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Tilapia genetics

A guide by Lakeway Tilapia

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Understanding Tilapia Genetics

My primary goal with this page is to teach you the genetics behind whether a tilapia fingerling becomes a male or a female. It is not intended to explain every aspect of genetic inheritance. I've done my best to keep it as simple as possible and stay focused on the conclusions, rather than the mechanics.

Genetics common to all tilapia

When tilapia are first hatched they are gender-less; neither male nor female. During the first 21 days or so, the amount of estrogen or testosterone in their blood stream, will determine whether they develop ovaries or testicles. It is a common practice for tilapia farmers to feed their tilapia fry with a food that has been laced with masculinizing hormones, such as dehydroepiandrosterone or 17a-Methyltestosterone, to increase the odds that they will develop as males. Male tilapia grow faster than females, which makes them more desirable as production stock. In addition, a predominately-male tilapia population reduces the likelihood of spawning, which can result in plant roots being eaten in aquaponic raft systems and wasted economic resources in aquaculture ponds. Unfortunately, there are a lot of people who have no understanding of the science behind hormone usage, or how it works, so they give in to imagined fears, steering clear of tilapia altogether.

The good news is, there's an alternative to hormone usage, and even better, the process is completely natural. The process is known as cross breeding, and it works like magic. Cross breeding, or hybridization, however you prefer to say it, has nothing to do with genetically modified organisms (GMO's). Genetically modified tilapia are created in the laboratory, where genetic material is altered by artificial means. The only genetically modified tilapia that I'm aware of, is a genetically modified Nile tilapia male, which has been altered in the lab to carry a pair of YY chromosomes, instead of its natural XY. Ultimately, this YY chromosome pair will determine the gender of their progeny. Some people have also heard of the fact that Wami tilapia males carry a ZZ chromosome pair, but that's not a genetic modification, it's what they carry naturally, the same as Blue tilapia. But I'm getting ahead of myself here. Let's get back to our gender-less tilapia fry.

So as I was saying, during their first three weeks of life tilapia fry are gender-less, at least outwardly. But that doesn't mean that their sex hasn't already been pre-determined by natural genetics. Barring any interference by humans, each tilapia will develop into whatever gender it is supposed to be, according to the level of estrogen or testosterone present in its bloodstream. So the question is, if the gender has already been determined by natural genetics, then how and when was it decided? The answer is kind of cool, even if it is only high school level biology.

The cells of a female tilapia have 44 chromosomes. When she reaches her reproductive age, some of those cells will form into eggs. When the cells that form into the eggs start their meiosis, the two chromosomes that determine gender, as well as the other 21 paired chromosomes, are separated into the different eggs. When all is said and done, each egg will have 22 chromosomes, that include either a single chromosome for male, or a single chromosome for female. Simply put, an unfertilized tilapia egg is either half-way to being a male or half-way to being a female.

The cells of the male tilapia also have 44 chromosomes, and at his reproductive age, some of those cells will form into sperm. And, just like with the female's eggs, when the cells start their meiosis, the same separation of chromosome pairs also happens. So each sperm is either half-way to becoming a male or half-way to becoming a female. Got it so far?

When the female deposits her eggs, and the male fertilizes them with his sperm, the chromosomes combine back into pairs, producing a fry made out of cells containing 44 chromosomes each. Within these new 44 chromosomes, there are 2 that will determine the gender of the tilapia. One chromosome that came from the egg, and one chromosome that came from the sperm. There are only two possible outcomes - male or female. Or more specifically, more estrogen than testosterone, or more testosterone than estrogen.

Now that we have that established, mother nature tosses a monkey wrench into the works. There appears to be some conditions, such as temperature, or the proportion of males to females, that can override the levels of estrogen or testosterone, and cause tilapia fingerlings to develop as the opposite sex, or even switch genders several weeks after they have developed. And, as if it can't get any stranger, tilapia can even develop as hermaphrodites, with both male and female reproductive organs. Fortunately for me, the writer, and you, the reader, I choose to not explore these rare occurrences in an effort to keep confusion to a minimum. The truth is, nobody really knows why these anomalies occur. The best that anyone can do is record their observations, and make educated guesses.

Okay, so let's get down to the specifics of some individual species.

