POPULATION STRUCTURE AND HETEROSIS - Hill Temple Collective - International Cannagraphic Magazine Forums

OG bub · 07-10-2006, 10:32 PM

Inbreeding is regularly deleterious in many species (e.g., alfalfa, corn, drosophila) and sometimes deleterious in other species (onions, sunflowers),.but it has little or very little effect in still other species (cucurbits and the self-pollinated species). Many investigators have postulated that these differences depend on mating systems and the influence they have on the kind, the amount, and the ways in which genetic variability is organized in populations. In species that reproduce by self- fertilization, homozygosity is normal; deleterious mutants become homozygous soon after their origin and are eliminated promptly. Accordingly, populations of such species ultimately become adapted to homozygosity and develop a genetic organization that Mather (1943) called "homozygous balance." The type of population structure characteristic of self-pollinated species also appears to exist in some cross-pollinated species that have very small effective population sizes. Thus, a single farmer may require only a few plants to satisfy his needs, so that population size is restricted generation after generation to very few interbreeding individuals. Under such circumstances most loci would be expected to soon become and remain homozygous, thus developing so-called homozygous balance. This may explain why some outcrossing species such as sunflowers and cucurbits show little inbreeding depression.

Populations that suffer severe inbreeding depression have consistently been found to carry large numbers of recessive alleles sheltered in heterozygotes. In Drosophila pseudoobscura, for example, every fly among the many thousands that have been tested has been found to harbor at least one recessive lethal, and lethals are by no means the only component of the load of unfavorable alleles carried by such species. The load of deleterious mutations in alfalfa appears to be at least as great as that in various Drosophila species, and the load in corn is perhaps nearly as great. However, many investigators doubt whether the heterotic responses observed in such species can be accounted for by the dominance hypothesis. One reason for skepticism is that there is little convincing evidence that such commonly observed mutants have much effect on yield. Wentz and Goodsell (1929), for example, compared the frequencies of defective seed, seedling, and mature-plant recessives in 19 open-pollinated corn varieties with the yields of these 19 varieties and detected no relationship. Woodward (1931) self-pollinated a large number of vigorous plants of three open-pollinated varieties of corn and classified the resulting selfed lines for defective mutants. Seeds from selfed ears free from defective mutants were then composited to reconstitute the three varieties. The yields of the "purged stocks" were compared with the yields of the original stocks and found to be nearly exactly the same.

Crow (1984) calculated the maximum heterosis possible under the dominance hypothesis and concluded, "It seems probable that (the dominance hypothesis) may explain a major part of the loss in vigor with close inbreeding of random mating strains and the recovery of vigor on crossing.... However, it cannot account for increase in vigor following the crossing of artificially inbred strains much beyond the level of the equilibrium population from which the inbred lines were derived."

07-10-2006, 10:48 PM

I must correct you on the curcurbits not showing inbreeding depression. I grow AG pumpkins. There is a network of folks who have been inbreeding the AG for size for the past 30 years---its starting to take its toll. In the past 5 years especially. The plants are suddenly not as strong as AGs of the past. A mutant called "ribbon-vine" is becoming more prevalant, which pretty much is a suicidal plant. And it seems it is only going to get worse....

Good post,
Seed

OG bub · 07-11-2006, 03:15 AM

The information above is by Allard, 1999, Principles of Plant Beeding.
not my observations, but my studies.

thanks for the interest. Ill be posting more that you will enjoy, soon.

Peace, bub.

suzycremecheese · 01-20-2007, 10:14 AM

Quote:

I must correct you on the curcurbits not showing inbreeding depression. I grow AG pumpkins. There is a network of folks who have been inbreeding the AG for size for the past 30 years---its starting to take its toll. In the past 5 years especially. The plants are suddenly not as strong as AGs of the past. A mutant called "ribbon-vine" is becoming more prevalant, which pretty much is a suicidal plant. And it seems it is only going to get worse....

Hi,

Good stuff here. Lots of potential for discussion in this forum. I'm of the opinion that every species suffers from some degree of "inbreeding depression," but because they all benefit differently from overdominance/ heterosis, and suffer differently from inbreeding.

I believe that almost any species can suffer from inbreeding if it is too severe. This is because every species has its own deleterious alleles. When heavily inbreeding, especially with small effective breeding populations, if these arent eliminated during selection the frequency will increase and increase until you find a situation like the "ribbon vine" you mention.

Nice real world example to bring up for discussion.

Buddle · 09-20-2008, 06:35 PM

I agree SCC..nice to have a real life exmple.
I may be reading too much into this or its gone way over my head w/o me knowing..LOL..BUT...isn't this kind of the same thing as Darwin's "survival of the fittest " evolution theory? Nature breeds the less desirable characteristics from the plants over time to secure their survival and ultimately their existence.

MHHSP · 10-12-2008, 04:11 AM

Yes that is true buddle until we tamper with species like the dog or pot. haha

In fact population isolation like scc brought up, is one factor needed for evolution to work. The same condition that would lead to inbreeding and higher adverse allele rates among the population... possibly another stress factor driving selection.

The article also mentions yield may not be adversely affected by inbreeding behavior, however this may vary well lead to problems later down the line. Variety among the given species would be reduced and all traits would become somewhat concentrated.
Susceptibility to disease also increases as the plants gene pool narrows.

