Reverse evolution?

| 28 Comments

When I bought my house, I inherited a weed (which some consider a ground cover) known as snow on the mountain (Aegopodium podagraria “variegatum”). The plant has a variegated leaf, and I assume it is a cultivar. It is very aggressive and propagates by rhizomes.

One day, a green plant appeared and rapidly took over. No surprise, inasmuch as its leaves are all green, with no white border, and substantially darker than those of the cultivar. The photograph shows the green-leafed plant driving out the variegated plant (sorry—I meant “weed”).

IMG_3507SnowOnTheMountain_600.jpg

The variegated plant is plainly not the result of a single mutation, and evolution is supposed to be irreversible anyway. So where did the green plant come from?

I am sure the answer is “obvious,” but it is not obvious to me, and other readers may be interested in the phenomenon.

28 Comments

It’s not easy being green…

also possible that the variegated phenotype is heterozygous - seeds from the weed (if it self polinates) may produce offspring expressing an allele that is solid green (I don’t know which would be dominant genetically but one may be more vigorout thatn the other based on the amount of sun in the spot they are growing)

-jasonmitchell

Green on the mountain.

Not a great name.

Glen Davidson

I had a similar thing happen with my hostas.

I’m pretty sure the variegation in Snow on the Mountain is an example of chimerism, as Paul Burnett guessed. That means it’s not a trait passed on by seed, and even some kinds of cuttings won’t preserve it. The white parts are variations of a cell type that don’t synthesize chlorophyll due to not expressing certain genes, while the green parts are a different variation which does express the genes necessary for chlorophyll synthesis. There are two genotypes in the same plant, right next to each other in the same tissue.

Regardless, as you noticed and Jason Mitchell said the pure green individuals are more robust under the same conditions because they have a phytosynthetic advantage. Eventually they’ll take over the whole patch (and then some). If you want to preserve the variegated weeds, you’ll have to remove the green ones. If you want to preserve the lawn, it’s probably best to take them all out. And don’t just dump the rhizomes somewhere, or they’ll simply turn into somebody else’s problem.

Here’s some more information about chimerical plants, including methods of propagation and tissue cultures: http://aggie-horticulture.tamu.edu/[…]himeras.html

I remember reading about how a popular cultivar of the Empress Tree, Paulownia tomentosa, had a wonderful fragrance, but, that all clonal descendants of that cultivar are scentless, apparently due to some mutation in one of the cuttings.

Incidentally P. tomentosa is another invasive, weedy species.

My favorite “invasive weedy species” is Tradescantia pallida - lookitup on Google Images - a gorgeous purple ground cover. I found it growing in the sidwalk cracks in New Orleans in 2004 - several pieces found their way into my pocket… It has taken over several flowerpots in my balcony “garden”.

First, what makes you say that this isn’t the result of a single mutation? And who said evolution can’t go backwards. It can always mutate alleles back.

Quite aside from the fact that attributing “backward” or “forward” to evolution tends to assign a false sense of intentionality…

Ooh,ooh! I know this one!

Variegated plants like the original one here have three layers of tissue in their leaves (and other shoot organs.)One or more of these layers (I cant tell whether it is one or two from this photo)has a mutation that prevents it from making chlorophyll, so it that layer is white. At least the L1 layer (the outer layer of the leaf making up its margin) has this mutation. The L3 layer, the inner one makes normal chlorophyll and is green. The middle L2 layer may or may not be normal. If it shares the mutation with the L1 layer, then there should be a light green zone between the white and dark green zones.

The L3 layer normally gives rise to all of the flower tissues, so any seeds from the variegated plant would produce all green plants.

Because this plant also is reproduced vegetatively from rhizomes, it appears that at least one of these rhizomes was produced exclusively from the fully green L3 layer. The advantage of three fully photosynthetically functional layers of leaf tissue is obvious. No back mutation needed (not that it couldn’t happen.)

More speculation: LOH through HDR involving SCE.

In other words: Loss of heterozygousity through homology-directed repair involving sister chromatid exchange. If this were the (IMO unlikely) case “white” would be the dominant and “green” the recessive allele.

First, what makes you say that this isn’t the result of a single mutation? And who said evolution can’t go backwards. It can always mutate alleles back.

I almost said the odds against that are a few billion to one against. But, that’s per individual. If the mutation is a simple point mutation, then once the new allele becomes commonplace, i.e., in several billion individuals, then it would be possible for a few offspring to get the previous allele by mutation.

