Barbara McClintock (June 16, 1902 – September 2, 1992) was an American scientist and cytogeneticist who was awarded the 1983 Nobel Prize in Physiology or Medicine. McClintock received her PhD in botany from Cornell University in 1927. There she started her career as the leader in the development of maize cytogenetics, the focus of her […]
Barbara McClintock (June 16, 1902 – September 2, 1992) was an American scientist and cytogeneticist who was awarded the 1983 Nobel Prize in Physiology or Medicine. McClintock received her PhD in botany from Cornell University in 1927. There she started her career as the leader in the development of maize cytogenetics, the focus of her research for the rest of her life. From the late 1920s, McClintock studied chromosomes and how they change during reproduction in maize. She developed the technique for visualizing maize chromosomes and used microscopic analysis to demonstrate many fundamental genetic ideas. One of those ideas was the notion of genetic recombination by crossing-over during meiosis—a mechanism by which chromosomes exchange information. She produced the first genetic map for maize, linking regions of the chromosome to physical traits. She demonstrated the role of the telomere and centromere, regions of the chromosome that are important in the conservation of genetic information. She was recognized among the best in the field, awarded prestigious fellowships, and elected a member of the National Academy of Sciences in 1944.
During the 1940s and 1950s, McClintock discovered transposition and used it to demonstrate that genes are responsible for turning physical characteristics on and off. She developed theories to explain the suppression and expression of genetic information from one generation of maize plants to the next. Due to skepticism of her research and its implications, she stopped publishing her data in 1953.
Later, she made an extensive study of the cytogenetics and ethnobotany of maize races from South America. McClintock’s research became well understood in the 1960s and 1970s, as other scientists confirmed the mechanisms of genetic change and genetic regulation that she had demonstrated in her maize research in the 1940s and 1950s. Awards and recognition for her contributions to the field followed, including the Nobel Prize in Physiology or Medicine, awarded to her in 1983 for the discovery of genetic transposition; she is the only woman to receive an unshared Nobel Prize in that category.
Zea is a genus of grasses in the family Poaceae. Several species are commonly known as teosintes and are found in Mexico, Guatemala, and Nicaragua. There are five recognized species in the genus: Zea diploperennis, Zea perennis, Zea luxurians, Zea … Continue reading →
Zea is a genus of grasses in the family Poaceae. Several species are commonly known as teosintes and are found in Mexico, Guatemala, and Nicaragua.
There are five recognized species in the genus: Zea diploperennis, Zea perennis, Zea luxurians, Zea nicaraguensis, and Zea mays. The last species is further divided into four subspecies: huehuetenangensis, mexicana, parviglumis, and mays. The first three subspecies are teosintes; the last is maize, or corn, the only domesticated taxon in the genus Zea. The species are grouped into two sections, sect. Luxuriantes, with the first four species, and sect. Zea with Zea mays. The former section is typified by dark-staining knobs made up of heterochromatin that are terminal on most chromosome arms, while most subspecies of sect. Zea may have 0 to 3 knobs between each chromosome end and the centromere and very few terminal knobs (except Z. m. huehuetenangensis which has many large terminal knobs). The two perennials are thought to be one species by some.
Teosintes are critical components of maize evolution, but opinions vary about which taxa were involved. According to the most widely-held evolutionary model, the crop was derived directly from Z. m. parviglumis by selection of key mutations ; up to 12% of its genetic material came from Z. m. mexicana through introgression. Another model proposes that a tiny-eared wild maize was domesticated, and after being spread from east-central Mexico, this cultigen hybridized with Z. luxurians or Z. diploperennis resulting in a great explosion of maize genetic diversity, ear and kernel forms, and capacity to adapt to new habitats, as well as increased yields. A third model suggests that the early maize resulted from a cross between Z. diploperennis and a species of Tripsacum; support for this is minimal. A fourth model posits that teosinte resulted from hybridization between an early wild form of Z. m. mays and Tripsacum.
All but the Nicaraguan species of teosinte may grow in or very near corn fields, providing opportunities for introgression between teosinte and maize. First- and later-generation hybrids are often found in the fields, but the rate of gene exchange is quite low. Some populations of Z. m. mexicana display Vavilovian mimicry within cultivated maize fields, having evolved a maize-like form as a result of the farmers’selective weeding pressure. In some areas of Mexico, teosintes are regarded by maize farmers as a noxious weed, while in a few areas farmers regard it as a beneficial companion plant, and encourage its introgression into their maize.