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Thursday, July 22, 2010

Hybridation pros and cons


An issue that has arisen more than once in discussions as of late, specially in relation with the recently discovered
Neanderthal admixture in Eurasian Homo sapiens, is that of hybrid vigor or lack of it thereof. This is addressed at a new paper in PLoS Biology:

Ulises Rosas et al., Cryptic Variation between Species and the Basis of Hybrid Performance. PLoS Biology 2010. Open access.

Author summary:

A major conundrum in biology is why hybrids between species display two opposing features. On the one hand, hybrids are often more vigorous or productive than their parents, a phenomenon called hybrid vigor or hybrid superiority. On the other hand they often show reduced vigour and fertility, known as hybrid inferiority. Various theories have been proposed to account for these two aspects of hybrid performance, yet we still lack a coherent account of how these conflicting characteristics arise. To address this issue, we looked at the role that variation in gene expression between parental species may play. By measuring this variation and its effect on phenotype, we show that expression for specific genes may be free to vary during evolution within particular bounds. Although such variation may have little phenotypic effect when each locus is considered individually, the collective effect of variation across multiple genes may become highly significant. Using arguments from theoretical population genetics we show how these effects might lead to both hybrid superiority and inferiority, providing fresh insights into the age-old problem of hybrid performance.

A news article synthesizing the findings can be found at Science Daily:

The results show that hybrids might be expected to exhibit increased performance in basic traits such as growth. However, they also show that in the longer term, other traits such as those involved in sexual reproduction might be expected to perform less well, accounting for reduced fertility of hybrids.

8 comments:

Maria Lluïsa said...

Here are my thoughts:
First of all: what is an hybrid? I thought it was the offspring of two individuals belonging to different species. Hybrids are by definition infertile, much like donkeys, ligons and tigons. The cause of the cases I mention is the different number of chromosomes between the two species.
That doesn't seem to be the case of neandertals, it is quite clear they had 46 chromosomes, much like modern humans.
Their offspring with modern humans was not so infertile, because their genes are still within us.
Moreover, according to their genome, neandertals fall within human genetic diversity. They can be considered a deeply divergent human lineage. They (Pääbo's team) told this in 2006, and again in 2010.
In a John Hawks' weblog post talking about the genome, I read that the genome of the European individual was close to those of neandertals than to the San genome.
There are many deeply divergent lineages in Africa, although I never heard anything being said about mixed children.
Mixed people of, for example an European and a sub-saharan African don't seem to be stronger, nor healthier nor taller than other humans, and they have definitely no infertility problems.
If the data is correct, then should we call neandertal offspring with modern humans "hybrids"? Where's the proof of their infertility?
The only way to prove it is having a population of neandertals and another of modern humans, but actually this isn't possible, and genome can't tell us if there were infertility problems nor hybrid vigour.
Apparently, it was not the case.

Maju said...

Hi and welcome, Maria Luïsa.

That's the "classical" concept of species but today the barrier is very much blurry as we understand that there is no simple yes/no answer to what is a species.

As former gardener I am familiar with hybrids, often fertile, not just across the species barrier but even across higher taxonomic levels.

Recently it was acknowledged that polar and grizzly bears occasionally mate and produce fertile offspring, th offspring of tigers and lions is variedly fertile, and there are growing number of known more or less rare interspecies fertile offspring.

I believe that nowadays the common understanding of what "species" mean is a taxon that does not normally mate in the wild. It's not absolute impossibility of reproduction but rather a practical reproductive isolation.

In this sense it seems that, in spite intense interaction early in Palestine and later in all West Eurasia, Neanderthals and Sapiens only inter-mated (at least with effective success) very occasionally.

"In a John Hawks' weblog post talking about the genome, I read that the genome of the European individual was close to those of neandertals than to the San genome".

I don't think that's correct (please provide a link so I can trace that info to the source). What was discovered this Spring is that all Eurasians (Europeans the same as Chinese or Papuans) are slightly closer to Neanderthals than most Africans (Yorubas and Bushmen). See here and here and in general the category Neanderthal in this blog.

In my understanding the two species were pretty much separated but still close enough that some hybrids could survive and transmit their genes, which were eventually homogenized in the initially small founder population of Eurasian H. sapiens (as in the old H. sapiens sapiens I mean, ok?)

It is also important to understand Atapuerca researchers have been disputing the "Anglosaxon" claims (Trinkaus specially but Paabo is also feeding that idea, wrongly in my opinion) of Neanderthals and Sapiens diverging so recently and I agree with them. While we are very similar, all the available archaeological evidence suggests a divergence c. one million years ago, with the dispersal of Acheulean.

The similarities hence derive from both of us stemming from H. ergaster and having a convergent evolution (towards larger brains and cognition specially). But there are key elements that make us very different too, not just the teeth that Aida Gómez has researched but also other parts of the body, notably the torax, the femur and even the skull.

Tiger and lion also had convergent evolution in some aspects (lions are closer to other Panthera but tigers to the Uncia genus, smow leopard, was known recently) but they are still different. They can produce fertile hybrids in zoos but they are most unlikely to produce them in the wild.

...

Maju said...

...

"Mixed people of, for example an European and a sub-saharan African don't seem to be stronger, nor healthier nor taller than other humans, and they have definitely no infertility problems".

Absolutely.

However now and then we stumble onto some more or less controversial stuff pondering the advantages and disadvantages of mating with distantly related individuals. For example East Asian women have average smaller pelvis and may have more birthgiving problems when having children with West Eurasians, who are generally somewhat larger. Another paper on Danish couples suggested that certain genetic closeness optimizes chances of reproductive success but it was unclear if this was a psychological or biological effect.

