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From: iayork@panix.com (Ian A. York)
Newsgroups: alt.folklore.urban
Subject: Rock apes
Date: 19 Jul 1996 14:26:39 -0400

In article <31EFC742.6EE@oas.omron.com>, Bryan Beatty <beatty@oas.omron.com> wrote: >Robert B. Kennedy wrote:
>
>Also, if it's not all that difficult to do, I'm surprised that it hasn't been
>done already, somewhere. Such an experiment would be extremely unethical,
>even (pardon the expression) "inhuman," but there certainly doesn't seem to
>be any shortage of unethical people in the world.

There is, however, a shortage of chimpanzees.

>> I can see no reason why a Chimp/Human
>> cross couldn't be done. We are genetically similar enough to cross.
>
>Based on what? Unless someone can provide hard numbers to quantify just how
>much genetic difference is and isn't tolerable to allow a cross, then we
>can't say one way or the other whether such a cross is, in fact, possible.

That's a rather simplistic answer. There's unlikely to be a threshold of genetic similarity at which hybrids can or cannot form; the understanding of interspecies crosses is only rudimentary at the moment. I append a previous post of mine that gives some cites on this.

The separation of humans and chimps into different genera is no help; first, as others have pointed out, the division is artificial and at the time it was done it would have been unthinkable to lump them into the same genus. (There may be other reasons for the separation, but it was originally done for non-scientific reasons.) Second, biologically the genus is not a particularly meaningful category - as opposed to the species, which is biologically meaningful (but, as I've pointed out elsewhere, doesn't mean different species cannot interbreed, or even interbreed fertilely).

Consider this: humans and chimps had a common ancestor some 4 - 7 million years ago. Horses and donkeys, which diverged some 3 - 5 million years ago, interbreed like nobody's business. I can't find the percent similarity of horses and donkeys, but I suspect it's not significantly better than between humans and chimps, especially as the microsatellite DNA has evolved very rapidly in horses vs. donkeys [Genomics. 18(1):113-7, 1993] but less so in humans vs. chimps [American Journal of Physical Anthropology. 96(1):63-71, 1995]. I find it unlikely that horses and donkeys are more than 98% identical at the DNA level, and as previously noted they (like humans and chimpanzees) have different numbers of chromosomes. For that reason, I see no obvious reason to beleive that chimps and humans cannot interbreed, though their offspring would almost always be sterile, I presume.

But as I say, the whole thing is very complex, and simple answers are probably going to be wrong. There's little point arguing about it.

From a previous post:

Now blows the east wind Nature (1996) 360:105-106

Chung-I Wu

During the Three Kingdom period towards the end of the mighty Han dynasty, the three forces were ready for their Armageddon at the Vermilion Cliff in eastern China. Through shrewd manoeuvers, the Southern Alliance was able to dupe its enemy on the west flank into locking all of its warships into a tight formation. What was then missing on this tranquil water was a wind from the east, to send down fireships to set the whole enemy fleet ablaze.

Since then,"all ready but the east wind" has become a metaphor for action for which all preparations humanly possible have been made, but which requires nature to yield just one more thing. The discovery by Davis and co-workers of fertile hybrids between the two best-studied species of _Drosophila_, described on page 157 of this issue(1), may be the east wind in the genetic analysis of speciation.

How one species splits into two is one of the central questions in evolutionary biology. Among all differences between species, the traits contributing to reproductive isolation are the most intriguing. Although in itself the phenomenon seems to make no sense (what good does it do to produce sterile progeny?), reproductive isolation is crucial for nascent species to continue their divergence without their unique innovations being lost by blending through gene migration. What sort of genetic changes underlie this process?

Studying the genetics of reproductive isolation may seem to be an exercise in self-defeat; after all, we are attempting to study the sterility of crosses between species by means of crossing. Cases of unisexual sterility, generally obeying what is known as Haldane's rule, do however allow geneticists to practise their trade. For example, the yak-cow cross results in sterile males and fertile females. So do crosses between guinea pigs and cavies.

A more serious problem is that the traits of reproductive isolation are genetically complex, as has been known since the 1930s (refs 2, 3) and confirmed recently(4). What this means is that even the best genetic tools available may be only barely adequate. Unfortunately, _Drosophila melanogaster_, the species for which we have one of the best genetic toolkits, cannot be used in research on reproductive isolation because it produces sterile hybrids with all known relatives, including the closest one, _D. simulans_.

This is a topic with a long history. As Davis and colleagues remark, back in 1920 A.H. Sturtevant must have been disappointed by the sterility between _D. melanogaster_ and _D. simulans_. There were more disappointments to come. H.J. Muller, together with G. Pontecorovo, devised a clever way of bypassing sterility in the first filial generation (F1) to obtain genotypes equivalent to the backcross F2 generation. They were essentially all sterile(3). In the 1970s, Tsacas a colleagues searched for more relatives of _D. melanogaster_ and found a pair of sibling species from islands in the Indian Ocean, _D. mauritiana_ and _D. sechellia_(5). Both, like _D. simulans_, produce sterile hybrids in their hybridizations with _D. melanogaster_ (although in other respects the three species have been useful in research on reproductive isolation(4,6)). A third attempt also failed to oversome the sterility barrier, but it finally led to the new discovery(1). Watanabe(7) first identified a mutation that can rescue the otherwise inviable hybrids between _D. simulans and _D, melanogaster_. A total of four such mutations have been discovered since(8,9). Again, however, none of the hybrids is fertile.

The existence of viability rescue mutations did offer a ray of hope: the possibility of rescue of female fertility. It was during the pursuit of one such mutation that Davis _et al._ discovered one last piece that may finally bring _D. melanogaster_ into work on reproductive isolation. They found that females of a few _D. simulans_ strains, when mated to males of a particular _D. melanogaster_ line, produced F1 hybrid females that are fertile. There have been unconfirmed rumours that this has been done before, but the report of Davis _et al._ is the first to show convincingly that the strains belong in the correct species and that the F1 and F2 are true hybrids.

Regardless of the genetic basis of the fertility rescue, which is very interesting in itself, the discovery of this phenomenon is promising. If chromosomal segments can be individually introduced from _D. simulans_ into _D. melanogaster_ through fertile F2 offspring, rigorous genetic analysis can be carried out for all aspects of species differentiation, including inviability, sterility, sexual behaviour, and so on. The availability of chromosomal rearrangements, mutation and molecular tools in _D. melanogaster_ will make this feasible. There are still some difficulties; for example, the range of genotypes of the fertile F2 females is not known yet. Nevertheless, surmounting them will take only human efforts.

Finally, the observation that the best rescue strain from _D. simulans_ is of African origin is intriguing. Lines of _D. melanogaster_ from some parts of Africa exhibit substantial divergence in DNA variation from strain from other continents(10), and together these results support the view that this species group originated in that continent. Could Africa contain a hidden wealth of materials for speciation research?

Our understanding of the nature of species difference as a genetic phenomenon is still primitive(4), in comparison with the spectacular advances that have been made in understanding development., Lack of suitable materials has been an impeding factor. But with the appearance of the paper by Davis _et al._, there is good reason to believe that the east wind is blowing (as indeed it did at the Vermilion Cliff) and that we will see significant progress in our knowledge of reproductive isolation.

--

      Ian York   (iayork@panix.com)  <http://www.panix.com/~iayork/>
      "-but as he was a York, I am rather inclined to suppose him a
       very respectable Man." -Jane Austen, The History of England

--

      Ian York   (iayork@panix.com)  <http://www.panix.com/~iayork/>
      "-but as he was a York, I am rather inclined to suppose him a
       very respectable Man." -Jane Austen, The History of England