Are Invasive Crayfish in Europe the Result of Parthenogenesis?

[BillTre]

Here is a NY Times article and a Science magazine news article about the crayfish taking over Europe.

Genomes have been sequenced.
It has 3 sets of chromosomes from two different animals (both belonging to the same species). The two parental animals were not closely related.
The two sets of chromosomes from the same animal were virtually identical indicating that they were the result of a doubling of a single chromosome set (or from a very inbred population).
Two chromosome sets from one parent and one from the other makes the resulting crayfish a triploid (three chromosome sets). Triploids are often unfertile.
This crayfish is somehow just making triploid eggs that can start developing without any genetic input from the males. Thus they are doing parthanogenisis. (Some parthogenetic species are require sperm from a closely related species to poke the egg and get development going, some do not.)
These crayfish are supposed to be large, fecund (makes a lot of eggs), and better able to adapt to differeing conditions. They are looking at why that is.
Because they are being produced parthanogenetically, they are all genetic clones of the original. they have lost the ability to mix their genomes up when they breed. This is supposed to limit their long term evolutionary viability because useful new mutations arise rarely and are limited to the clonal lineage of the animals they arose in rather than mixing more widely in the population.

From fish breeding studies it known that triploid fish are made in some fish farms in part because they make bigger fish. That are also sterile (non-invasive) and divert resources to meat production (profit).
Another parallel to fish breeding is they are the result of crossing two distinct genetic groups together (the two parental crayfish). The resulting animal has a lot of heterozygosity at lots of different genetic loci. These are the conditions for producing hybrid vigor, Where you cross together two different genetic lines and the first generation does super well because to has so much heterozygosity, but crosses among the first generation can not maintain the levels of heterozygosity, and those animals are more like their parents. (There are arguments about whether its is really the heterozygosity driving this, but its an OK description).
Both of these conditions are preserved in these crayfish since the are only making clones of themselves. They are triploids (good) and very heterozygous (also good) forever.

 

[Drakkith]

From the science article:

More important than the crustacean’s origin may be that this clone thrives in a wide variety of freshwater habitats, with different temperatures, salinities, and acidity. Clones are supposed to be at a disadvantage because they lack the genetic variation to adapt to new situations. But, “This paper suggests that an animal species can rapidly invade a large geographical area despite reproducing without sex and being clonal,” Danchin says.

The marbled crayfish’s three sets of chromosomes may be key, containing enough variety for adapting to different conditions.

If each offspring is a clone of the parent, how is it that there is "variety" leading to adaption for different environments? I thought there needed to be multiple variations in the traits expressed in offspring to lead to adaption. How does a population of clones adapt to an environment?

 

[BillTre]

They variety they refer to is the variety of alleles for any individual gene (on average). Because they have a gene copy from one of the evolutionary lines and a different gene copy from the other distantly related evolutionary line. The mechanics of the cross that generated the triploid has forced a situation where there are, for every gene, at least one copy from each parental line.
For anyone gene, there will be two distinct versions (Alleles) of that gene. The will be two copies of one of those versions due to the chromosomal doubling.
The important point is having two versions which could have been adapted to different environments. This (in theory) will provide genes to take care of a wider range of conditions.
Since there are 21,000 (protein encoding) genes, there only has to be small effects (on average) on individual genes to get a big effect on overall phenotype.

This could be something like a gene encoding an enzyme having two different versions with different optimal temperatures. Or if could be different control mechanisms, or other things... It doesn't have to be a simple dominant-recessive like situation.

 

[Drakkith]

This could be something like a gene encoding an enzyme having two different versions with different optimal temperatures. Or if could be different control mechanisms, or other things... It doesn't have to be a simple dominant-recessive like situation.

Ah, I see. Thanks Bill!

 

[256bits]

The important point is having two versions which could have been adapted to different environments. This (in theory) will provide genes to take care of a wider range of conditions.

