The Rise of Human Chromosome 2: The Fertility Problem

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This essay is the second of a series authored by Dave Wisker, Graduate Student in Molecular Ecology at the University of Central Missouri.

As I wrote in the previous essay in this series, Intelligent Design advocate Casey Luskin doesn’t think the fusion which produced human chromosome 2 could have become fixed in the human population:

Miller may have found good empirical evidence for a chromosomal fusion event. But our experience with mammalian genetics tells us that such a chromosomal aberration could have created a non-viable mutant, or a normal individual who could not produce viable offspring. Thus, Neo-Darwinism has a hard time explaining why such a random fusion event was somehow advantageous.

Luskin (and other ID/creationist apologists I’ve seen on internet discussion fora) maintain that the fusion which resulted in human chromosome 2 must have had drastic negative effects on the fertility of heterozygotes for the fusion. This reduction in fitness, they argue, would effectively prevent the propagation of the fusion throughout the population. On the surface, this sounds like an effective argument, since it is known that translocations and fusions can have such a negative effect by producing non-balanced gametes (see PZ Myers’s article on his blog Pharyngula for a detailed explanation). However, anyone familiar with the cytogenetic literature of mammals knows that one cannot claim that these rearrangements greatly decrease fertility with any certainty, since there are numerous examples where such an expected reduction does not occur.

For example, daMota and da Silva (1998) observed centric fusions in goats that had no discernable effect on fertility:

The results suggest the involvement of chromosomes 6 and 15 in the fusion demonstrated by G-banding in prometaphase cells. The Brazilian sample of animals carrying structural rearrangements did not present any reduction in fertility, suggesting the existence of prezygotic selection against unbalanced gametes

They also cite several other goat studies which found the same thing. In rodents, Nachman and Myers (1989) found a similar situation in a population of marsh rats:

The observation of karyotypic uniformity in most species has led to the widespread belief that selection limits chromosomal change. We report an unprecedented amount of chromosomal variation in a natural population of the South American marsh rat Holochlus brasiliensis. This variation consists of four distinct classes of chromosomal rearrangements: whole-arm translocations, pericentric inversions, variation in the amount of euchromatin, and variation in number and kind of supernumerary (B) chromosomes. Twenty-six karyotypes are present among 42 animals. Observations of the natural population over a 7-year period and breeding experiments with captive animas indicate that heterozygous individuals suffer no detectable reduction in fitness. This is at odds with a central assumption in current models of chromosomal speciation and provides a firm rejection of the view that selection necessarily restricts chromosomal change.

In another study, Bardhan and Sharma (2000) studied heterozygote fertility for numerous translocations and fusions in mice, and concluded:

The three chromosomal species exhibit a high incidence of polymorphisms for Robertsonian fusions and pericentric inversions. Breeding experiments and histological analysis of testis show that heterozygosity for pericentric inversions and Robertsonian fusions had no effect on fertility.

Nachman and Myers (1989) reported similar results in marsh rats:

The observation of karyotypic uniformity in most species has led to the widespread belief that selection limits chromosomal change. We report an unprecedented amount of chromosomal variation in a natural population of the South American marsh rat Holochlus brasiliensis. This variation consists of four distinct classes of chromosomal rearrangements: whole-arm translocations, pericentric inversions,variation in the amount of euchromatin, and variation in number and kind of supernumerary (B) chromosomes. Twenty-six karyotypes are present among 42 animals. Observations of the natural population over a 7-year period and breeding experiments with captive animals indicate that heterozygous individuals suffer no detectable reduction in fitness. This is at odds with a central assumption in current models of chromosomal speciation and provides a firm rejection of the view that selection necessarily restricts chromosomal change.

Researchers in speciation are interested in the fertility effects of chromosomal rearrangements because reduced fertility in heterozygotes can be an effective barrier to gene flow between populations differing by such rearrangements. But they have come to the conclusion that this reduced fertility is not as common as cytogenetic theory predicts. Coyne and Orr (1998) summed up the situation this way:

A further problem with chromosomal speciation is that it depends critically on the semisterility of hybrids who are heterozygous for chromosome rearrangements. It is not widely appreciated, however, that heterozygous rearrangements theoretically expected to be deleterious (e.g. fusions and pericentric inversions) in reality often enjoy normal fitness, probably because segregation is regular or recombination is prevented (see discussion in Coyne et al. (1997)).

Spirito (1998) pointed out that the type of rearrangement was not a reliable indicator of its effect on fertility:

[quote] In conclusion, the reduction in fitness due to the presence of a chromosomal rearrangement (especially in the case of inversions and Robertsonian rearrangements) is not foreseeable a priori solely on the basis of the nature of the structural rearrangement. The absence of definite rules means that it is necessary to experimentally analyze the level of selection against the heterozygote for each particular rearrangement of evolutionary interest. (p.321).