The issue is not really clear and that's why I echoed this paper. Extreme inbreeding is dangerous and can lead to simple extinction. On the other hand when the populations have been diverging for a million years, as happened between Neanderthals and Sapiens, the biological differences may be too extreme to allow effective hybridization in most cases. And let's not forget psycho-cultural differences and the too common xenophobia (or ethnophilia if you wish: prefering the familiar to the exotic).

Anne Gilbert said...

Maju:

Several comments here: First of all, it is not uncommon for closely related species to mate and produce perfectly fertile offspring. Where I live, there are two species of gull that have done just that. The Puget Sound gulls are considered a hybrid species. It's known that wolves x coyotes do that, too. The genetic results are, um interesting.

Pããbo and his team started our as very much "separate species" people re Neandertals x "modern" humans. He has gradually becme more open to other possibilities, culminating in hes most recent foray here. The results, of course, could be interpreted several ways, and have been. I don't know about Trinkaus for sure, but I've seen reports that claim Trinkaus thinks there was more hybridization , and perhaps later than later. I haven't seen any actual Trinkaus comments though, so, for now I favor Pääbo, pending further research.

Maju said...

Hi, Anne.

Nothing to add to your comment on seagulls and wolves+coyotes. It's totally within my understanding of "the species problem".

However re. Paabo, I find he is too caught up in the most unlikely short chronology for the Neanderthal-Sapiens divergence. The paper in fact considers a wide range of chronologies from c. 800,000 years to c. 350,000. All are taken from the genetic molecular clock literature and what, IMO, is an artificial shortening of the Pan-Homo divergence, which is at the root of most problems in molecular clock age estimates.

Archaeology and palaeoanthropology (excepting the unlikely hypothesis of Trinkaus - famous but isolated) don't seem to agree with such short chronological frames. The 800,000 date would be close but still a little short because we know a lot now of the evolution towards Neanderthal from H. ergaster nowadays, thanks to the rich Atapuerca site, and it began at least 900,000 years ago (up to 1.3 million years by Gómez' work).

Personally I understand this is the correct dating because the molecular clock is wrongly calibrated and the Atapuerca team are most serious.

Nowadays they have dispatched a work team to East Africa in order to attempt to clarify the parallel H. ergaster - H. sapiens evolution with fossils. Possibly in five or ten years we will know something more but I'm pretty sure it will confirm the long chronology, because all solid evidence points there.

"I haven't seen any actual Trinkaus comments though"...

I mentioned him here. He made comments for Quo, a Spanish language magazine grunting against genetics and the molecular clock even if it seems to support his hypothesis somewhat. I also link there to his 2007 paper, which is a very arguable study only on skull shapes that I found not convincing.

"so, for now I favor Pääbo"...

Paabo and his team have done an excellent work with the Neanderthal Genome. However when it come to chronological interpretation, Paabo is ranting in the void. He's arbitrarily choosing the less reasonable possible interpretation on his own personal favoritism and that way acting irresponsibly and misleading the public.

He's pushing for his pet theory without any new evidence in support of it.

terryt said...

"Hybrids are by definition infertile, much like donkeys, ligons and tigons".

Depends on how different the 'species' are. Many different-looking populations, defined as being separate 'species', are quite capable of producing fertile offspring.

"Extreme inbreeding is dangerous and can lead to simple extinction. On the other hand when the populations have been diverging for a million years, as happened between Neanderthals and Sapiens, the biological differences may be too extreme to allow effective hybridization in most cases".

That sums it up pretty well, although I tend to accept a date of around half a million years for separation of Neanderthal and modern human ancestry. I strongly suspect Homo heidelbergensis is the common ancestor.

Maju said...

"I strongly suspect Homo heidelbergensis is the common ancestor".

This kind of theory always makes me think of a reggae song of the 1980s, "Miguelín el casero" (Mikey the farmer, a real character, who sold marihuana along with other farm products at the city market of Vitoria for a time), which I happily replace with "Miguelón", who lived also in the area long ago. :D

But there are serious issues with this theory, as we have discussed before:

1. It's simply not conceivable that whatever the direction of the H. heidelbergensis migration, they did not carry some sort of techno-culture with them. This is a key evidence that is lacking AFAIK. You will say it's the hyper-diffuse concept of Levallois technique but I find hard to swallow that, really, specially because there's nothing like a direct correlation between this Eurasian and this African site, as happens with Acheulean or MSA, clear cases of migration.

2. There is strong evidence in Atapuerca for a local evolution in Europe from H. ergaster to H. neanderthalensis, via H. antecessor and H. heidelbergensis. This would imply a Europe to Africa migration but the alleged H. heidelbergensis (H. rhodesiensis probably, though they are somewhat similar) is in fact older in Africa (Bodo, 600 Ka ago) than in Europe (400 Ka ago)

3. There are scientists (the already mentioned Dr. Gómez) arguing for a tooth-based divergence of more than one million years, which is coincident with Acheulean scatter.

4. The morphological differences, specially the thorax, between H. sapiens and H. neanderthalensis are too wide to be within just doubly the range of H. sapiens.

5. The Denisova finger points to an older divergent mtDNA lineage that simply does not fit in the short chronology model but totally does when applied the long chronology (it'd be an H. erectus lineage).

So I think that the short chronology is merely a fetish, a fashion. It will be dispelled in due time because everything that really matters points in the opposite direction.

Anne Gilbert said...

Maju:

Of course you may be right. This study is essentially in its infancy. Gut for now, I'm accepting this "short" chronology, because that's what there is. As for Trinkaus, I don't know. He's long been a proponent of the idea that Neandertals and "modern" humans probably mixed, whether at 60000 years or maybe later. And no, he doesn't consider N's a "separate species". IN any case I agree with you that there's plenty more to be done.