Inbreeding Depression

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Jim Tjepkema
Inbreeding Depression

Inbreeding depression is a topic which I think needs some consideration.  Many of the crops that seed savers grow are know to be subject to I D.  To avoid I D a fairly high plant population is needed when saving seeds.  I have seen suggestions that one should save seeds from 80 plants to avoid this problem for plants that can expeience I D.  In the early days of the Seed Savers Exchange there wasn't much attention paid to growing plant populations large enough to avoid I D.  Now that I am aware of this need for planting larger plant populations to avoid I D I am making larger plantings.  I have grown some of my seeds through numberous generation planting only smaller populations of much less than 80 plants and I wonder if I have caused some of them to decline due to I D.  I don't see any that have dramaticly declined.  I would be interested in hearing the opinions and experiences of other seed savers regarding I D.   Jim Tjepkema

inbreediung depression

I'm really glad Jim brought up this point. I know it depends completely on the plant specie type, and sometimes on the variety type as well. For example, with self-pollinating annuals like peas and beans, ID is much less of a concern than with crossers like corn, which contains a lot of genetic plasticty, and even 80 plant may be woefully short of containing the genepool for that variety. Even though peas require many fewer plants than corn, some "varieties" such as Third World landraces and market samples may contain a wide range of genetic diversity, even within phenotypes (plants or seeds which LOOK the same) That's why plant workers often refer to those as "populations", since they almost defy the definition of "variety". Most European and American pea varieties are descended from original "pure-line" or single-seed selections or very narrow "mass selections", so the genetic variation is very small to begin with, and therefore easily captured with a rather small number of plants. The extreme, of course, is a clonal variety like potato, in which case a single plant contains all of the genetic diversity of the whole population (barring mutation). It helps to know what you're working with at the outset, which isn't always possible.

Joseph Lofthouse
Joseph Lofthouse's picture


Joseph Lofthouse
Joseph Lofthouse's picture
As I look at the heirloom

As I look at the heirloom seed movement, I see a tremendous problem with inbreeding depression. I suppose that it's to be expected if a variety is maintained in a "pure" state for 60 generations.

I avoid inbreeding depression in my crops by encouraging promiscuous pollination in every crop. Even among the mostly inbreeding crops I keep a careful watch for the occasional natural cross pollination and give them a special place in my

oxbowfarm's picture
In my opinion, the best way

In my opinion, the best way to avoid inbreeding depression is to have massively more people saving each variety, rather than giving hard and fast rules on population sizes.  When every gardener/farmer saved their own seed, very few/if any crops had variety names, and I strongly suspect seed was traded widely among neighbors and family.  This allowed for gene transfer among sub-populations while allowing home gardeners the ability to save seed from smaller grow-outs.  I just cannot beleive that great grandma mabel kept 80 -100 beets for running to seed etc.  The minimum population "rules" and isolation requirments that are widely taught to potential seed savers discourages so many folks from moving beyond beans and tomatoes.

Kathleen Plunke...
inbreeding depression

I've also been concerned about inbreeding. and would like to hear how others are dealing with it.

When I first joined SSE in 1984  I was hearing advice like "minimum of 6 to 12 plants" for cross-pollinated crops, that number got higher over the years, now I hear John Navazio recommending 200 plants!  

There was a cabbage variety I liked that was disappearing from catalogs years ago, I tried saving seed from the last 6 plants in my cellar, it went downhill rapidly within a few generations.

Most of my cross-pollinated crops have been through several generations of less than 50 plants, (self-poll. much smaller than that.)  I've been trying to increase population sizes and also to  have more years between generations. (See my separate query on freezers.)  Some of my strategies have been using the same parent seed for several years and then combining those for the next generation. (For example, parsley seed saved  from 10-18 plants each year in 2009, 2010, and 2012, all from  seeds from 2006 plus 2007.  Next generation will use 09 + 10 + 12.  I'm hoping the different growing conditions in different years select for slightly different genetics and so I keep more of what is there. 

Where possible, I've also brought in seed of a variety I'm concerned about  from other growers in the SSE  (or sometime purchased if commercially available) to make up what I've probably lost during those small-population generations.   I contributed seed for Roberta's Lutz  Beet project, then got seed back from her with the added genes from the other sources  she used, which I will add to my next generation. This sounds like what oxbow farm is talking about,only at the moment not enough seed growers in the immediate neighborhood to make it work with only local.

All of which takes a lot more planning and garden time, of course, and also results in more seed than I really need.  I start to see why, along with basic seed saving being something most people understand and practice, there's also pretty  long tradition of folks who've specialized in growing seed of these more complicated crops to share.  Besides SSE and now GSN I sell a few seeds locally at the food coop and through a tiny catalog but still less than what I grow in many cases.