So, it should be clear that Casey Luskin’s remarks are simply not reflective of the current thinking in mammalian cytogenetics. In the next essay, I will take Spirito’s advice and examine the situation in mammals and specifically in humans more closely, so that we can get a better picture of the factors affecting the fixation of Human Chromosome 2.

References:

Bardhan A and T Sharma (2000). Meiosis and speciation: a study in a speciating Mus terricolor complex. J. Genet. 79: 105-111

Coyne, JA and HA Orr (1998).The evolutionary genetics of speciation. Phil. Trans. R. Soc. Lond. B 353: 287-305

da Mota LSLS and RAB da Silva (1998). Centric fusion in goats (Capra hircus): Identification of a 6/15 translocation by high resolution chromosome banding . Genet. Mol. Biol. 21(1): S1415-47571998000100012(online publication)

Nachman MW and P Myers (1989). Exceptional chromosomal mutations in a rodent population are not strongly underdominant. PNAS 86: 6666-6670

Spirito, F (1998). The role of chromosomal change in speciation. In Endless Forms: Species and Speciation, DJ Howard and SH Berlocher, eds. Oxford University Press.

20 Comments

Arthur Hunt said: …Casey Luskin’s remarks are simply not reflective of the current thinking in mammalian cytogenetics.

Well, duh! As a degreed geologist and a lawyer (not to mention a paid shill for the Dishonesty Institute), what else would you expect?

Excellent. I look forward to the next installment.

Luskin and other creationists frequenly use a logical fallicy in attacking evolution.

It is that they say that the scientists have not explained A or B or have a hard time explaining C or D. The implication is that this is some find of a refutation. Science, unlike creationism which starts with the answers, finds what it finds when it finds it. The absense of evidence or an explanation today does not mean that there will not be one on the future. It is possible that the explaation will never be forthcoming because the undlying theory is defective, but in science this is always true.

The absense of evidence now is not evidence of absense’

It figures, Luskin tries to refute something that’s been proven for about a decade. What I’d like to know is what’s the significance of the chromosomes? Why is it that a fusion plays a big part in making us human? (see, if we were IDers, we wouldn’t care about finding this out)

Feste said:

It figures, Luskin tries to refute something that’s been proven for about a decade. What I’d like to know is what’s the significance of the chromosomes? Why is it that a fusion plays a big part in making us human? (see, if we were IDers, we wouldn’t care about finding this out)

There is no evidence that I’m aware of suggesting the fusion has much to do with “making us human”, if anything. It’s certainly something that differentiates us from the great apes, but I don’t think it defines us as being human.

I don’t know that I’d consider this a logical fallacy. Luskin (and creationists generally) are uncomfortable with uncertainty and too proud to admit even temporary ignorance. So they keep hammering that science isn’t where to put your faith because science suffers two flaws faith can’t accept - it’s hopelessly incomplete, and it makes mistakes. Magic suffers neither of these flaws, making it a much sturdier vehicle into which to place your faith.

Glad you are posting on this. As a creationist who now accepts evolution, I have always wondered how an event like this gene fushion can take place. It seems I was told it is in part what makes us reproductively isolated from chimps, but then I wondered how the first person who had it wasn’t reproductively isolated from all other humans. Is it that when quite a few of these these events have taken place, separately, and became fixed into our genome over time, that the sum total of them eventually make us reproductively isolated from a sister species.

Wild Horses 66, Domestic Horses 64, and Donkeys 62 Chromosomes When I confront creationists on the issue of Human/Ape chromosome numbers, I use all the science mentioned in this blog, plus I add information from the genus Equus.

I first ask creationists if Horses, Donkeys, and Zebras are related. They often reply “yes, they are of the same created kind”, often noting that Horses, Zebras, and Donkeys can breed with each other and that they are all descendants of the 2 horses on Noah’s ark 4000 years ago. Then I reveal their chromosome numbers.

Wild Horses (Equus ferus przewalskii) have 66 chromosomes. Domestic horses (Equus caballus) have 64 chromosomes, and Donkeys have 62 chromosomes. The cross of a domestic horse and a donkey produces a mule or hinney with 63 chromosomes. Crossing a wild horse with a domestic horse produces a horse with 65 chromosomes.

If Equus species that range in chromosome number from 62 to 66 can all descend from a common ancestor, why can’t apes (48 chromosomes) and humans (46 chromosomes) descend from a common ancestor?

I also ask them if any humans have more than 46 chromosomes. Every time but once, they have said no. Then I inform them that people with Down’s Syndrom have 47 (due to 3 copies of chromosome 21).

