Does polyploidy always isolate the polyploids from the diploid source population?

Question

I am receiving conflicting information while researching polyploid speciation. On the one hand, some sources state that a polyploid can only reproduce sexually with polyploids having the same number of chromosomes. But I am also finding information claiming tetraploid x diploid reproduction in orchids, or tetraploid x triploid reproduction of orchids (for example,see American Orchid Society, AOS.org). Is polyploid sexual isolation always the case, sometimes the case, or never the case? Can someone clarify this?

Answer 1

Hi there,

This question has been around for a while, and you have probably moved on since last year, but I wanted to answer this one so that anyone else out there with the same question could have good information.

The answer is that yes, it's possible for polyploids and diploids to hybridize. It's not super common, but it's possible, and very well documented in a lot of high quality research. Here are some papers that talk about it; such papers can be found on Google Scholar:

https://academic.oup.com/biolinnean/article/82/4/537/2643085

https://www.sciencedirect.com/science/article/abs/pii/S0169534799016080

https://www.pnas.org/content/112/9/2806.short

A quick read of the abstracts from any one of these papers might lead to even more confusion, since as you can tell the field of polyploid research is very complicated, and a lot of factors come into play. For example, it is possible for a population of a single species to contain both diploid and polyploid individuals. This is actually how most polyploid species originate. If a population of diploids contains individuals that produce un-reduced gametes (gametes with a ploidy of 2n rather than 1n), and one of these combines with a normal gamete, then the progeny will be triploid. If two 2n gametes join together, then the progeny will be tetraploid. In some cases, the tetraploid could become a separate species. Such a species will still retain the ability to hybridize with the diploid "parent" species, usually forming triploid progeny. Most triploids are a dead end, since they usually have lower fitness than either diploids or tetraploids. However, a triploid can produce both 1n (normal) and unreduced (2n) gametes, which could result in either more triploid or more tetraploid progeny when they combine with gametes from normal diploids.

Hopefully you understood what I described above, because it can get a lot more complicated if you add in other factors. For example, in nature there are two kinds of polyploids, so-called "autoploids" and so-called "alloploids." The autoploids are polyploids in which all of the genomes come from the same species (like the above example), while in alloploids the genomes come from different species. These two kinds of polyploids have different likelihoods of hybridizing with diploids, due to incompatibilities that can arise among the genomes.

This is a very deep and complicated subject, so I encourage you to look into the biology of polyploidy so that you can understand all of the interesting and counter-intuitive consequences of polyploidy. A good place to start would be with this paper, which is open access:

https://academic.oup.com/jhered/article/109/3/283/4827622

Good luck!

Dylan

Answer 2

Hi "Asked",

There are many different origins of polyploidy and related conditions. Sometimes the resultant organisms are viable, and sometimes you can find comparable, compatible polyploid individuals to cross them with! Induction of tetraploidy, frequently with colchicine, and backcrossing with the 2n, routinely generates triploids with commercially advantageous properties (must be vegetatively propagated, seedless, and cannot easily cross viably with wild-type populations). Case in point: banana, though this was originally, perhaps naturally, generated by a 2n x 4n interspecies cross. The Moraceae are particularly egregious at that -- I think 22n ploidy is the record there (natural, too!).

It's frequently morphology that drives such selection -- polyploid plants have reduced internode distances, much thicker leaves, bulkier flowers, etc. Desirable horticulturally.

Certainly deliberate crosses in which the offspring have fixed ploidy (2n x 4n = 3n) will be more likely fertile than those with "unstable" ploidy (4n x 3n). However, there may still be an "error" in segregation, and you may get a few offspring.

In nature, though, ploidy is stable within populations, as far as I am aware.

-- Cheers, -- Mr. d.