On the other hand, if the reason the new allele spread in the population was because it conferred an advantage, then the reverse mutation would be detrimental, and wouldn’t spread. Well, unless the environment also reverted to what it was before the new allele became advantageous.

There’s too many variables! :)

But the above analysis (if you call it that) is for a single point-mutation in a common allele. The odds of reversing some set of point mutations would be exponentially lower, especially if they were selected for along the way.

Henry

https://www.google.com/accounts/o8/[…]vtiF0BBqF10Q said: If this were the (IMO unlikely) case “white” would be the dominant and “green” the recessive allele.

I don’t think a white plant - no green, no chlorophyll - would pass that trait on to its descendents.

The variegated plants are born out of chloroplastic mutations, not mutations from nucleus (on chromosomes).

Basically, they are “heteroplasmic”, i.e. their cells is initially a population of usual functional chloroplasts and unusual (mutated) non-chlorophyllian chloroplasts (called leucoplast, because not being green make them appear white, hence the leuco- greek root). When the leaf is built up, some cells will only inherit leucoplasts and some only classical chloroplasts, by mere luck, while the others will inherit both plastids in various proportions. Then all subsequent cells from these lines will keep the signature of this random distribution: if you only have leucoplasts, all daughter cells will only have leucoplasts, thus making the produced leaf areas white. (or green if you only have regular chloroplasts, or inbetween if you have both kinds of plastids). This is how variegation pattern is produced at leaf level.

Sometimes when the complete segregation of plastids is happening early, you may have whole plant parts that are “albinos” (say a whole stem for e.g.), or reversed to fully chloroplastic (green). See also comment on layers a bit above, because the exact pattern may be further controlled differently depending on the plant species. But the basics is always based on heteroplasmy. Genetics of variegation is also interesting in that chloroplasts are paternally inherited in plants, so that it doesn’t breed easy or classically. And leaf patterns still partially rely on random events during cytokinesis (cell division) when plastids are divided among mitotic daughter cell lines.

So complete reversion may happen when an early cell ends up with only regular chloroplasts, or are produced when dad’s pollen has. (I suspect this is what happened here –actually reversion is common in Aegopodium podagraria “variegatum”).

What happens then is an obvious advantage of completely green plants that are fully chloroplastic (and more efficient in photosynthesis) and which take over competitors that still need to sustain large tissue loads that consume resources without producing sugars.

Can FL or IBIG or SteveP understand any of this?

Just Bob said:

Can FL or IBIG or SteveP understand any of this?

They can at least “explain” it: God made it.

Even they might admit that more has happened than just that (“microevolution”), but that’s all that matters.

Glen Davidson

You know, a god that would piss around with flipping an allele in a weed for no particular reason, let alone keeping track of every thought of every person, the actions of every sparrow (and presumably all other animals), and indeed every molecule on Earth (to say nothing of the ENTIRE UNIVERSE) is NOT an anthropomorphic god. In fact, it is nothing like a human being. We are obviously NOT made “in the image of” a being with those capabilities and proclivities. What human being would even want to listen in on all the thoughts of all the people on Earth, all the time, let alone keeping track of every other living creature?

Just Bob said:

You know, a god that would piss around with flipping an allele in a weed for no particular reason, let alone keeping track of every thought of every person, the actions of every sparrow (and presumably all other animals), and indeed every molecule on Earth (to say nothing of the ENTIRE UNIVERSE) is NOT an anthropomorphic god. In fact, it is nothing like a human being. We are obviously NOT made “in the image of” a being with those capabilities and proclivities. What human being would even want to listen in on all the thoughts of all the people on Earth, all the time, let alone keeping track of every other living creature?

Don’t know what that being might be called when it’s at home, but in public these days its initials are NSA.

seedsaside said:

The variegated plants are born out of chloroplastic mutations, not mutations from nucleus (on chromosomes).

Basically, they are “heteroplasmic”, i.e. their cells is initially a population of usual functional chloroplasts and unusual (mutated) non-chlorophyllian chloroplasts (called leucoplast, because not being green make them appear white, hence the leuco- greek root). When the leaf is built up, some cells will only inherit leucoplasts and some only classical chloroplasts, by mere luck, while the others will inherit both plastids in various proportions. Then all subsequent cells from these lines will keep the signature of this random distribution: if you only have leucoplasts, all daughter cells will only have leucoplasts, thus making the produced leaf areas white. (or green if you only have regular chloroplasts, or inbetween if you have both kinds of plastids). This is how variegation pattern is produced at leaf level.

