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Author Topic:   Hybrids and Evolution
Dr Jack
Member (Idle past 185 days)
Posts: 3507
From: Leicester, England
Joined: 07-14-2003


(1)
Message 16 of 26 (532570)
10-24-2009 7:27 AM
Reply to: Message 1 by Straggler
10-21-2009 5:04 PM


The biochemistry of hybrids
Why are hybrids such as mules and ligras infertile?

The exact reason why a particular hybrid is infertile varies, but is commonly down to failure of meiosis to function correctly. During one of the early stages of meiosis, all chromosomes need to be paired up so they can then be divided into the haploid daughter cells.

Where there are a differing number of chromosomes in the two parents species the chromosomes obviously can't pair up; this means that the gametes produced do not contain a correct number of chromosomes and are thus not viable. This can also occur simply because the chromosomes present are sufficiently different.

Why are hybrids even biologically possible from an evolutionary point of view?

Hybridisation can occur because of the commonalities in mating systems, and because there is no selective pressure to prevent them except in cases of sympatric evolution. Related species from the same location are more likely than related species from different locations to be unable to hybridise and to behaviourally avoid hybridisation.

What interesting examples of hybrids are known to exist or be possible?

Poeciliopsis monacha-lucida and Poecilia formosa are wonderfully interesting examples.

P. monacha-lucida is a female only species that is produced by the crossing of a female P. monacha and a male P. lucida. It can mate with P. lucida to produce offspring, however the genetic material passed on by P. monacha-lucida only comes from P. monacha, the entire P. lucida half of the genome is simply discarded. This is called hybridogenesis, it has been observed in other species, such as some frogs (that link also has a helpful diagram).

Poecilia formosa is also all female; it is formed from the hydridisation of P. latippina and P. mexicanuum. It reproduces by gynogenesis; it must mate with a male P. mexicanuum in order to reproduce but the offspring produced is a genetic clone. The sperm from the father has its genetic material entirely discarded. This is called gynogenesis

I'll return to the other questions in another post.


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Replies to this message:
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RAZD
Member
Posts: 19759
From: the other end of the sidewalk
Joined: 03-14-2004
Member Rating: 6.4


(1)
Message 17 of 26 (532573)
10-24-2009 8:02 AM
Reply to: Message 15 by Blue Jay
10-24-2009 2:37 AM


Varieties, Hybrids, Hybrid Zones, and Speciation
Hi Bluejay.

Given all the subjectivity involved in biological definitions, I’m not sure we can actually say that hybridization really means all that much. “Hybrid” is, itself, a rather vague term, defined as the fusion of two other things that were previously classified as distinct from one another based on equally vague and subjective terms.

I'm surprised that nobody has mentioned hybrids between varieties within a species, as this is well known and often used in breeding.

http://en.wikipedia.org/wiki/Heterosis

quote:
Heterosis is a term used in genetics and selective breeding. The term heterosis, also known as hybrid vigor or outbreeding enhancement, describes the increased strength of different characteristics in hybrids; the possibility to obtain a genetically superior individual by combining the virtues of its parents.

Heterosis is the opposite of inbreeding depression, which occurs with increasing homozygosity. The term often causes controversy, particularly in terms of the selective breeding of domestic animals, because it is sometimes believed that all crossbred plants or animals are genetically superior to their parents; this is true only in certain circumstances: when a hybrid is seen to be superior to its parents, this is known as hybrid vigor. When the opposite happens, and a hybrid inherits traits from their parents that makes them unfit for survival, the result is referred to as outbreeding depression. Typical examples of this are crosses between wild and hatchery fish that have incompatible adaptations.


Dogs are probably the best example of hybridization between varieties.

The other place where we see hybridization being important is in hybrid zones between varieties of a species, where gene flow occurs between what would otherwise be isolated populations in a small area where the populations overlap. You could have a linear distribution of population centers and multiple hybrid zones between them to maintain gene flow, however the rate of flow is unlikely to keep the ends of the distribution from diverging. When we talk about ring species we see situations where the linear distribution folds back on itself and ends up with varieties that overlap but don't hybridize.

Perdition, Message 7: As a complete layman, I've always assumed hybrids are a result of two recently diverged lineages. If they're both still near the "fork in the river" so to speak, they have enough similarities to overcome the differences that are building up.

