Saving seeds, avoiding inbreeding depression

DavidP · July 19, 2013

I've been saving seeds for a few years now with pretty good success and the resultant plants look and seem pretty OK.

I read in the book "Seed to seed" by suzanne ashworth that for inbreeding species like tomatoes you need to take seeds from about 20 individuals to really maintain enough genetic variation over time to provide a decent population in the long term. I really am not doing that in that I don't think I've ever grown 20 plants of one single variety.

Is this something I should be worried about and perhaps I should really just be buying seed instead from suppliers who hopefully are maintianing the diversity by using 20 or more plants as there seed source. I know at the moment its probably not much of a concern in a generation or so. I if I keep doing this from a small number of plants will I eventually run into a general deterioration of the variety that I'm trying to save.

I guess I probably shouldn't worry too much as perhaps I should just periodically buy new seeds to get a better gene pool into my seed collection. I'd be interested in any experiences of a variety declining in successive generations and what peoples thoughts were who got much more experience than I do as I've only really been saving seeds for a few years and with no problem as yet.

TZ-OH6 · July 19, 2013

Someone applying population genetics models to tomatoes does not know tomatoes.

Theoretically, there is no inbreeding depression in a tomato variety because a tomato variety is not a genetically mixed population. It is more akin to a clone because it has been selected to be homozygous at all alleles All of the variation has been bred out of it to stabilize the best qualities, so there are no hidden bad recessive alleles to get concentrated and expressed through inbreeding.

What can happen is just the opposite; somewhat like genetic drift -- when you accidentally get crossed seeds when you don't want them. The fewer seeds you save the more likely it is that you will accidentally save many seeds from a bee cross because tomato flowers do not outcross evenly. One tomato/ tomato flower can have 0% outcrossing, and the next can have 30% depending on where the bees have been. Most tomatoes are on the low end, 0%-10% crossed seeds, so save a couple to make sure you didn't get one of the rare 30%-40% crossed fruit.

There is also the chance that you are saving seeds from a hybrid and don't know it, so saving from more than one plant is advisable. It also helps to know what the variety is supposed to look like, so grow more than one plant.

joseph · July 20, 2013

My take on tomatoes, is that when they left their native ecosystem that their native pollinators did not get taken with them, and therefore they have adapted, after a fashion to growing as highly inbred plants, but they are still highly inbred plants. Heirlooms and OP tomatoes suffer the common symptoms of highly inbred plants: lack of vigor, poor fruit set, etc... There is a very good reason that hybrid tomatoes are so highly sought after and widely planted. The effects of the inbreeding depression have been partially undone.

In my garden, I am working towards a promiscuously pollinating population of tomatoes. I have 2 or 3 species of bumblebees that pollinate tomatoes. So I am selecting for tomato plants with extended stigmas that drop clouds of pollen on the bumblebees, so that the plants are highly attractive to bumblebees, and thus the plants can cross pollinate willy-nilly like they did before they were removed from their native ecosystem.

Heritage · July 20, 2013

David,

Good question!

If you aren't able to maintain a population of ~20 plants, here are a couple of alternatives:

1. Save seed from 4 plants for 5 years and keep each batch of seed (from individual plants) separate. Start all 20 of the plants from the original seed source and not from the seed saved the last season. In year 6, plant seed from each of the 4 plants you grew in year one, etc... This method requires a lot of record keeping but, otherwise, is practical. Since tomato seed stays viable for many years, this method could be applied to any number of years ( e.g., 2 plants for 10 years, all from the original seed source)

2. Trade or buy seeds from growers who are maintaining a genetically diverse population of the variety you want to grow.

3. Add seed from other sources to your own population of that variety. For this, Tania's database and SSE yearbooks are good references to determine the seed history/lineage.