I've used the "let it all cross and save the best" for a few crops such as my Swiss Chard, for others I like having distinct varieties where I know what I will be getting.  I would like to hear more from folks doing this. . . 

population size

Tim makes an extremely good point here. While population size certainly deserves consideration, there can be no hard and fast rule re. numbers. For one thing, it varies immensely according to the reproductive stategy of the specie, such as selfer vs crosser, but also the nature of the original sample population: a clone, pure-line selection, mass-selection, market sample, landrace, etc. I share his concern that if people  are obsessive about population size, it will have a chilling effect on seed-saving in general. John Navazio has been highly critical of heirlooms for that reason, yet were it not for those populations, however small, being maintained, then he would never have had the raw material to work with in his breeding in thw first place. Great gandma Mabel's beet strategy may have been something like an old local (here) custom for saving diversity in flint corn seed: before the days of silage corns, there was a local early flint type that everyone grew, but no one named (my own selection i call Byron Yellow from its particular origin). Folks usually grew a small patch for the chickens and porridge, a fraction of an acre. Yet they had a custom of once a year - New Year's Day, i believe - the farmer would take a scoop of corn out of his feed bin, and go pay the neighbours a holiday visit, in the course of which he would dump that scoop into the neighbour's bin, take another scoop out, and proceed to visit the other neighbours in like manner. In the course of that day, every bin would have a slight mixture of other kernels (the same "variety", if such a word applies here). Every farmer maintained his basic selection, while effectively enjoying the germplasm diversity of hundreds of acres. Just one strategy for dealing with a potential problem.

Jim Tjepkema
It might be possible to deal

It might be possible to deal with inbreeding depression by mixng seeds grown by several seed savers.  However, if each of these seed savers is only saving seeds from small populations, all of them might be creating seeds with I. D.  You might have some recovery from ID by mixing ID seeds from several source, but you might not have complete recovery fom ID 

I wonder if intermixing with wild population might have played a role in preventing ID in the past when most or all of the seeds used by gardeners and farmers were ones they saved themselves.  Open polinated varieties created by plant breeders that have a potential for ID may not be very suitable for seed saving by any one who has limited gardening space.  I might have to cut back the number of seeds of this kind that I am saving and grow larger populations of the best ones that I think are worth the extra effort needed to prevent ID.  However, I wonder if the population sizes given as necessary to prvent ID are acurate and if one really always needs such large population sizes to prevent ID.   

I am grateful for this

I am grateful for this discussion.I am new to seed saving and would like to move beyond self pollinators but have certainly felt discouraged by population size reccomenations. I have limited garden space and try to grow as much food as i can for my family but want to start producing more of my own seed.

This discussion, especially

This discussion, especially Tim's comment is especially inspiring to me. Here most of us have always looked at seed saving exchanges as vehicles for preserving VARIETIES, as opposed to preserving diversity within a variety (as Joseph rightly complains about). In future i'm thinking of doing a lot more periodic exchanges with folks who have the same variety, but who, like myself, would like to increase their number of parent plants. I've often mentioned this "community strategy" in my talks but frankly have rarely done it.

Joseph Lofthouse
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My protocol for avoiding

My protocol for avoiding inbreeding depression is:

  • Add small amounts of new genetics to the genepool from time to time
  • Include a small amount of 2 and 3 year old seed in each year's planting
  • Grow a sufficently large population to maintain genetic diversity
  • Be liberal during selection: for example by saving fruits of different sizes, shapes, colors, textures, flavors, and maturity dates.
  • Swap seeds with the neighbors to enhance local adaptability
Russell Crow
Inbreeding Depression

As far as I know inbreeding plants such as tomatos, beans, peas, peppers are self pollinating, and don't suffer from inbreeding depression. They are naturally inbreed already. It's the outbreeding types like corn that need to pollinate by wind or other plants that are bee pollinated and  need to have pollen from other plants of it's own species that you need to make sure you have high enough populations to preserve enough genetic diversity to keep the species healthy.  

Joseph Lofthouse
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Tomatoes may show dramatic

Tomatoes may show dramatic hybrid vigor when hybridized. Researchers in Pakistan found that yields increased as much as 50% in tomato hybrids compared to the best yielding heirloom parents of the cross. (They also found reductions of yield of similar magnitude with other crosses, so specific combining ability is important.) The researchers also found that hybrids bore fruit up to 8 days earlier than either heirloom parent. In my garden that is very important. It seems to me that the dramatic market share of hybrid tomatoes is due to their superior performance. And that's for tomatoes, that have a very narrow genetic base. I suspect that if other mostly-inbreeding crops were subjected to similar tests that we would find similar inbreeding depression.

I love growing hybrid beans and peas because of the glorious yields.