The fungal geneticist Professor Lorna Casselton CBE FRS died on the 14th February 2014. Professor Casselton was best known for her pioneering research in understanding the genetics of mating in basidiomycete fungi.
Lorna Casselton (née Smith) was born on the 18th July 1938, and was the daughter of William Charles Henry Smith and Cecile Smith (née Bowman). Lorna’s parents ran a smallholding with a retail outlet, which sold produce and seeds. William Smith was a keen amateur naturalist and encouraged his daughters Lorna and Pamela to become biologists. He also had an interest in genetics so there were several appropriate books in the house, such as William Bateson’s, Mammalian Genetics, actually given to him by someone who worked in Bateson’s laboratory. From an early age Lorna became very interested in planting and propagating crops and developed a passion for biology and genetics in particular, which would last for her whole life.
Lorna first attended Southend High School for Girls and then attended University College London where she gained a BSc in Botany and a PhD in 1964. She and her sister were the first generation of their family to attend University. Lorna’s PhD studies were carried out under the supervision of Professor Dan Lewis FRS, who was a pioneer in fungal genetics having carried out work, for instance, in George Beadle’s laboratory, the co-discoverer with Ed Tatum of the one gene-one enzyme hypothesis, based on their studies of Neurospora crassa. In Dan Lewis’s laboratory, Lorna started working on the basidiomycete fungus Coprinopsis cinerea (then known as Coprinus lagopus and subsequently C. cinerea). She became interested in how basidiomycete fungi are able to find successful mating partners, when there are so many individual mating types (or ‘sexes’) in nature. She soon realized that this was a case of these fungi having developed a sophisticated method of distinguishing ‘self’ from ‘non-self’ and she set out to define the genetic basis of this recognition.
In Dan Lewis’ laboratory, Lorna developed a successful technique for generating diploids of Coprinopsis, which greatly facilitated genetic analysis and therefore provided a deeper insight into the genetics of mating. Her years at UCL as a PhD student indeed led to many of the techniques that were subsequently utilised by Lorna in her independent scientific career and she was both a quick learner and a determined student with ambitions to lead her own research group. Lorna first moved to a position as Assistant Lecturer in Royal Holloway College London, before becoming a lecturer and subsequently, Professor of Genetics at Queen Mary College, University of London in 1989.
Lorna was later awarded an AFRC/BBSRC Postdoctoral Fellowship, which was followed by BBSRC Senior Research Fellowship in 1995. It was during this period that Lorna moved to the Department of Plant Sciences at the University of Oxford where she remained for the rest of her career. She was appointed Professor of Fungal Genetics in 1997 and a fellow of St Cross College, Oxford between 1993 and 2003, and was an Honorary Fellow of St Hilda's College, Oxford from 2000.
Lorna Casselton's pioneering contributions were to work out the genetic basis of mating in Coprinopsis, as she had set out to do upon graduating from the Lewis laboratory at UCL. Lorna set about identifying the genes involved in sex determination and through painstaking genetic analysis and molecular biology over more than two decades, she was able to identify the genes that specify each sex and then show how their products interact with each other to allow recognition between different sexes to occur and for mating to proceed.
Lorna and her students identified the genes residing at the A and B mating type loci, which are unlinked chromosomal loci, each containing multiple genes. Critically, in a series of seminal research papers Lorna and her colleagues, most notably Ursula Kües, showed that the A loci contain two genes that encode homeodomain proteins which form heterodimers which are necessary for successful mating and which transcriptionally regulate sexual reproduction and the onset of meiosis. Interestingly, homeodomain proteins are involved in embryonic development in animals (including humans) where they specify overall body patterning, such as the position of the thorax or antennae of an insect.
Indeed, such homoeotic genes had originally been discovered in fruit flies and subsequently found in vertebrates too, but their identification in yeasts and filamentous fungi, which was reported by several groups during this period, was highly significant. To find such proteins in much simpler organisms, such as fungi, revealed much about their evolution and the precise function of these important regulators of multicellular development.
Professor Casselton was also able to determine how different mating type proteins interact with one another to form heterodimers and in this way to show how compatible mating couples could have sex successfully. She also showed how the B locus contains multiple genes that are involved in pheromone production and a single gene that encodes a cognate pheromone receptor, a Gprotein-coupled receptor. Therefore in order to mate successfully Coprinopsis has to have different alleles at both A and B loci, which are necessary for the correct pheromone-receptor combination to effect formation of a dikaryon and for an appropriate heterodimeric transcription factor to be produced to regulate sexual reproduction and meiosis.
It was for these key discoveries into the operation of tetrapolar mating systems in fungi that Professor Casselton was elected a Fellow of the Royal Society in 1999. She was also honoured by the British Mycological Society, who made her an Honorary Member in 2002, and by Societies of Mycology and Fungal Genetics from across the globe.
Lorna was a tenacious and skillful scientist, who pursued a problem of enormous technical difficulty and genetic complexity. The fact that she overcame these challenges was a testament to her determination, her strength of character and her ability to motivate those around her. She was relentlessly enthusiastic about her science, which influenced many other fungal geneticists (including myself), and she also had very exacting standards. Lorna was a formidable critic of sloppy thinking and poorly designed experiments and did not suffer fools gladly. Her unremitting commitment to scientific excellence was the principal reason she was so successful in studying such a difficult research question. She also trained some highly successful scientists, including the late Norman Todd (her first PhD student), and leading fungal biologists Ursula Kües and Meritxell Riquelme.
Lorna was also, however, fantastic company with a very keen sense of humour– indeed she had a sparkling wit. The fact that Lorna worked on the most intimate details of the ‘private lives’ of fungi was certainly not lost on her and she could add the appropriate level of innuendo into her presentations to keep the audience fully engaged. At the Asilomar Fungal Genetics Conferences, of which she was a regular attendee and Plenary Speaker, she was once given the honour of delivering an after dinner speech in which regaled the audience with tales of her life in science and also read us a love poem between two Coprinopsis individual meeting for the first time. Lorna was an incredibly influence on many of the leading fungal biologists working today across the world and was arguably the most important and prominent fungal biologist of the UK in the last 30 years.
Following her election to the Royal Society in 1999, Lorna served on their governing Council from 2002–2003 and in 2006 she was elected Vice-President and Foreign Secretary of the Royal Society. As Foreign Secretary, Lorna travelled very widely, visiting 27 different countries during her three and a half years in office. She was an immensely popular and active Foreign Secretary of the Royal Society, a fantastic ambassador for Fungal Biology and Biology more widely, as well as a strong proponent of the importance of women in science and a recognized leader in her field. She became a Member of the Academia Europaea in 2008 and was awarded an Honorary Doctorate of Science of University College London in September 2010. She was appointed Commander of the Order of the British Empire (CBE) in the 2012 Birthday Honours for services to fungal genetics and international science.
Lorna was very happily married to William Tollett, a pilot, with whom she shared a passion for flying and adventure. She is sadly missed by all who knew her and who respected her approach to science, her great skill and tenacity, her commitment to excellence and her sheer force of energy.
Professor Nicholas J. Talbot
School of Biosciences
University of Exeter