Question on Hybrids and fertility

[DHF]

Hello everyone,

just looking for a little clarification on Hybrids. I have read that Any Hybrid species is by default Sterile. Does that mean its only sterile with other hybrids or would it be sterile if it mated with a non hybrid animal?

Thanks for any input.

 

[Doug Huffman]

I do not believe that hybrids are in general sterile.

 

[jim mcnamara]

I guess we should clarify what a hybrid is.

Doug H is clearly referencing hybridization. As in hybrid crops, e.g., hybrid sweet corn, Big Girl Tomatoes.
The hybrid produces viable seed, but the offspring do not exhibit a lot of the parental traits.

DHF is posing the question from species isolation point of view, e.g., a wild ass mating with a horse, producing hybrid offspring (mule) which is sterile.

So consider that everybody is correct so far. And 'Any Hybrid' species I am taking to mean inter-specific hybrids.
This means that we are crossing elm trees with oak trees (this is just crummy example) and maybe getting some seeds that actually grow into trees.

The primary reason this kind of organism is sterile is that during gamete formation (meiosis), chromosomes do not usually match up into identical pairs because there are not very many identical ones to start with. This is true in plants particularly. Animals often have all kinds of mating interactions that are incompatible with any other organism even if meiotic failure did not ruin things earlier. Plant have sort of the same thing - but it usually involves flower timing, available pollinators, production of allelopathic compounds, and so on.

 

[DHF]

Yes my curiosity was centered around interspecific hybrids. My curiosity wasn't centered on anyone specific animal but on the process as a whole. In general I was wondering if there were exceptions to the sterility that hybrids are attributed with.

 

[jim mcnamara]

Mammals, birds, and reptiles - hybrids are almost always going to be sterile. Note the 'almost always'. This is because there are always oddball things.

Plants use all sorts of mechanisms to isolate species from one another, so that while interspecific hybrids can appear, they are often sterile. Fern species often have LOTS of copies of chromosomes, and this prevents closely related species from creating reproducing hybrids. This is called polyploidy. Humans have 2 sets (23 chromsomes in a set) of chromosomes. n is "one set". We humans are therefore 2n - which is diploid (di- means two). Some species of grasses have more than 16n. Very similar looking species in the Poaceae can vary from 6n -> 16n. Some plants that we put into a single species, Example: switchgrass (Panicum virgatum), cannot really be a single species because some individuals are 4n and some are 8n. It is a mess - our problem, not the plant's problem.

http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1003215

 

[mes314159]

Whether interspecific hybrids are fertile or sterile depends to some extent on the definition of a species. If one defines a species as organisms that look very different, then things someone has decided to call different species may actually not be, and may interbreed perfectly well - it depends on what one means by "look different" which is basically impossible to quantify. If one defines a species as a group of organisms that can interbreed productively, then by definition interspecific hybrids have to be sterile. The latter definition, while attractive (as per Ernst Mayr) does have problems in practice. What about populations of organisms that are very similar physically and perhaps genetically, but live in different regions so they never have a chance to meet. If one forces mating and the offspring are viable (and fertile), then by the latter definition these would not be different species. But for various other reasons it may make more sense to think of them as different species. Also, this species definition is by its nature inapplicable for extinct organisms, fossils and such. It seems pretty extreme to say that Tyrannosaurus and Triceratops can't be defined as different species because we have no way to test whether their hybrids could be made.

Even in living organisms, there are some intriguing situations, for example certain well-studied fish populations in certain lakes, that prefer to stay near the shore. Any fish is able to mate with fish living nearby, but fish on opposite sides of the lake are not fertile together. It's impossible to define where two (or more) different "species" boundaries are, it's more a quantitative effect of increasing genetic incompatibility in taking pairs of fish at increasing distance apart (up to half-way around). I'm not an evolutionary expert but a number of such situations have been described.