07-10-2006, 10:32 PM	#1
OG bub ~Cannabis-Resinous~ Join Date: Jun 2005 Location: in the woods by the sun Posts: 1,949	POPULATION STRUCTURE AND HETEROSIS Inbreeding is regularly deleterious in many species (e.g., alfalfa, corn, drosophila) and sometimes deleterious in other species (onions, sunflowers),.but it has little or very little effect in still other species (cucurbits and the self-pollinated species). Many investigators have postulated that these differences depend on mating systems and the influence they have on the kind, the amount, and the ways in which genetic variability is organized in populations. In species that reproduce by self- fertilization, homozygosity is normal; deleterious mutants become homozygous soon after their origin and are eliminated promptly. Accordingly, populations of such species ultimately become adapted to homozygosity and develop a genetic organization that Mather (1943) called "homozygous balance." The type of population structure characteristic of self-pollinated species also appears to exist in some cross-pollinated species that have very small effective population sizes. Thus, a single farmer may require only a few plants to satisfy his needs, so that population size is restricted generation after generation to very few interbreeding individuals. Under such circumstances most loci would be expected to soon become and remain homozygous, thus developing so-called homozygous balance. This may explain why some outcrossing species such as sunflowers and cucurbits show little inbreeding depression. Populations that suffer severe inbreeding depression have consistently been found to carry large numbers of recessive alleles sheltered in heterozygotes. In Drosophila pseudoobscura, for example, every fly among the many thousands that have been tested has been found to harbor at least one recessive lethal, and lethals are by no means the only component of the load of unfavorable alleles carried by such species. The load of deleterious mutations in alfalfa appears to be at least as great as that in various Drosophila species, and the load in corn is perhaps nearly as great. However, many investigators doubt whether the heterotic responses observed in such species can be accounted for by the dominance hypothesis. One reason for skepticism is that there is little convincing evidence that such commonly observed mutants have much effect on yield. Wentz and Goodsell (1929), for example, compared the frequencies of defective seed, seedling, and mature-plant recessives in 19 open-pollinated corn varieties with the yields of these 19 varieties and detected no relationship. Woodward (1931) self-pollinated a large number of vigorous plants of three open-pollinated varieties of corn and classified the resulting selfed lines for defective mutants. Seeds from selfed ears free from defective mutants were then composited to reconstitute the three varieties. The yields of the "purged stocks" were compared with the yields of the original stocks and found to be nearly exactly the same. Crow (1984) calculated the maximum heterosis possible under the dominance hypothesis and concluded, "It seems probable that (the dominance hypothesis) may explain a major part of the loss in vigor with close inbreeding of random mating strains and the recovery of vigor on crossing.... However, it cannot account for increase in vigor following the crossing of artificially inbred strains much beyond the level of the equilibrium population from which the inbred lines were derived." __________________ Resin Enhancer Founder: HillTempleCollective HTC will focus on Medical Cannabis seed production first, recreational Cannabis seed production afterwards. If the two happen to swim in the same pool, then, beautiful. in theory, there is no difference between practice and theory... In practice, there is.. Tao Te Ching
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07-11-2006, 03:15 AM	#3
OG bub ~Cannabis-Resinous~ Join Date: Jun 2005 Location: in the woods by the sun Posts: 1,949	The information above is by Allard, 1999, Principles of Plant Beeding. not my observations, but my studies. thanks for the interest. Ill be posting more that you will enjoy, soon. Peace, bub. __________________ Resin Enhancer Founder: HillTempleCollective HTC will focus on Medical Cannabis seed production first, recreational Cannabis seed production afterwards. If the two happen to swim in the same pool, then, beautiful. in theory, there is no difference between practice and theory... In practice, there is.. Tao Te Ching
	Quote

09-20-2008, 06:35 PM	#5
Buddle Senior Member Join Date: Jan 2008 Posts: 1,946	I agree SCC..nice to have a real life exmple. I may be reading too much into this or its gone way over my head w/o me knowing..LOL..BUT...isn't this kind of the same thing as Darwin's "survival of the fittest " evolution theory? Nature breeds the less desirable characteristics from the plants over time to secure their survival and ultimately their existence. __________________ Looking for that awesome smoke from Hawaii I purchased from a surfer in 1991.No pun intended.Miss that weed.
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07-10-2006, 10:48 PM	#2
Guest Guest Posts: n/a	I must correct you on the curcurbits not showing inbreeding depression. I grow AG pumpkins. There is a network of folks who have been inbreeding the AG for size for the past 30 years---its starting to take its toll. In the past 5 years especially. The plants are suddenly not as strong as AGs of the past. A mutant called "ribbon-vine" is becoming more prevalant, which pretty much is a suicidal plant. And it seems it is only going to get worse.... Good post, Seed
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10-12-2008, 04:11 AM	#6
MHHSP Holy Fuck! Join Date: Dec 2007 Posts: 441	Yes that is true buddle until we tamper with species like the dog or pot. haha In fact population isolation like scc brought up, is one factor needed for evolution to work. The same condition that would lead to inbreeding and higher adverse allele rates among the population... possibly another stress factor driving selection. The article also mentions yield may not be adversely affected by inbreeding behavior, however this may vary well lead to problems later down the line. Variety among the given species would be reduced and all traits would become somewhat concentrated. Susceptibility to disease also increases as the plants gene pool narrows.
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