How does a species evolve to have more or less chromosomes?

[wywong]

I cannot figure out how the first member of such a new species breeds. I can think of only three possibilities:

i. it breeds with an unrelated partner with exactly the same changes in their chromosomes, which is highly unlikely,
ii. it breeds with a sibling with similar chromosome makeup,
iii. it breeds with a partner with different number of chromosomes, but somehow produces fertile offspring with similar chromosome makeup as the former.

For example, did the first hominid with 46 chromosomes find a mate among those with 48 chromosomes, or one with 46 chromosomes as well?

 

[jim mcnamara]

To start with, consider altering some of your assumptions. There wasn't a first hominid. There was an isolated population of individuals that over time diverged.
One of the successful traits new species can have is to prevent hybridization with the 'parent line'. To prevent 'undoing' favorable changes with gene flow from existing older populations.

So over a long period of time some early hominids lived apart and responded to environmental changes (Natural Selection). And those changes were 'permanent' because the new hominids were genetically isolated.

:: the terms I used with ' characters around them are there for understanding, there are other more precise terms. Which need long-winded definitions.

 

[Drakkith]

I'm also curious as to how chromosone numbers can change. I'm certain that a mutation that changes the number of chromosomes is usually harmful and usually results in an inability to breed, but there must be some method by which this can occur and spread.

To start with, consider altering some of your assumptions. There wasn't a first hominid. There was an isolated population of individuals that over time diverged.

I think the OP just meant the first hominid with 46 chromosomes, not the first hominid of all time.

 

[jim mcnamara]

@Drakkith
Chromosome number is not conserved quite the way you think. Read about polyploidy and chromosome fusion:
http://www.talkorigins.org/origins/postmonth/jan99.html

But you are right - many individuals may fail to reproduce before the 'new' chromosome number takes over.

 

[Drakkith]

@Drakkith
Chromosome number is not conserved quite the way you think. Read about polyploidy and chromosome fusion:
http://www.talkorigins.org/origins/postmonth/jan99.html

My apologies for not being clear. I'm aware that chromosome fusion occurs, I'm curious as to how such an offspring can be fertile (and with apparently few or no major negative traits because of this fusion) and how it can then spread to become the dominant/only number in a population.

 

[wywong]

I think of a 4th possibility. It is quite convoluted and only work for animals that care for their young:

An individual acquires a mutation (let's call it M) that increases the chance of some offspring having certain chromosomal change. At the beginning those mutant offspring are unable to pass on their genes due to lack of mating partners and disabilities. However, their normal siblings can still pass on M. Incidentally that parent also has some other good genes close to M, so that it has more descendants that others, which in turn means a small but considerable portion of the population are infertile mutants with chromosomal change. Still the chance that two such mutants can produce a healthy offspring is extremely small. We need a mechanism by which those infertile mutants can evolve to remove their negative traits.

Suppose a parent with M acquires another mutation (N) that compensates for one negative trait associated with the chromosomal change. As a result, that parent needs less resources to cater for its mutant offspring, and can thus bring up more healthy offspring, which help to spread N. Thus, even though N is only beneficial to infertile mutants, it is selected for. After many generations, there are enough compensatory genes in the gene pool so that some mutants have so few disabilities that they can produce viable offspring.

Does the above make sense?

 

[Ygggdrasil]

PZ Meyers has a good explanation on his blog about how new species with different chromosome numbers might evolve: http://scienceblogs.com/pharyngula/2008/04/21/basics-how-can-chromosome-numb/

For example, when talking about individuals of the ancestral species that carry a chromosome fission or fusion:

The net result is that although this individual is fine and healthy, a significant number of his or her gametes may carry serious chromosomal errors, which means they may have reduced fertility. They aren’t sterile, though; some of their gametes will have the full complement of genes, and can similarly produce new healthy individuals who will probably have fertility problems. (Note: the significance of those fertility problems will vary from species to species. Organisms that rely on producing massive numbers of progeny so that a few survive to adulthood would be hit hard by a change that cuts fecundity; species that rely on producing a few progeny that we raise carefully to adulthood, like us, not so much. So you have to have sex 20 times to successfully produce a child instead of 5 times; that won’t usually be a handicap.)

