I had some questions that I figured some knowledgeable people here might be able to give some input on. It’s regarding the fertility of various Lepomis hybrids. From what I’ve read, many different F1 hybrids are fertile and capable of producing both F2s and backcrosses. The thing is, I can’t find much info on F3 hybrids or offspring produced by backcrossed fish (either backcrossed again, outcrossed to something else, or 2 backcrosses breeding together).

I’ve seen a few papers showing that F1 hybrids between Lepomis species produce unreduced diploid gametes, including one example of a population of green sunfish/pumpkinseed hybrids where all observed males were sterile and the females consistently produced unreduced diploid eggs, resulting in sterile triploid backcrosses (1st link below). It looks like at least some hybrids only produce the unreduced gametes at a low frequency (12% in BG x GSF according to the 2nd paper linked below) so, assuming at least some of the remaining gametes are fully functional haploids, they should be able to produce fertile diploid backcrosses and F2s. However, I can’t find many reports of fertile fish past the F1 generation (just one paper, the 3rd link below). So does anyone know of any examples? If it does happen, which combinations are possible? Does the sex of the parent species matter? What about the exact population the parents are from? Are tetraploids viable/fertile? Green sunfish apparently differ in chromosome count (either 2n=46,47,or 48); so does that have any bearing on hybrid fertility/viability? I’m sure there’s other factors I’m not even considering.

My main reason for being interested in this is just plain curiosity; but if fertile hybrids are possible, they could be useful for making selectively bred aquarium strains (similar to what's been done with fish like peacock cichlids). Who knows how many crazy color schemes you could breed for with the available gene pools that exist in wild species. Fertile hybrids would also have some implications for fishery management, especially stocked waters like bass ponds. If hybrids are able to introgress into a parent species’ population, it might be worth considering stocking a mix of species and/or hybrids for purposes of genetic diversity. Or maybe the opposite is true; it might be worth avoiding hybrids to prevent the spread of undesirable traits. For example, maybe someone wants a self-sustaining population of bluegill in their pond, but throws in a few BG x GSF hybrids as well in an effort to get some larger or just interesting looking fish. It might be that in the long term, those hybrids introduce some undesirable GSF traits (smaller adult size, larger mouth which increases competition with young bass, etc.) into the bluegill population, leading to a worse fishery in the long-term.

If anyone has something to add, I’d very much appreciate it. This has been a topic that’s interested me for a while and I’d like to learn more about it.

Some further reading for anyone interested: 

Triploid backcross hybrids of green sunfish and pumpkinseeds:

https://www.jstor.org/stable/1445768

Page 56 (page 18 of the PDF) on this paper has several references to papers about sterility of sunfish hybrids: https://bolnicklab.files.wordpress.com/2015/10/bolnick-2009-in-biology-of-centrarchid-fishes.pdf

The only record I can find of any F3 hybrid Lepomis (warmouth x green sunfish and the reciprocal cross). Not genetically confirmed or produced from artificial spawning, but from ponds stocked exclusively with F2 hybrids; could potentially be other fish mistaken for F3s. The paper also mentions fertile backcrosses and 3-way hybrids when it says on page 187 (33 of the PDF) “BW x B, G x GW, and B x RG populations produced large numbers of young.”, but those were also produced by stocking ponds with hybrids and were not genetically confirmed:

http://www.nativefishlab.net/library/textpdf/16131.pdf

Green Sunfish Chromosome count:

https://www.karger.com/Article/PDF/129907

The first we tried to replicate roughly a decade ago where we produced F1 female green x male pumpkinseed. F1 males were effectively sterile. Females we could not get to breed over many attempts with many females in ponds and in tanks. Author of study may have had important differences in stocks observed compared to mine.

Crossing female greens with male warmouth yields sterile offspring of both sexes. All hybrids I have made with warmouth have been sterile. Polyspermy might be what is making for what appears to be Fx hybrids.

I wanted to make a sterile triploid that did not require direct ploidy manipulation.

Proper response requires more time than I have, although I used to a know of a green by bluegill that what many FX's already by 35 years ago. It may still persist.

Interesting that all of your warmouth hybrids were sterile when 3/4 of the supposedly fertile 2nd generation hybrids reported in the paper by Childers were part warmouth. My understanding of all of this is pretty basic, but polyspermy would result in a triploid F1 which could be easily mistaken for a triploid backcross produced from unreduced gametes (both having 2 sets of chromosomes from one species, 1 from the other), correct? From what I understand, the fish that Childers reported as F2s and F3s were just small fish taken out of ponds stocked exclusively with hybrids of the previous generation, and none of them were tested for ploidy. I'm not sure what else he did to verify that they were actually the offspring of the fish stocked; it could be that some of the fish reported in the paper were misidentified, though that seems like a lot of different bad IDs for one study. Maybe there's just some genetic difference in the original pure fish affecting hybrid fertility, hard to say unless someone else manages to produce fertile hybrids.

Regarding the green x bluegill you knew of; what was the reason for believing they were later FX hybrids?

I've seen it stated on places like the Pond Boss forums that the commonly stocked hybrid green x bluegill can reproduce over multiple generations; I just haven't found studies clearly showing that. In the case of ponds stocked with hybrids, it might be that any observed reproduction is just multiple year-classes of sterile F2s produced by the original stocked fish, giving the impression of multi-generation breeding. Or maybe one of the parent species was accidentally introduced at some point and were later mistaken for hybrids. It seems like fertile diploid F2s of this cross should be possible based on the report of low frequency of unreduced gametes in the F1s, I'd just like to see something showing beyond a reasonable doubt that they occur.