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Mogens Christian Wanning Westergaard (12 June 1912 – 6 August 1975)^[1] was a Danish geneticist and cytogeneticist known for pioneering studies of sex determination in plants, mutagenesis in fungi, and the cytological mechanisms of meiosis.^[2][3] His research on the dioecious plant Melandrium album helped establish that the presence of a Y chromosome determines male sex in the species.^[3]

Westergaard was born in Denmark on 12 June 1912. He studied biology and genetics at the University of Copenhagen, where he became a student of the prominent Danish geneticist Øjvind Winge at the Carlsberg Laboratory.^[3][2]

Early in his career Westergaard developed an interest in cytogenetics and the genetic mechanisms underlying sex determination in plants.^[3]

Westergaard belonged to a prominent intellectual lineage in genetics. In a genealogical analysis of the field by Alfred Henry Sturtevant, Westergaard’s scientific ancestry was traced through Øjvind Winge to the Danish geneticist Wilhelm Johannsen, one of the founders of modern genetics.^[1]

Westergaard served as the first professor of genetics at the University of Copenhagen and he played a central role in developing the university’s Genetics Institute as a major center for genetics research in Denmark.^[3][4]

Westergaard’s early research focused on the genetic basis of sex determination in dioecious plants, particularly Melandrium album (now Silene latifolia).^[3][2]

At the time he began this work, many geneticists believed that plant sex determination operated through a balance between X chromosomes and autosomes, similar to the mechanism described in Drosophila.^[2] Through cytogenetic studies of polyploid and aneuploid plants, Westergaard demonstrated that male sex in Melandrium is determined by the presence of a Y chromosome rather than solely by the ratio of X chromosomes to autosomes.^[3][2] His experiments showed that several X chromosomes are required to counterbalance the male-determining genes carried on a single Y chromosome.^[3]

Using plants carrying fragmented or partially deleted Y chromosomes, Westergaard identified different functional regions of the Y chromosome responsible for suppressing female development and initiating male reproductive structures.^[3] These experiments demonstrated that the Y chromosome carries multiple linked factors involved in sex determination and male fertility.^[3][2]

Based on his experimental findings and comparisons across plant species, Westergaard proposed that the evolution of separate sexes in flowering plants involves at least two closely linked genetic factors: one suppressing female development and another promoting male function.^[5] This model helped explain the origin of plant sex chromosomes and predicted the evolution of suppressed recombination between X and Y chromosomes.^[4]

Related studies in Silene demonstrated that the Y chromosome carries a strong male-determining factor capable of directing male development even in individuals with multiple X chromosomes.^[4] This discovery in plants preceded the identification of a male-determining factor on the human Y chromosome by several decades.^[4] These studies helped establish Melandrium/Silene as a major model for the genetic and evolutionary study of plant sex chromosomes.^[2]

Following the Second World War, Westergaard spent time at the California Institute of Technology working with Herschel K. Mitchell studying fungal genetics.^[3] Westergaard and Mitchell developed a culture medium that facilitated the formation of perithecia in Neurospora crassa, which enabled improved genetic analysis of this model organism.^[3]

Westergaard also pioneered the use of back-mutation assays to study the mutagenic effects of chemical and physical agents in fungi.^[3][2] Work by Westergaard and collaborators using adenine-requiring mutants of Neurospora became an important experimental system for investigating mutagenesis in eukaryotes.^[3]

Later in his career, Westergaard’s work on Neurospora included cytological investigations of meiotic chromosome structure and the synaptonemal complex.^[2][3]

Working with the fungus Neottiella rutilans and later with lily (Lilium), Westergaard and collaborators, including Diter von Wettstein, examined the organization of meiotic chromosomes and the timing of recombination events.^[3] In his later years, he continued studying meiotic chromosome behavior in Lilium, investigating chromosome pairing and crossing-over.^[1]

These studies connected chromosome pairing and crossing-over and provided evidence that DNA replication occurs before karyogamy in ascomycete fungi.^[3] His cytological work helped demonstrate how microscopic analysis of chromosomes could characterize genetic processes.^[2]

During the Second World War he participated in the Danish resistance movement and was imprisoned by German authorities.^[3][6] In 1951, he became the subject of international attention when the United States denied him entry to attend a genetics conference because of a previous Communist Party affiliation.^[7] During the German occupation of Denmark in World War II, Westergaard joined the Danish resistance movement.^[3]

According to records from the Danish Resistance Database, he participated in underground resistance activities and was arrested by German authorities in 1944, spending time in the Frøslev internment camp.^[6][3]

• Westergaard, M. (1946). “Aberrant Y chromosomes and sex expression in Melandrium album.” Hereditas. 32 (3–4): 419–443. doi:10.1111/j.1601-5223.1946.tb02784.x. PMID 20998142.

• Westergaard, M.; Mitchell, H. K. (1947). “Neurospora V. A synthetic medium favoring sexual reproduction.” American Journal of Botany. 34: 573–577. doi:10.1002/j.1537-2197.1947.tb13032.x.

• Westergaard, M. (1958). “The mechanism of sex determination in dioecious flowering plants.” Advances in Genetics. 9: 217–281. doi:10.1016/S0065-2660(08)60163-7.

• Jensen, K.; Kirk, I.; Kølmark, G.; Westergaard, M. (1951). “Chemically induced mutations in Neurospora.” Cold Spring Harbor Symposia on Quantitative Biology. 16: 245–261. doi:10.1101/SQB.1951.016.01.020.