Piranhas and Hybridization

Recently in the geographic variant morphology thread the topic of piranha cross breeds came up. I wanted to respond to a few posts there, as well as invite other piranha enthusiasts to share their opinions/experience on the topic. The posts are below. I hope everyone will participate.

Trigga;3416065; said:

Show me one piranha cross breed.. It's never been documented to happen you can't have reasearched in the right place if that's the info your getting. rhombeus are some of the most widely spread serrasalmus species, found in almost every country piranha are found.

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SharkAquarium;3416738; said:

relax, Trig. There is very little black and white here, and let's not start name calling because of a little confusion. I believe everyone should be entitled to their opinion, whether right or wrong (without out being attacked or insulted).

I would agree with you that piranha crosses, to my knowledge, have not been documented; BUT I would not agree with you that 'rhombeus' is found 'in every country piranha are found'. It is generally agreed that 'rhombeus', as used on these species location lists, are NOT one species; rather a complex of species yet to be split out, and identified.

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Trigga;3421078; said:

no worries g no name calling here I was just questioning his source of reasearch

also I said almost every country not every country that has piranha has rhombeus.

I agree with the whole rhombeus complex thing too and I think my Bolivian rhombeus is probably a member of that complex. Even so I don't think that different species inside the complex breed of course this isn't based off any reasearch but just my opinion... There are other fish complexes ( many African cichlids) and they don't naturally interbreed so I dont see piranha doing so either.

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In my opinion, the reason why we see so many geographic variants of piranhas, whether it's ternetzi or diamond rhoms, is because there is not a lot of outbreeding between wild populations in different areas. It is my theory that perhaps all piranhas evolved from a common ancient ancestor (megapiranha??) and the division into two genera (pygocentrus and serrasalmus) and all the myriad species of piranha is a direct result of adaption to specific niche feeding behavior. This adaptation expressed in the genes over generations results in varying species of fish.

What we have in the geographic variants like ternetzi and S. rhombeus are breeding populations caught between species. They haven't evolved enough to qualify as a different species of fish, but they have differentiated themselves from the broader species through natural selection to the point that they have distinguishable differences in morphology. That is to say they look different, which causes all sorts of confusion among hobbyists like ourselves.

Could a piranha crossbreed happen? I think absolutely yes. This is pure conjecture because as far as I know there have been few experiments to verify that this is the case, but likewise there hasn't been much evidence to the contrary either. We do know that in the species S. spilopleura (or maculatus) DNA analysis has shown that there are some hybrid gene alleles in the genetic makeup of the species. I will not revisit that species in this post, but for more info you can click here.

Due to the difficult nature of breeding S. rhombeus in captivity it may be a long time before this question is resolved, but I maintain that lacking evidence to the contrary, if you physically removed eggs from a Guyana rhom, and sperm from a blue diamond, and used the one to fertilize the other, a spawn would occur. Would they create sexually viable offspring? I have no idea, but I would think so.

What do you think? Is hybridiztion possible with P's? If so what sort of genetic crosses would you like to see? Piraya/irritans? Marginatus/brandtii? RBP/rhoms? ok I might be stretching it a bit...

Trigga;3424420; said:

Also care to quote the part in the opefe article where it states that there are some hybrid gene alleles in the genetic makeup of s.spilo I could not find it when i skimmed through it just now.

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Gradual frequency differences in the A*125 and B*210 alleles at two GPI loci detected in S. spilopleura caught between the upper Paraná River (cytotype 'a') and the lower Paraná River (cytotype 'b' and cytotype 'c') led Cestari (1996) to suggest that there may be interbreeding between fish from these two sites, supporting the hypothesis of a hybrid origin for the 'c' cytotype. Several cases have been described in the literature where genetic polymorphism seems to be shared between a pair of species while closely related species might be expected to show higher levels of shared polymorphism (see Clark, 1997). Nakayama et al. (2001) considered S. rhombeus to be a cryptic species, with imperceptible morphological differences among the three cytotypes examined by us and it follows that the occurrence of a Est-D3 locus polymorphism shared among these cytotypes would reasonably be expected to follow the same pattern as that seen for the 2n = 58 cytotype i.e. segregation of alleles following a Mendelian model which did not occur. Additionally, our data may suggest that these Central Amazon piranhas karyotypic groups partially represent isolated populations, or populations which have been isolated for an insufficient period of time for the fixation of different cytotype-specific alleles. A character applied for identifying taxonomic units with species status should occur in all members of the species and not in other species, i.e., be a unique fixed allele or its product. Consequently, various distinct genetic and molecular techniques such as chromosome, DNA and protein studies should be complemented with meristic-morphometric studies in order that the taxonomic status of Central Amazon rhombeus complex can be elucidated. (Aylton Saturnino Teixeira et al.,).

