We end the year with the plant genus Cannabis, belonging to the family Cannabaceae, which also includes hops (Humulus sp.) and Hackberries (Celtis). Cannabis is most famous today for its use as a recreational drug, aka marijuana, although the legalization of this plant as a drug has been quite controversial in the United States and across the world. Despite its notoriety, however, the origins, chemical properties, reproductive strategies and dispersal of Cannabis across the globe are quite fascinating and this plant genus has been impacting human culture for ages. Cannabis is one of the oldest domesticated plants and various ancient human cultures have used it for spiritual rituals, medicinal purposes, and fiber for rope or clothing that has been extracted from hemp plants (Li 1973, 1974; Russo, 2007, 2008). The genus most likely originated in Central, South or Eastern Asia but the exact origin of Cannabis is difficult to determine because of shifts in its distribution between glacial cycles (Clarke & Merlin 2013). Humans brought this species to Europe and later to Africa and the Americas where it was cultivated and domesticated into different varieties (Clarke & Merlin 2013).
Carolus Linnaeus, the founder of the binomial species system, was the first person to classify the Cannabis genus in 1753, and only identified a single species, Cannabis sativa L. However, Jean Baptiste Lamarck (yes, the one who came up with a first theory of adaptation, now known and disregarded as “Lamarckian evolution”) described a second species, Cannabis indica, in 1785 (Watts, 2006). While the validity of the second species is debated, the groupings “sativa” and “indica” are still commonly used. Interestingly, Cannabis has an unusual amount of genetic diversity when compared to other plant groups (Sawler et al. 2015; Lynch et al. 2016; Vergara et al. 2016). Recent scientific research has found that there are genetic clusters, and thus it is possible that several other species will be described. However, these do not seem to reflect Lamarck’s classification of C. sativa and C. indica (Sawler et al. 2015; Lynch et al. 2016; Vergara et al. 2016).
What molecular properties and processes make this plant so popular as a recreational drug? Cannabis produces cannabinoids, which interact with our own endocannabinoid system within the brain and nervous system (Gertsch 2008). The endocannabinoid system is involved in regulating multiple physiological processes, including sleep and hunger. One of the primary and most widely known cannabinoids produced by the Cannabis plant isΔ-9-tetrahydrocannabinolic acid (THCA), which is converted to the neutral form Δ-9-tetrahydrocannabinol (THC) once heated. This neutral form interacts with the endocannabinoid system producing a psychoactive effect (gets us “high”). THC also seems to have important medical uses potentially serving as treatment for Parkinson’s disease (Carrol et al. 2012), dementia (Walther et al. 2006), and autoimmune disorders (Lyman et al. 1989). The other well-known cannabinoid in Cannabis is cannabidiolic acid (CBDA), which produces cannabidiol (CBD) when heated. Data suggest CBD, which is not psychoactive, may mitigate some of the negative effects of THC (such as anxiety and paranoia) and has potential uses in treating cancer (Soliman et al. 2015) and epilepsy (Mechoulam et al. 2002; Devinsky et al. 2014). Besides THC and CBD, Cannabis produces around 74 different cannabinoids (ElSohly et al. 2005; Radwan et al. 2008; ), which are present at varying potencies and ratios across particular cultivars and may also have medical importance, including cannabigerol (CBG) Borelli et al., 2014), cannabichrome (CBC) (Izzo et al. 2012) and Δ-9-tetrahydocannabivarin (THCV) (McPartland et al. 2015).
Cannabis also has interesting reproductive strategies. It can be either dioecious, meaning that there are male and female plants, similar to what we see in humans and other animals (Soltis et al. 2005; Bell et al. 2015). However some Cannabis varieties are monoecious, and thus produce both males and female flowers in the same plant. To make this even more confusing, when environmentally stressed, some male plants can produce female flowers and some female plants can produce male flowers. Additionally, sex is determined by two chromosomes, X and Y (thus males are XY and females are XX), but the hermaphrodites have undifferentiated chromosomes (Hirata 1929; Yamado 1943; Sakomoto et al. 1998). Interestingly, males, females and hermaphrodites can cross with each other and produce fertile offspring.
About the Author
Dr. Daniela Vergarais post-doctoral researcher at the University of Colorado, Boulder, where she is working in Dr. Nolan Kane’s lab Cannabis Genomic Research Initiative. Specifically, Daniela has been exploring the cannabinoid genes in the genome and understanding the how these genes relate to the chemotypes. Daniela also founded and is the director of a non-profit organization The Agricultural Genomics Foundation that holds a 501(C)(3) status (AGF; AgriculturalGenomics.org) and aims in becoming a genomic repository (“library of genomes”) helping CGRI perform their research. AGF also educates the public about science, Cannabis, evolutionary biology, and genomics, through public talks.
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The purpose of this and future volumes of the Handbook of Genetics is to bring together a collection of relatively short, authoritative essays or annotated compilations of data on topics of significance to geneticists. Many of the essays will deal with various aspects of the biology of certain species selected because they are favorite subjects for genetic investigation in nature or the laboratory. Often there will be an encyclopedic amount of information available on such a species, with new papers appearing daily. Most of these will be written for specialists in a jargon that is be wildering to a novice, and sometimes even to a veteran geneticist working with evolutionarily distant organisms. For such readers what is needed is a written introduction to the morphology, life cycle, reproductive be havior, and culture methods for the species in question. What are its par ticular advantages (and disadvantages) for genetic study, and what have we learned from it? Where are the classic papers, the key bibliographies, and how does one get stocks of wild type or mutant strains? The chapters devoted to different species will contain information of this sort. Only a few hundreds of the millions of species available to biologists have been subjected to detailed genetic study. However, those that have make up a very heterogeneous sample of the living world.
bacteria biology evolution fungi genetics mutant
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