We also observe the equivalent sort of thing in the human population all the time - Robertsonian translocations between acrocentric chromosomes, for example, are very common, and while they increase the miscarriage rate, they do so without a terribly noticeable effect on fertility. In a small population, there is no reason why such an effect would not drift to fixity (after all, they happen de novo all the time), or if some positively selected mutation happened close to the centromere on such a fusion chromosome, it could drive itself quite easily to high levels in even a large population.

But creationists ignorant about biology?? Could that ever happen? Surely not!

[I should have added that people with such a translocation have precisely the same amount of genetic material as everyone else, but only 45 chromosomes in total.]

Edwin Hensley said:

Wild Horses 66, Domestic Horses 64, and Donkeys 62 Chromosomes When I confront creationists on the issue of Human/Ape chromosome numbers, I use all the science mentioned in this blog, plus I add information from the genus Equus.

I first ask creationists if Horses, Donkeys, and Zebras are related. They often reply “yes, they are of the same created kind”, often noting that Horses, Zebras, and Donkeys can breed with each other and that they are all descendants of the 2 horses on Noah’s ark 4000 years ago. Then I reveal their chromosome numbers.…

…If Equus species that range in chromosome number from 62 to 66 can all descend from a common ancestor, why can’t apes (48 chromosomes) and humans (46 chromosomes) descend from a common ancestor?

I also ask them if any humans have more than 46 chromosomes. Every time but once, they have said no. Then I inform them that people with Down’s Syndrom have 47 (due to 3 copies of chromosome 21).

I used to be a really sympathetic guy, but my sympathy for the constant attempts by creationists to find a way to “disprove” some aspect of evolution has worn out. I now relish watching them walk into their own traps. I now thoroughly enjoy watching embarrassment overtake them when confronted with evidence that strips them of whatever meager ammunition they thought they had.

Edwin Hensley said:

Wild Horses 66, Domestic Horses 64, and Donkeys 62 Chromosomes When I confront creationists on the issue of Human/Ape chromosome numbers, I use all the science mentioned in this blog, plus I add information from the genus Equus.

I first ask creationists if Horses, Donkeys, and Zebras are related. They often reply “yes, they are of the same created kind”, often noting that Horses, Zebras, and Donkeys can breed with each other and that they are all descendants of the 2 horses on Noah’s ark 4000 years ago. Then I reveal their chromosome numbers.

Wild Horses (Equus ferus przewalskii) have 66 chromosomes. Domestic horses (Equus caballus) have 64 chromosomes, and Donkeys have 62 chromosomes. The cross of a domestic horse and a donkey produces a mule or hinney with 63 chromosomes. Crossing a wild horse with a domestic horse produces a horse with 65 chromosomes.

If Equus species that range in chromosome number from 62 to 66 can all descend from a common ancestor, why can’t apes (48 chromosomes) and humans (46 chromosomes) descend from a common ancestor?

I also ask them if any humans have more than 46 chromosomes. Every time but once, they have said no. Then I inform them that people with Down’s Syndrom have 47 (due to 3 copies of chromosome 21).

Only problem is, as most people know, Mules can’t infertile and breed. That’s why we don’t have a mule genus in nature.

Mules are infertile and can’t breed.

Stud Horse said:

Only problem is, as most people know, Mules can’t infertile and breed. That’s why we don’t have a mule genus in nature.

We wouldn’t have a “mule” genus, anyhow, given as how both donkey and horse are in the same genus, Equus.

Furthermore, it is possible for female mules to breed and have foals, though, there have only been about 60, maybe 63 documented cases of fertile (female) mules mating and producing foals in the past 500 years.

You have presented good evidence that translocations and Robertsonian fusions do not have a negative effect on fertility.

But the putative fusion of chromosome 2 would have been a telemere-telemere fusion. Is there evidence to suggest that such fusions do not have a negative effect on fertility?

Besides us, you mean? :-)

Seriously, they may well have. I wonder if this fusion is related to the proposed/putative “bottlenecks” in human ancestor populations.

Seriously, they may well have. I wonder if this fusion is related to the proposed/putative “bottlenecks” in human ancestor populations.

That’s an interesting thought - fusion (or its reverse) might (sometimes?) reduce fertility between those with and without the change, but not affect couples who both have that change?

Henry

http://www.pubmedcentral.nih.gov/ar[…]rtid=1288329

I’m slowly wading through this, but I think they’re saying what I knew I couldn’t be the first to suggest.

This is closer:

http://www.asa3.org/ASA/PSCF/2004/P[…]04Wilcox.pdf

If you can filter out the religious contortions…

About this Entry

This page contains a single entry by Arthur Hunt published on February 8, 2009 7:32 PM.

The Rise of Human Chromosome 2: The Dicentric Problem was the previous entry in this blog.

Florida: Reliving the Past is the next entry in this blog.

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