Or diverging populations, where speciation has not occurred yet but is immanent. If the isolation of the main populations increases and the hybrid zones become smaller so gene flow slows, the interfertility between the populations decreases as they accumulate mutations that are increasingly incompatible.

Thus hybridization can be seen as attempts to reunite populations that have become estranged, possibly resulting in some varieties showing hybrid vigor that can also increase and either draw the populations back together or meld with one or the other population. Lots of possibilities here.

Enjoy.


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This message is a reply to:
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Coragyps
Member
Posts: 5377
From: Snyder, Texas, USA
Joined: 11-12-2002


Message 18 of 26 (532587)
10-24-2009 12:40 PM
Reply to: Message 16 by Dr Jack
10-24-2009 7:27 AM


Re: The biochemistry of hybrids
Where there are a differing number of chromosomes in the two parents species the chromosomes obviously can't pair up; this means that the gametes produced do not contain a correct number of chromosomes and are thus not viable.

But, as noted upthread, domestic and Przewalski's horses are interfertile, and they have differing chromosome numbers - diploid #'s are 64 and 66, IIRC. So a hybrid has 65, but is still interfertile. I'm guessing that's because the 2n=66 is from a recent fission in a 2n=64, and the resulting broken chromosome hasn't accumulated many mutations.

But I'm not a biologist.....


This message is a reply to:
 Message 16 by Dr Jack, posted 10-24-2009 7:27 AM Dr Jack has responded

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Dr Jack
Member (Idle past 185 days)
Posts: 3507
From: Leicester, England
Joined: 07-14-2003


Message 19 of 26 (532594)
10-24-2009 1:43 PM
Reply to: Message 18 by Coragyps
10-24-2009 12:40 PM


Re: The biochemistry of hybrids
But, as noted upthread, domestic and Przewalski's horses are interfertile, and they have differing chromosome numbers - diploid #'s are 64 and 66, IIRC. So a hybrid has 65, but is still interfertile. I'm guessing that's because the 2n=66 is from a recent fission in a 2n=64, and the resulting broken chromosome hasn't accumulated many mutations.

I don't know the details of this for certain, but I will speculate a little.

While the hybrids are fertile, they're less fertile than either normal domestic or Przewalski's horses. I would suggest this is probably because a significant number of the gametes produced by meiosis are not viable. Probably what's happening is that matching up can occur correctly with most of the chromosomes (because they are sufficently similar), and then providing that either the combined domestic chromosome and not the extra Przewalski, or both Prezwalski chromosomes and not the combined chromosome end up in one cell - and, of course, crossing over doesn't mess it up - then the resulting gamete is viable. This would happen in roughly 1 in 4 gametes, explaining the reduction in fertility.

I suspect this has probably been investigated at some point though

I'm not a biologist...

I'm not a biologist either... yet.


This message is a reply to:
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Dr Jack
Member (Idle past 185 days)
Posts: 3507
From: Leicester, England
Joined: 07-14-2003


Message 20 of 26 (532598)
10-24-2009 2:31 PM
Reply to: Message 1 by Straggler
10-21-2009 5:04 PM


More answers
How closely related do species have to be in order to successfully breed a hybrid?

There is no clear criteria of the basis of relatedness for this. Especially once you leave the large mammals. Some very closely related species can't hybridise; some remarkably remote species can.

What do we actually mean by "closely related"? What does this mean in terms of DNA, genetics, chromosomes etc. etc.?

There isn't a specific definition. It could mean several different things. The most meaningful from an evolutionary point of view is probably the number of generations seperating them, via the last common ancestor population (usually more easily - but less accurately - expressed as a time, often the time to the seperation), but you could also look at genetic similarity, phenotypic similarity, and so on. Common usage is pretty casual on what exactly you're talking about.

Where is the line that divides the ability to reproduce fertile offspring, infertile offspring and the inability to breed at all? Are there definite dividing lines or is it a graduated scale?

It's very graduated, and isn't even a scale (see gynogenesis and hybridogenesis as in my earlier post). Quite often, even "fertile" hybrids are actually less fertile than pure bred examples from either parent strain.

What does the existence of hybrids tell us about evolution as a theory?

I can't see that it tells us much at all.

Ignoring the moral questions how feasible is a chimp-human hybrid?

I do not believe a humanzee is possible. Certainly there appears to be no evidence of one ever having actually occurred; despite experiments apparently being performed and documented cases of female chimps being used as prostitutes. From a biological point of view, it would seem unlikely that a chimp/human hybrid would be viable because of the huge differences in development timings and physical morphology; it seems quite probable that an embryo, even if created, would not reach term.


