By Dr. Daniela Vergara
Mitochondria and chloroplasts are special parts of plant and animal cells. Mitochondria are like tiny power houses make energy the cell can use. Chloroplasts are found only in plants and help them make food from sunlight through a process called photosynthesis. What makes these two parts interesting is that they have their own DNA, separate from the DNA in the cell’s nucleus, and it is a circular DNA.
These organelles are usually passed down from the mother, this is called maternal inheritance. That’s because when an egg cell and a sperm cell join, most of the cell parts come from the egg. The sperm mostly just gives its DNA, not the other parts of the cell. This is important because by studying mitochondrial or chloroplast DNA, the maternal line can be traced. These organelles also play key roles in health, energy use, and even sex determination in some plants.
The Cannabis sativa Chloroplast Genome
When DNA is extracted from a plant, it includes not only the nuclear genome (the main set of instructions found in the cell’s nucleus) which we have previously discussed, but also the smaller genomes from the chloroplast and mitochondria.
Hops (Humulus lupulus) and cannabis (Cannabis sativa) are close relatives. They both belong to the same plant family, Cannabaceae, and share a common ancestor that lived about 25 to 28 million years ago.
In 2016, three complete chloroplast genomes were assembled and annotated: one from a hops variety called “Saazer” and two from C. sativa varieties, “Carmagnola and “Dagestani”.
The two C. sativa genomes were each 153,871 base pairs long and only differed from each other by 16 small DNA changes called SNPs (single nucleotide polymorphisms).
The hops genome was slightly shorter, at 153,751 base pairs, and had over 1,700 SNPs when compared to cannabis (Vergara et al. 2015). This shows that while hops and C. sativa are related, their chloroplast DNA is still quite different.
Circular map of the C. sativa chloroplast genome (153,871 bp).The genome is divided into four regions: Large Single Copy (LSC), Small Single Copy (SSC), and two Inverted Repeats (IRa and IRb). Genes are color-coded by functional category. This circular map reflects the structure of the chloroplast genome, which is naturally circular in most plants. This particular chloroplast is from the hemp variety Carmagnola.
Like most chloroplast genomes, the C. sativa chloroplast is composed of four main regions: a large single-copy (LSC) region, a small single-copy (SSC) region, and two inverted repeats (IRs).”
The C. sativa Mitochondrial Genome
The mitochondrial genome is also a circular genome but larger than the chloroplast. For C. sativa, the mitochondrial genome is 415,499 base pairs long and contained 54 genes: 38 protein-coding, 15 tRNA, and 3 rRNA genes. This particular mitochondrial genome shown below is from the same Carmagnola plant whose chloroplast we saw above (White et al. 2016).
Circular representation of the C. sativa mitochondrial genome, color-coded by gene function.
CMS- Cytoplasmic Male Sterility
Cytoplasmic male sterility (CMS) is a condition in plants where they cannot produce functional pollen, making them unable to reproduce as males. This trait is usually inherited from the mother because it involves the mitochondria, which are passed down through the maternal line.
CMS occurs when there is a conflict between genes in different parts of the cell—specifically, between mitochondrial DNA (inherited from the mother) and nuclear DNA (inherited from both parents). When these two sets of genetic instructions don’t work well together, the plant can become male sterile, meaning it cannot make healthy pollen.
CMS in C. sativa?
Studies of both the mitochondrial and chloroplast genomes of C. sativa have so far shown no evidence of cytoplasmic male sterility (CMS).
The mitochondrial genome from different C. sativa varieties is very similar, and there’s no strong evidence of the typical changes seen in other plants with CMS. This could mean a few things: CMS might not happen in cannabis at all; or it might happen in a different way that hasn’t been discovered yet. Another possibility is that the genes that cause CMS in C. sativa are actually found in the nucleus (the main DNA center) rather than in the mitochondria (Attia et al. 2021).
More research is needed to find out whether C. sativa has its own unique version of CMS, or if it uses a completely different system for deciding plant sex.
Why This Research Matters
Knowing the full chloroplast and mitochondrial genomes allows researchers to:
- Trace how different C. sativa varieties evolved
- Study how sex evolved in C. sativa
- Improve breeding programs for both marijuana and hemp
References:
Attia, Z., C. Pogoda, D. Vergara, and N. C. Kane. 2021. Mitochondrial genomes do not appear to regulate flowering pattern / reproductive strategy in Cannabis sativa. AoB PLANTS.
Vergara, D., K. H. White, K. G. Keepers, and N. C. Kane. 2015. The complete chloroplast genomes of Cannabis sativa and Humulus lupulus. Mitochondrial DNA:1-2.
White, K. H., D. Vergara, K. G. Keepers, and N. C. Kane. 2016. The complete mitochondrial genome for Cannabis sativa. Mitochondrial DNA Part B 1:715-716.