Among the piranhas, genus Serrasalmus is probably the most studied genus in Dna chomosomal research. Serrasalmus spilopleura, a widespread species found throughout the Amazon basin, the Paraná-Paraguay and Uruguay basin, 5 cytotypes(=cell types) have been described so far. Three of them were found in Paraná-Paraguay basin (CESTARI and GALETTI 1992a; CESTARI 1996) and two in the Catalão Latke in the Amazon basin (NAKAYAMA et al. 2000). There is variability due to chromosomal rearrangements and to understand these alterations and its association to the speciation process is one of the major challenges in karyoevolutionary (= evolutionary change in the chromosome set) studies (WHITE 1977; JOHN 1980; GUERRA 1988). By recognizing that S. spilopleura is chromosomaly polymorphic (=gene varying body shape) and that might exist a species "complex" (NAKAYAMA et al. 2000), the present study is aimed to increase the sampling sites in the Central Amazon basin and to document the cytogenetic variation found in nominal forms of S. spilopleura. Sixty-two (62) specimens of species S. spilopleura were collected for this test. Three different areas in the Central Amazon basin: (1) At the confluence of Solimões and Negro Rivers (Catalão Lakes), 31 specimens (13 males, 16 females and 2 undetermined sex ones); (2) in the lower Manacapuru River, a tributary of Solimões River, 15 speciemens (3 males and 12 females); (3) 16 specimens (10 males and 6 females) in the Amazon River, near to the Itacoatiara township (CENTOFANTE, PORTO, FELDBERG 2002).

All specimens displayed 60 chromosomes and FN 108. What was interesting is that they detected structural polymorphism (varying body shapes) without sexual chromosomal heteromorphism (= changes in development that lead to changes in the end result), leading them to propose new cytotypes for this species. According to JOHN (1980), the occurrence of rearrangements is not a common event, happening seldom on homologue (=similar) chromosomes. Thus, these body (or structural) changes are introduced in wild populations by interbreeding. In areas like this lake the hypothesis is that they should not exclude the possibility that this type of situation can favor the fixation of individuals bearing chromosomal arrangements. NAKAYAMA, et al. (in press) in S. rhombeus and by FELDBERG et a. (1999) in Plagioscion sp. this chromosomal arrangement was detected. According to GUERRA (1988), some chromosomal arrangements are more frequent under certain environmental conditions, since higher levels of polymorphism occur for species inhabiting their most favorable environment. Generally speaking, all species of genus Serrasalmus present chromosomal pair with heterochromatic blocks on long arms, which is considered to be a cytogenetic marker for genus Serrasalmus (NAKAYAMA 1997). However in some (C and D cytotypes), this pair appears to not be composed by homologue chromosomes. This lack of homology might be related to rearrangements (CENTOFANTE, PORTO, FELDBERG 2002). What this work provides is a definition of how important the role is non Robertsonian chromosomal rearrangements in the chromosomal evolution of the subfamily Serrasalminae. Further what these investigations show is how the chromosomal polymorphism in Serrasalmus spilopleura may indicate a speciation process when compared to samples from Amazon and Paraña-Paraguay basins (CENTOFANTE, PORTO, FELDBERG 2002).

Cytotype information by populations and individual (s)
Cytotype A: 3 populations from Catalaõ, Manacapuru and Itacoatiara.
Cytotype B: During collection trips from July to December, 1998, in these 3 areas, was not found.
Cytotype C: Was present in 7 individuals only at Catalaõ Lake population.
Cytotype D: A second pair was formed by different sized chromosomes, and it was found only in one individual from Manacapuru River mouth.
The Rehabilitation of S. spilopleura vs. S. maculatus: OPEFE readers may access the actual .pdf by following this link.

Trigga;3424867; said:

cool never knew that. It makes sense though i guess when you think about it because they look so similar.

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hmmm... after re-reading this I could be mistaken. I would like to see if Frank can clarify this. Normally, in biology, hybrid is a specific term that relates to offspring produced by the union of two different species. I wonder if here it is used to describe offspring of the union of two different cytotypes of the same species.

I should email...

Diogenes;3444954; said:

I should email...

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sent. we can see what the guru says.

Looks like I was wrong on both counts. =) Frank from OPEFE:

I'm a bit fuzzy on remembering that original document, those are filed away in storage and I don't have access to them at the moment. What I can tell you is in a later reviews, the species was determined to be polymorphic (varying body shape) that had to do more with pH values changes that effected the zygotes than actual hybridization. These changes were observed more on fishes that became land-locked from ancient flood and receding waters.

The paper (Cestari), 1996 suggested hybridizing between Cytotype A and Cytotype B which produced Cytotype C. But as I wrote above that was not the case afterall.

so....back to square one.