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caffeine
Member
Posts: 1602
From: Prague, Czech Republic
Joined: 10-22-2008
Member Rating: 2.5


Message 21 of 26 (532735)
10-26-2009 5:26 AM
Reply to: Message 18 by Coragyps
10-24-2009 12:40 PM


Chromosone number and fertility
Thinking about it, isn't it essential that at least some combinations of different numbers of chromosones are capable of producing viable offspring? Otherwise, it wouldn't be possible for secually reproducing organisms to evolve different numbers of chromosones in the first place. The first mutant with a duplicated or deleted chromosone would be necessarily infertile except in the extremely unlikely case of bumping into a member of the opposite sex with exactly the same mutation.
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Dr Jack
Member (Idle past 185 days)
Posts: 3507
From: Leicester, England
Joined: 07-14-2003


Message 22 of 26 (532821)
10-26-2009 4:48 PM
Reply to: Message 21 by caffeine
10-26-2009 5:26 AM


Re: Chromosone number and fertility
That would follow in animals, yes.
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derwood
Member
Posts: 1457
Joined: 12-27-2001


Message 23 of 26 (534265)
11-06-2009 11:56 AM
Reply to: Message 21 by caffeine
10-26-2009 5:26 AM


Re: Chromosone number and fertility
There are a number of species that maintain karyotypic polymorphism in their populations. A nice example of the ability of species with differing karyotypes being able to interbreed and produce viable offspring are the domestic horse and Przewalski's horse.
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Godless Dave
Junior Member (Idle past 3320 days)
Posts: 1
Joined: 11-22-2009


Message 24 of 26 (536346)
11-22-2009 4:02 AM
Reply to: Message 8 by greyseal
10-22-2009 11:50 AM


Re: they do happen...
The post I'm replying to is a month old but...

quote:
There are also documented occurences of half-cow-half-moose births, although I don't know how long they live and if they're fertile either.

Half-cow half-moose? Did you mean to say bison instead of moose? Because I'm pretty sure cattle and moose can't interbreed!


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Iblis
Member (Idle past 1976 days)
Posts: 663
Joined: 11-17-2005


Message 25 of 26 (536348)
11-22-2009 4:49 AM
Reply to: Message 24 by Godless Dave
11-22-2009 4:02 AM


Re: they do happen... (NOT)
Hi Dave, welcome to the great monkey-or-not poop toss.

Yeah, the moose-cow is a tabloid myth. Ranchers in Minnesota occasionally blame deformed calves on the horny bull moose; the two examples that got some press turn out to be endocrine problems, probably caused by a combination of recessive genes.
http://jhered.oxfordjournals.org/cgi/pdf_extract/30/10/457

Beefalo, on the other hand, are delicious, and have moved on from problematic attempts to a succesful breed of cattle. They have totally sabotaged "pure" bison conservation, though, as they fill the niche better than their endangered kinfolk.
http://en.wikipedia.org/wiki/Beefalo

Remember that old Star Trek where they talk about the buffalo being extinct? It's the one with the sexy salt-sucker in it who puts the make on everyone in the ship, McCoy's ex-girlfriend, tells Uhura he is a "Swahili" etc.

Anyway I won a huge bet on that one time, I had a buddy who really believed they were gone! He had faith in Mr. Spock ...


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greyseal
Member (Idle past 1942 days)
Posts: 464
Joined: 08-11-2009


Message 26 of 26 (536349)
11-22-2009 5:01 AM
Reply to: Message 25 by Iblis
11-22-2009 4:49 AM


Re: they do happen... (NOT)
Yeah, the moose-cow is a tabloid myth. Ranchers in Minnesota occasionally blame deformed calves on the horny bull moose; the two examples that got some press turn out to be endocrine problems, probably caused by a combination of recessive genes.
http://jhered.oxfordjournals.org/cgi/pdf_extract/30/10/457

Well I'll have to ask my farmer-friend who keeps cows for some evidence then.

It came up in a discussion where we were talking about calves since it was that time of the year up here - they'd just had one that was born "inside out" and he'd had to put it down.

There are lots of moose where we live and the cows here are let out to roam around so there's more than enough chance for it to happen.

I don't think he was deliberately lying to me and feel it more than likely that he believed what he was saying...


This message is a reply to:
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