Most plants reproduce via sexual production. Like humans, they combine genetic information from two parents to form a zygote, and then a growing embryo. This embryo, also known as a seed, can develop into a mature plant, with half its genes from each parent.

This strategy of taking two unique sets of DNA from each parent is great for genetic diversity, but can be limiting in agriculture. Thanks to sexual reproduction, classic apple lines cannot be produced from seed, as genetic recombination would result in offspring with different genes from their parents. Take a look at this very simplified example:

There are some plants that can reproduce genetically identical offspring, such as pepper. True-breeding plants can reproduce via asexual reproduction because they are genetically pure, meaning they are entirely homozygous. Unlike apples and humans, the chromosomes from the mother and father are completely identical, so genetic recombination has no effect on the resulting genome. It doesn’t matter what alleles are taken from each parent, because they’re all the same.

This principle is how hybrid corn is made. By crossing two genetically pure corn lines, the offspring hybrids will be genetically uniform. Since only one genotype is made, breeders can select for the most robust lines and consistently produce their seeds at scale. Most importantly, hybrids experience heterosis, or hybrid vigor, in which plants with diverse genotypes outperform what would be expected from the sum of both parents.

This is explained through two major models. As described by the dominance model, each inbred parent has undesireable alleles that aren’t present in the other, so in the hybrid, the inferior allele from one parent is suppressed by the superior allele from the other parent. This explains how weakness from either parent can be avoided, but doesn’t clarify how heterozygous plants can outperform the better of both parents. This phenomenon is explained by the overdominance model, which states that interactions between diverse alleles are superior to those between homozygous alleles. This is thought to be an evolutionary response to promote genetic diversity. Though the molecular mechanism responsible for this effect remains uncharacterized, it is true that inbreeding depression would be prevented by punishing highly homozygous individuals.

The adoption of hybrid corn is partially responsible for the sevenfold jump in corn yield between 1930 and the 1990s. However, crossing hybrids doesn’t produce the same hybrids; it results in the same genetic recombination as crossing honeycrisps. The only way to consistently produce hybrid lines is to cross the genetically pure parents. This is why hybrid corn seed must be bought by farmers instead of collected and used in the next season.

However, true-breeding lines aren’t the only way plants produce genetically uniform offspring. Some plants perform apomixis, in which seeds are produced via mitosis, yielding genetically identical cells, rather than meiosis, the method of sexual reproduction. These apomictic plants can produce seeds that are identical to the plants they came from. With a robust apomictic hybrid corn line, farmers would be able to collect its seeds and plant the same crop year after year.

However, apomixis doesn’t happen naturally in major agricultural crops like corn and sorghum. Of course, researchers have been working for years to see if it can be mimicked through genetic engineering, and the field is rapidly advancing.

Synthetic apomixis has been achieved by preventing meiosis and initiating parthenogenesis, the process of forming a zygote from an egg without fertilization. Creating the Mitosis instead of Meiosis (MiMe) phenotype required three mutations: spo11, rec8, and osd1. Mutating spo11 disrupts homologous recombination, the first step of meiosis in which chromosomes swap some of their DNA. Combined with the rec8 mutation, the chromosomes divide as they do in mitosis, without exchanging genetic information. At this point, the chromosomes have divided as they would in mitosis, but meiosis includes a second step that further divides the DNA. Mutating osd1 avoids this issue by cancelling meiosis II, effectively replacing the normal meiotic process with mitosis.

Although these mutations cause mitosis to occur instead of meiosis, the process will not be initiated on its own. Under normal conditions, seeds will only form upon fertilization, so parthenogenesis must be induced. This can be done with genes that induce embryogenesis such as those in the BABYBOOM (BBM) family. This pathway was utilized in a paper published earlier this month with the engineering of apomictic sorghum, an important grain with outstanding resilience to adverse conditions and poor soil. These researchers induced parthenogenesis through the insertion of a BABYBOOM-LIKE2 transgene, and the MIME phenotype was created by using CRISPR/Cas9 to knock out spo11 and rec8, along with both osdL1 and osdL3, which play similar roles to osd1 in other species. Two methods were employed: a two-step approach in which parthenogenesis was established before making MIME edits, and a single-step approach that made all the changes at once. Although other experiments in rice found the best results in the single-step method, it produced less consistent results in parthenogenesis than the two-step method in sorghum.

Unfortunately, the apomictic sorghum produced significantly less seed than the sexual control, which would make it unrealistic for farmers to use. Of course, this research is early. Over the next several years, further innovations will be made to bring synthetic apomixis closer to implementation, which has immense agricultural potential. Producing hybrid corn requires the growth and pollination of inefficient pure lines every year, which is expensive. This cost is why most rice isn’t hybrid, despite the benefits hybrid vigor would bring. This technology can increase the accessibility of hybrid seeds for farmers, which will result in higher yields that can feed more people. If applied, these innovations stand to increase the productivity and accessibility of food for the future.

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I had the pleasure of hearing Dr. Venkatesan Sundaresan, a professor at UC Davis with distinguished work on apomixis, present for the Plant & Microbial Biology department at UC Berkeley a few weeks ago. I had heard about apomixis before, but hadn’t fully understood it until his presentation, which I found extremely interesting. He earned a Wolf Prize for his research on apomixis, a highly prestigious award considered by some to be the agricultural equivalent of a Nobel. His presentation focused on early embryogenesis and the genes involved, including how expression differs in paternal and maternal DNA, which is important when engineering a system without any paternal genes. Turns out, since the sperm does not apply as much mRNA as the much larger egg, paternal mRNA makes up a lot of zygotic transcription. This is crucial for genes like BBM, because in an all-female system, they might need to be synthetically induced. Interesting to think about!

I’ve had a few conversations about apomixis with others, some of whom have raised concerns about its financial incentive. This advancement stands to take power away from seed companies, as without the need to buy seeds every season, farmers can be more self-reliant. Sounds great for the farmers and food security in general, but less profit for corporations. However, major agricultural company Corteva Agriscience seems to be on board. Apomictic plants would make hybrid seed production a lot more affordable, and I’m guessing the ability to patent their lines would protect their intellectual property, preventing farmers from collecting and using seeds.

It’s hard to say for sure how these advancements will change the seed market over the next few decades, but I’m hoping it will advance both productivity and accessibility. If a humanitarian project like Golden Rice or the VIRCA project were to develop a hyperproductive crop line free for farmers to use, they would be able to keep their own seeds while also benefitting from enhanced yields. And who knows, maybe people will be able to buy honeycrisp seeds one day (though I think that’s lower on the priority list).

Stay tuned to learn with me!

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More on apomixis: Is this the future of food? ‘Sexless’ seeds that could transform farming

Hybrid vigor: Unraveling the genetic basis of hybrid vigor

Overdominance is the major genetic basis of lint yield heterosis in interspecific hybrids between G. hirsutum and G. barbadense

Apomictic sorghum paper: Induction of Synthetic Apomixis in Two Sorghum Hybrids Enables Seed Yield and Genotype Preservation Over Multiple Generations