Sometimes when the complete segregation of plastids is happening early, you may have whole plant parts that are “albinos” (say a whole stem for e.g.), or reversed to fully chloroplastic (green). See also comment on layers a bit above, because the exact pattern may be further controlled differently depending on the plant species. But the basics is always based on heteroplasmy. Genetics of variegation is also interesting in that chloroplasts are paternally inherited in plants, so that it doesn’t breed easy or classically. And leaf patterns still partially rely on random events during cytokinesis (cell division) when plastids are divided among mitotic daughter cell lines.

So complete reversion may happen when an early cell ends up with only regular chloroplasts, or are produced when dad’s pollen has. (I suspect this is what happened here –actually reversion is common in Aegopodium podagraria “variegatum”).

What happens then is an obvious advantage of completely green plants that are fully chloroplastic (and more efficient in photosynthesis) and which take over competitors that still need to sustain large tissue loads that consume resources without producing sugars.

That makes sense, it would be exactly analogous to the subset of human genetic mitochondrial disorders that are due to mutations in mitochondrial genes http://en.wikipedia.org/wiki/Mitoch[…]rial_disease

Paul Burnett said:

https://www.google.com/accounts/o8/[…]vtiF0BBqF10Q said: If this were the (IMO unlikely) case “white” would be the dominant and “green” the recessive allele.

I don’t think a white plant - no green, no chlorophyll - would pass that trait on to its descendents.

Actually, lack of chlorophyll in plants would be more akin to a mitochondrial disease in animals, i.e., lacking or passing on defective mitochondria.

And actually, yes, chlorophyll-deficient/lacking plants do pass on their condition to their offspring, as seen in saprophytes like the Indian Pipe and the underground orchids, parasites like dodder and various species of Rafflesia, and various sports that survive through direct human intervention, like the grafted red “albino” cacti one sees for sale in nurseries and supermarkets.

https://me.yahoo.com/a/JxVN0eQFqtmg[…]X_Zhn8#57cad said:

Just Bob said:

Can FL or IBIG or SteveP understand any of this?

They can at least “explain” it: God made it.

Even they might admit that more has happened than just that (“microevolution”), but that’s all that matters.

Glen Davidson

Heh, while reading this thread, before getting down to your post I was asking myself, do people like FL or Steve P bother?

A lot of variegated plants are due to chloroplast mutations as already noted.

One of my yard trees is a variegated holly.

It’s already differentially segregating its chloroplasts. Most branches are variegated. Some are yellow, the color of the variegation. Some are wild type green.

There are a few seedlings around. They are all green.

Does the variegated plant appear fully formed or is it (as I had assumed) a result of selective breeding?

Does the green plant grow from seed because the mutation is in the chloroplast rather than the nucleus? Is that the point I overlooked?

Many thanks for the explanations, some of which I actually understood or thought I did!

Matt Young said:

Does the variegated plant appear fully formed or is it (as I had assumed) a result of selective breeding?

Does the green plant grow from seed because the mutation is in the chloroplast rather than the nucleus? Is that the point I overlooked?

Many thanks for the explanations, some of which I actually understood or thought I did!

Some variegated strains appear fully formed due to a discovered mutant sport being clonally reproduced by their cultivators.

Others are produced through selective breeding.

Matt Young said:

Does the variegated plant appear fully formed or is it (as I had assumed) a result of selective breeding?

Does the green plant grow from seed because the mutation is in the chloroplast rather than the nucleus? Is that the point I overlooked?

Many thanks for the explanations, some of which I actually understood or thought I did!

As noted in another comment above, most of the time, variegation is directly selected in a “mutant” and then kept as such via asexual multiplication. To my knowledge, there’s only very little variegation that is worked out via selective breeding (but that’s also because there’s much lesser breeding for ornementals, because most of variation in ornementals is qualitative and there are no breeding programs like there is for food crops).

Reversed green forms can be produced from ‘somaclonal’ reversion (i.e., when part of a plant is experiencing reversion), but complete reversion assumes a very early developmental reversion, so that almost the whole plant has reversed back. This is possible, though with low probability happening. So the odds are greater that reversion actually happened via sex and seed production. (Apparently you let plants flower, as noticed on the picture at left).

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