Steve

July 19, 2013	#1
DavidP Tomatovillian™ Join Date: Jun 2011 Location: Riverside, Southern California, USDA 9b, Sunset 19 Posts: 63	Saving seeds, avoiding inbreeding depression I've been saving seeds for a few years now with pretty good success and the resultant plants look and seem pretty OK. I read in the book "Seed to seed" by suzanne ashworth that for inbreeding species like tomatoes you need to take seeds from about 20 individuals to really maintain enough genetic variation over time to provide a decent population in the long term. I really am not doing that in that I don't think I've ever grown 20 plants of one single variety. Is this something I should be worried about and perhaps I should really just be buying seed instead from suppliers who hopefully are maintianing the diversity by using 20 or more plants as there seed source. I know at the moment its probably not much of a concern in a generation or so. I if I keep doing this from a small number of plants will I eventually run into a general deterioration of the variety that I'm trying to save. I guess I probably shouldn't worry too much as perhaps I should just periodically buy new seeds to get a better gene pool into my seed collection. I'd be interested in any experiences of a variety declining in successive generations and what peoples thoughts were who got much more experience than I do as I've only really been saving seeds for a few years and with no problem as yet.

July 19, 2013	#2
TZ-OH6 Tomatovillian™ Join Date: Feb 2008 Location: Mid-Ohio Posts: 851	Someone applying population genetics models to tomatoes does not know tomatoes. Theoretically, there is no inbreeding depression in a tomato variety because a tomato variety is not a genetically mixed population. It is more akin to a clone because it has been selected to be homozygous at all alleles All of the variation has been bred out of it to stabilize the best qualities, so there are no hidden bad recessive alleles to get concentrated and expressed through inbreeding. What can happen is just the opposite; somewhat like genetic drift -- when you accidentally get crossed seeds when you don't want them. The fewer seeds you save the more likely it is that you will accidentally save many seeds from a bee cross because tomato flowers do not outcross evenly. One tomato/ tomato flower can have 0% outcrossing, and the next can have 30% depending on where the bees have been. Most tomatoes are on the low end, 0%-10% crossed seeds, so save a couple to make sure you didn't get one of the rare 30%-40% crossed fruit. There is also the chance that you are saving seeds from a hybrid and don't know it, so saving from more than one plant is advisable. It also helps to know what the variety is supposed to look like, so grow more than one plant.

July 20, 2013	#3
joseph Tomatovillian™ Join Date: May 2013 Location: Cache Valley, N/E of The Great Salt Lake Posts: 1,244	My take on tomatoes, is that when they left their native ecosystem that their native pollinators did not get taken with them, and therefore they have adapted, after a fashion to growing as highly inbred plants, but they are still highly inbred plants. Heirlooms and OP tomatoes suffer the common symptoms of highly inbred plants: lack of vigor, poor fruit set, etc... There is a very good reason that hybrid tomatoes are so highly sought after and widely planted. The effects of the inbreeding depression have been partially undone. In my garden, I am working towards a promiscuously pollinating population of tomatoes. I have 2 or 3 species of bumblebees that pollinate tomatoes. So I am selecting for tomato plants with extended stigmas that drop clouds of pollen on the bumblebees, so that the plants are highly attractive to bumblebees, and thus the plants can cross pollinate willy-nilly like they did before they were removed from their native ecosystem.

July 20, 2013	#4
Heritage Tomatovillian™ Join Date: Nov 2010 Location: San Diego Posts: 1,255	David, Good question! If you aren't able to maintain a population of ~20 plants, here are a couple of alternatives: 1. Save seed from 4 plants for 5 years and keep each batch of seed (from individual plants) separate. Start all 20 of the plants from the original seed source and not from the seed saved the last season. In year 6, plant seed from each of the 4 plants you grew in year one, etc... This method requires a lot of record keeping but, otherwise, is practical. Since tomato seed stays viable for many years, this method could be applied to any number of years ( e.g., 2 plants for 10 years, all from the original seed source) 2. Trade or buy seeds from growers who are maintaining a genetically diverse population of the variety you want to grow. 3. Add seed from other sources to your own population of that variety. For this, Tania's database and SSE yearbooks are good references to determine the seed history/lineage. Steve