So our two chromosome individual will have a reduced fertility as long as he or she is breeding with the normal one chromosome organisms, but those split chromosomes can continue to spread through the population. They are not certain to spread — they’re more likely to eventually go extinct — but by chance alone there can be continued propagation of the two chromosome variant. Which leads to another misconception in the question: something doesn’t have to provide a benefit to spread through a population! Chance alone can do it. We don’t have to argue for a benefit of chromosome fission at all in order for it to happen.

Such chromosomal abnormalities (for example, occurring in an isolated population of a species and reaching fixation through bottleneck or founder effects) could be the first step in establishing the reproductive barriers that split one species into two species.

 

[wywong]

If I understand correctly, one of our ancestors had 47 chromosomes due to fusion to two chromosomes. He/she was fit and healthy because he/she had the same set of genes. Some of his/her gametes had 24 normal chromosomes, some had 23 chromosomes with the same set of genes, and others had 23 or 24 chromosomes with extra or deleted genes. Those gametes with the 23 healthy chromosomes could successfully fuse with his/her partners' gametes (with 24 chromosomes), to produce healthy offspring with 47 chromosomes just like him/her. When two cousins (or distant cousins) with 47 chromosomes bred, some of their healthy children had 46 chromosomes like us. All of their gametes contained the same healthy set of 23 chromosomes; these individuals could produce healthy offspring with partners with 46, 47 or 48 chromosomes. Since individuals with 47 chromosomes are less fertile, successful populations tend to become either entirely 48-chromosome or entirely 46-chromosome.

 

[snorkack]

European horse has 64 chromosomes.
Ass has 62.
And hybrids of European horse and ass are mules who are infertile.
But Przewalski horse has 66 chromosomes.
And hybrids of Przewalski and European horse, while having 65 chromosomes, are fully fertile.
How do they manage that odd number of mismatching chromosomes without fertility problems?
Are there any other hybrids with mismatched chromosomes who are fully fertile?

 

[wywong]

As far as I can see, mismatch in chromosome number per se does not pose fertility problem, and mules' fertility problem is caused by the mismatch between the two sets of genes, not the odd number of chromosomes.

I don't know of other fertile hybrids with mismatched chromosomes but I just found that as much as 0.1% of humans have an odd number of chromosomes with no ill effects (see http://genetics.thetech.org/ask/ask409).

 

[BillTre]

There are a couple things going on here:
1)
Diploid organisms (having two sets of chromosomes) need close to a full sets of genes (some can be deleted, but not lots at once). Lacking one copy of many genes can be bad (resulting in death, infertility, or lesser problems).

2)
One of the main jobs chromosomes do is to drag around the sets of genes when cells divide. This ensures that each daughter cell gets either a full set of genes (two copies of each gene) after the chromosomes are duplicated (during mitosis: making normal cells) or one set of genes (a single copy of each gene) in meiosis: to make reproductive cells.
To do this, chromosomes pair up before the cell divides and then in successful divisions, one chromosome of each pair will randomly go to a each daughter pair. Normally, this would result in two copies of all the genes in normal cells or in one gene copy in each gamete (sperm or eggs).

Mismatched sets of chromosomes can carry all the necessary genes through normal divisions because each chromosome is duplicated before going to the separate daughter cells. However, there can be problems in meiotic divisions where only one set of the two chromosome sets go to each daughter cell. This can result in gametes with weird sets of chromosomes and therefore genes. These gametes may only make viable offspring when they combine with a gamete with a complementary weirdness. Not often likely, but possible.

Consider an animal with one normal set of chromosomes and a set with one chromosome split into two. The chromosomes will pair up nicely (pairing is something that occurs along the length). The one unsplit chromosome will be paired up with the two chromosomes that came from it by splitting in two. When the chromosomes are separated before cell division, the whole chromosome will go to one daughter cell and one of the derived parts will go to the other. The other unmatched chromosome will go to to one of the two daughter cells. If it ends up where the big chromosome is then that cell will have extra genes (not always good) and the other cell will be lacking some. If it goes to the daughter cell with the smaller chromosome then both cells will have a full set of genes.
If a gamete with two smaller chromosomes fuses with a similar gamete, than resulting new organism will have two complete sets of genes on two sets of chromosomes both with the split chromosomes. This might be a one in four occurrence, but a few of these could make a new breeding population of organisms with two sets of split chromosomes.
These things can get a lot more complicated.