How many seeds must you save to ensure a healthy gene pool in your plot?
I’m often asked how many plants of a particular cultivar should be grown and harvested if you plan to save seed for growing out in subsequent seasons. The answer to that question depends completely on the purpose of your seed saving, and it mostly relates to probability, convenience, and expected outcome.
When you think about a cultivar — say, your favorite open-pollinated roasting corn — you should stop to ponder the total number of viable corn seeds at any given time. And then back it up a generation (or 10) and consider how many corn plants went into producing all those seeds over the years. Now, multiply the number of seeds by the number of genes that the chromosomes carry (in corn, it’s 32,000 genes across 10 chromosome pairs), and account for the fact that each gene can have up to several different forms. Whew. You might need to invest in supercomputer time to sort out all the possible genetic combinations that collectively make up the total population of seeds belonging to your favorite open-pollinated roasting corn.
You might be led to believe that all those seeds are identical, because they look quite similar — they’re red and smooth, and the endosperm is white — and the plants they produce look morphologically similar, and all bear ears in the same time frame. They look nearly identical, and the collection of genes they contain deep inside will be virtually identical, but the forms of those genes will be all over the map! This is what genetic diversity is all about.
Why might it matter that all those forms of genes are included in that large population of seeds? Well, those gene forms, when present in the population, may confer benefits to members of the cultivar that contain them under a different set of environmental circumstances than those under which your corn thrives. The genetic diversity in the entire population of roasting corn might not matter to you at all, unless you move to a new growing Zone and carry seed along with you. At that point, you’ll hope your saved seeds contain the gene forms required for that corn to thrive in the new location. This is why you need to save seed from about 200 plants each year if your mission is to preserve the genetics of a specific heirloom, such as this hypothetical red-kerneled roasting corn. And don’t go selecting for the best-looking ears, because maybe those goofy ears would be the best-looking another climate could produce.
Probability is the reason it’s so important to collect and save a full cross-section of seed types from a full cross-section of plant types to preserve genetic diversity. Any one of us can only grow a small subset of all the seed for a given type of plant. The best scenario is for our seed to be selected randomly from the total population of seed. Of course, this is impossible in reality, but you get the point. We grow out a subsample of, say, 200 seeds pulled from the total population because it’s large enough, in the case of corn cultivars, to minimize the chance of omitting many of the different forms of all those different genes. But there’s still a chance that we’ll lose some gene forms.
The average home gardener is often uninterested in preserving the genetic variability of a particular heirloom, but rather wants to save seed from plants that thrived in their garden, or to keep the sunflower that great-great-grandma brought with her from Russia many generations ago. If that sunflower was grown out through the generations of your family, there’s a significant chance that it’s now quite genetically distinct from the cultivar that came here from Russia. This is because the sample size of seed that immigrated with your great-great-grandmother is probably too small to be fully representative of the population’s genetic variability. And there’s a good likelihood that the generations of your family who grew out the seed only grew a handful of plants, and collected seed from even fewer. There’s nothing wrong with this approach, which is really an inadvertent selection process. But in time you might end up with a highly inbred line, depending on how each generation saved the seed. There’s also a chance the plant is actually a landrace, derived from the original cultivar and genetically predisposed to thrive in your general vicinity.
So, if your approach to saving seed is to keep your favorite cultivars going year after year, and you don’t plan to move very far away, then selecting to favor the development of a landrace may well help improve your gardening experience — assuming that the local climate doesn’t change much and no influx of new pests is expected. Even then, there may be sufficient genetic diversity to allow the population to adapt to those changes. Conversely, there’s a possibility that the cultivar will suffer poor performance, or even die out, if it loses the gene forms that would allow it to adapt.
This discussion doesn’t take into account any flow of gene forms into your saved seed population — although flow can be useful to maintain diversity. First of all, your cultivar might receive pollen from wild plants or other domesticated cultivars growing in your vicinity. Pure seed folks aren’t generally interested in this kind of gene flow because it may “dilute” the gene form concentration that’s specific to your cultivar, or introduce unfavorable characteristics, such as proprietary pesticide resistance that will cause your seed to fail an organic certification test.
The other kind of gene flow can be elicited by routinely adding new seed to your population from other sources, grown in other locations. You can purchase a packet of the heirloom roasting corn, or trade for some with a friend living a couple of Zones away. Mix it with your saved seed and you’ll more than likely infuse some additional genetic diversity into your collection. This is great for home gardeners, but if you’re growing for seed to sell, or to preserve the genetic diversity of a cultivar, and plan to represent that seed as a known cultivar, then you really need to go the extra mile: Source your seed carefully, grow out that seed in a systematic way that will minimize undesirable gene flow, pay attention to the probabilities, and do your best not to inadvertently select when collecting seed to save.
Bio: Hank Will has a Master of Science in plant physiology and holds a doctorate in pigment biochemistry and genetics. He currently serves as Editorial Director for Heirloom Gardener.
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