I am just quoting you so as to refer to the quote that @Drakkith outlined, which is what also picked interest from the article.
I would have thought that anyone individual would be able to survive within a range of environmental factors, such as salinity, temperature, pH, as listed in the quote.

The marbled crayfish’s three sets of chromosomes may be key, containing enough variety for adapting to different conditions.

My two sets of chromosomes, ( I haven't checked so a basic assumption ) allow me to survive within a range of environments as a mature individual.
It is my progeny, as would also be the case for the crayfish, where environmental factors come into play as to the development of an egg sell to that of a viable individual able to survive and reproduce. It is as if the quote from the article is a gloss over something not completely investigated, and are just saying something that sounds good.

The marbled crayfish’s three sets of chromosomes may be key, containing enough variety for adapting to different conditions.

An educated guess - they do say "may be the key"

 

[BillTre]

The genetics of the new crayfish is "frozen" in that its genetics is an exact duplication of the parents (except for any NEW mutations that occur in an individual and for any NEW modifications in things like the DNA's methylation state).
The variability they speak of is among the 3 possible alleles of each particular gene (there are 21,000).
This is due to their unstated use of the hybrid vigor argument.

Here is an extreme example of a hybrid vigor argument:

Set-Up:
Two genetic lines are independently breed to complete homozygosity at all genetic loci. All of their genes all have two identical versions of the same allele. Thus, the total number of different alleles equals the total number of different genes (21,000 for the crayfish).
In the case of the crayfish, their separate lines were hypothesized to have been adapted to different environments, so the alleles they have at each of the 21,000 loci have an increased likelihood of being different between the two lines.

My two sets of chromosomes, ( I haven't checked so a basic assumption ) allow me to survive within a range of environments as a mature individual.
It is my progeny, as would also be the case for the crayfish, where environmental factors come into play as to the development of an egg sell to that of a viable individual able to survive and reproduce. It is as if the quote from the article is a gloss over something not completely investigated, and are just saying something that sounds good.

You would be like an individual of one of these crayfish lines adapted to your environment (where your lineage evolved). You may have different alleles at each loci, but they were both selected for the same environment and therefore would not be expected to provide good phenotypes for environments beyond that range. The other crayfish line might be like someone whose lineage evolved in a land far away and different environmental pressures (perhaps Tibet). The person from Tibet might then have genetically encoded different temperature tolerances beyond the range of you and your neighbors, or in the case of Tibetians, different hemoglobins with different oxygen carrying characteristics at different oxygen pressures. This in combination with your alleles, evolved form a non-Tibetan environment, could potentially cover and wider range of environments.

Hybridization:
In the extreme case, crossing two lines that are completely homozygous for different alleles will result in ALL of the offspring of such a cross being completely homozygous for each loci (two different alleles for each gene loci). Ideally, this would result in 42,000 (in a diploid example) total different alleles. This is the maximal heterozygosity an individual could support. In reality, in the crayfish, there would be some alleles that would have the same alleles between the two starting lines, so this is a high end estimate only.

Post Hybridization:
In normal animals (in the ideally totally homozygous diploids) the completely homozygous individuals of the first generation would breed among themselves generating offspring with a reduced amount of heterozygosity (statistically driven by the random choice of alleles at each gene (a 50-50 choice at each of the 21,000 loci)). This is a genetic drift type of argument. Take one allele for parent 1 (a choice of two possible alleles) and the you have a 50-50 chance of randomly picking the same same allele from the second parent. Repeated 21,000 times for each new individual of the second generation, this leads to a big reduction in heterozygosity. This is shy hybrid vigor fades after one generation.

However in the hybrid crayfish, its lack of sexual recombination at reproduction essentially freezes its genome (and all of it progeny) in this heightened state of heterozygosity.

 

[256bits]

This is shy hybrid vigor fades after one generation.
However in the hybrid crayfish...

That is something I will have to poke around with to gain a better feel.
thanks