The relationship between mankind and marijuana is one that spans thousands of years and just about every continent. Throughout this intimate affair, humans have continually tinkered with cannabis genetics in order to create cultivars that meet our various medical, spiritual and economic needs. Unfortunately, however, excessive genetic manipulation comes at a cost, and our intense efforts to selectively breed cannabis have resulted in a considerable narrowing of its gene pool.
While this has allowed us to design cultivars that express a small number of highly desirable traits – such as high cannabinoid content – it has also led to a loss of other genes that enable the plant to adapt to different environments and resist pests. Unless these genes are bred back into the cannabis genome, the species will continue to weaken, with potentially dire consequences.
From Landraces To Sinsemilla: A History Of Marijuana’s Genetic Diversity
Cannabis is believed to have originated in Asia, yet spread along human trading routes to all corners of the globe. In each new destination, the plant was forced to adapt to specific climatic conditions, which resulted in the emergence of numerous marijuana varieties in different locations around the world. Known as landraces, these early strains were allowed to grow wild, with large numbers of male and female plants existing side-by-side and breeding without human intervention.
Because no one was controlling which plants were allowed to reproduce, genetic diversity was high among these cannabis landraces. Yet this began to change in the twentieth century, when breeders stepped up their efforts to domesticate cannabis in order to create highly homogenised strains.
This involved the implementation of Mendelian breeding, whereby only those individuals that express certain desired characteristics are selected for reproduction. To prevent any unwanted pollination, all male plants had to be removed from the population, and pollen from only the very best of these would then be used to fertilise a few select females.
The elimination of all plants that didn’t meet the breeder’s needs resulted in a huge decrease in genetic diversity. After all, while these individuals may not have been particularly blessed in the cannabinoid department, they would each have possessed other genetic attributes, such as high resistance to pests or drought. Preventing these individuals from breeding therefore reduced the prevalence of these traits within the overall cannabis gene pool.
It was in the 1970s, though, that this process really accelerated, as breeders in the US ramped up their production of sinsemilla cannabis. The term sinsemilla comes from the Spanish word for ‘seedless’, and refers to unfertilised female plants. By this point, cultivators had already cottoned on to the fact that keeping males and females separate results in stronger bud, as seedless females are able to allocate more energy to cannabinoid production. Yet it now became fashionable to do away with males altogether by simply cloning the best females using cuttings.
While this enabled breeders to rapidly stabilise their strains by ensuring that all individuals possessed the same attributes, it also resulted in a huge decrease in the genetic diversity of their cannabis. To make matters worse, the illegality of marijuana prevented the creation of large-scale seed banks or genetic libraries, meaning that any genes that weren’t specifically selected by breeders simply disappeared over time. Today, the vast majority of landraces are more or less extinct, so restoring the original genetic diversity of cannabis is likely to be an extremely difficult task1.
What’s Wrong With Low Genetic Diversity?
To stabilise a strain, cannabis growers need to repeatedly in-breed their plants over several generations. Doing so fixes certain genetic characteristics within that strain, while simultaneously reducing the prevalence of other attributes that are of no use to the breeder or the user.
This can be a dangerous game to play, as it means that every plant in a particular crop will share the same strengths while also carrying the same weaknesses. Therefore, if a disease or pest comes along that these plants have lost the ability to resist, the entire crop can be wiped out.
Such a lack of genetic diversity was responsible for the Irish Potato Famine of 1854-49, which resulted in the deaths of almost a million people from starvation. Because potatoes are generally cloned from tubers rather than bred from seeds, the vast majority of potato plants in Ireland during this period were genetically identical. The most common strain at the time was known as the Irish Lumper, which was genetically predisposed to produce potatoes of a certain quality but had no genetic resistance to a particular form of blight.
When this disease broke out, it ripped through the country’s potato farms, eliminating a huge proportion of crops. Yet with a little more genetic diversity among Ireland’s potatoes, it’s likely that more plants would have retained the ability to resist the blight, resulting in less catastrophic losses.
As the cannabis genome becomes increasingly narrow – or homozygotic – the prevalence of genes that provide resistance to different environmental stresses decreases, making the species susceptible to the sort of disaster that devastated Ireland’s potatoes.
How To Increase Genetic Diversity Within Cannabis
As previously mentioned, breeders have spent decades in-breeding their plants in order to create cultivars that have a high cannabinoid content, largely because this has always been the most marketable trait. To cross these super strong plants with others that have a lower concentration of THC but happen to be resistant to certain weather conditions would not make financial sense, yet it may be necessary in order to safeguard the future of the species.
The process of crossing highly in-bred strains with unrelated plants is called crossing out, and is the main way in which the genetic diversity of cannabis can be increased. Similarly, it may be beneficial for cultivators to end their reliance on clones and to actually allow their female plants to be pollinated and produce seeds. Sowing these seeds will then give rise to plants that are more genetically diverse, and this heterozygosity can be further increased by breeding the resultant generation. The more unique plants that are created, the more genetically vigorous the crop will be.
The problem, however, is that we are essentially talking about sacrificing highly profitable traits in favour of others that have no immediate market value. After all, no one wants to buy weed that won’t get you high but which is great at resisting fungal infections. The challenge is therefore to incentivise commercial cultivators to create genetically diverse cannabis rather than continuing to select for a narrow range of genetic traits.
Previously this has been difficult to do, as the illegality of cannabis has meant that growers’ only financial rewards have come in the form of direct sales. Yet legalisation now opens the door to the creation of industry bodies that can reward breeders for enhancing the genetic diversity of cannabis, thereby ensuring the species continues to thrive into the future.
- Clarke RC, Merlin MD. Cannabis domestication, breeding history, present-day genetic diversity, and future prospects. Critical reviews in plant sciences. 2016 Nov 1;35(5-6):293-327. – https://www.tandfonline.com/doi/abs/10.1080/07352689.2016.1267498
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