of her works, Dr. Stevens worked on her PhD thesis where she collaborated with Thomas Hunt Morgan in Bryn Mawr College. During this time, she studied a variety of insects including the fruit fly, Drosophila Melanogaster (Wessel 2011). Dr. Stevens discovered four pairs of chromosomes, where one pair determines sex characteristics. Her discovery paved the way for future groundwork in Drosophila cytogenetics (Ogilvie and Choquette 1981).
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