Pure Strain Wami (Oreochromis urolpeis hornorum)

A female Wami carries two chromosomes that determine gender, they are W and Z. The W chromosome affects sites that produce a lot of estrogen and the Z chromosome affects sites that produce testosterone. The male Wami also carries two chromosomes that determine gender, and they are Z and Z. The two chromosomes affect the same sites and produce estrogen, but they also affect sites that produce an equal or greater amount of testosterone.

Egghead Alert: This is for the teacher that you have checking my work, everyone else ignore this part. The Z chromosome of the female also affects sites that produce a small amount of estrogen, but not enough to trigger the development of ovaries on their own.

When the female deposits her eggs for fertilization, they each have one gender determining chromosome, either W for female (by way of estrogen production), or Z for male (by way of testosterone production). When the male fertilizes the eggs, each individual sperm carries a Z chromosome, which is predominantly for male (testosterone), with a small chance of female (estrogen). Now is when the magic happens.

If the sperm, with its Z chromosome, gets into an egg with a W chromosome, the high level of estrogen that will be produced due to the W, combined with the low level of estrogen that will be produced by the Z, is just enough estrogen to overcome the amount of testosterone that will be produced, and the fry will develop as female with WZ chromosomes. But, if the sperm, with its Z chromosome, gets into an egg that also has a Z chromosome, the fry will be a male with ZZ chromosomes, due to the overwhelming amount of testosterone that will be produced in relation to estrogen.

And that my friends is where male Wami tilapia get their ZZ chromosome moniker. It's not some lab created phenomenon, it's a completely natural fact of the species.

Egghead Alert: There are a bunch of nit-wits on the Internet who try to explain this aspect of tilapia genetics by making statements such as, 'the Z chromosome is dominant over the X chromosome, so you get male fry', etc. When you consider that a chromosome is really just an efficient storage unit for DNA, and that it's the DNA that gives instructions to the cells, and it's the cells that will ultimately form into the structures that determine the amount of estrogen and testosterone produced in the blood stream, you have to wonder from which sixth graders homework they were copying. Which, thinking about it, is an insult to sixth graders, so my apologies. But, just to watch them dance with their own words, I'd enjoy seeing how they explain the fact that if you cross a Blue tilapia male, with his ZZ chromosome pair, with a Nile tilapia female, with her XX chromosome pair, you get mixed-gender offspring. According to their explanation, this paring should only produce male offspring. Cue dance the music...

Pure Strain Mossambica (Oreochromis mossambicus) and Pure Strain Nile (Oreochromis nilotica)

Please read the explanations for Wami above. What follows will only be the differences for Mossambica and Nile.

Female Mossambicas, and Niles, carry two X chromosomes (XX pair) for determining gender and male Mossambicas and Niles carry an XY pair. The XX chromosome pair of the females, affect sites that produce a little bit of testosterone, and sites that produce a lot of estrogen. The X and Y chromosomes of the males affect sites that produce a little bit of estrogen and a lot of testosterone respectively. When they spawn (within their own species), the resulting fry will either carry XX and develop as female, or XY and develop as male.

Now, let's make a predominantly-male hybrid.

Wami Tilapia Hybrid (Oreochromis urolepis hornorum x Oreochromis mossambicus) or (Oreochromis urolepis hornorum x Oreochromis nilotica)

When you cross breed (hybridize) a pure strain Wami, with either a pure strain Mozambique or pure strain Nile, the level of testosterone produced in the progeny as inherited from the Z chromosome of the male will be stronger that the level of estrogen produced in the progeny as inherited from the X chromosome of the female, and the offspring will primarily develop as male. Remember that female Mozambques and Niles both carry a XX chromosome pair for determining gender and the sites that they affect produce a small amount of testosterone and a large amount of estrogen. In contrast, the Wami carries a ZZ chromosome pair which affect sites that produce a small amount of estrogen and a an equal or greater amount of testosterone. When they combine, the level of testosterone over estrogen in the bloodstream is just enough that as many as 90 percent will develop as male.

Important point: I cannot emphasize this enough. In order to get predominantly-male offspring, both parents must be pure strain! Really pure strain without any variations or mutations. Anything less than pure strain might still produce more males than females, however it is unlikely (I hate the word impossible when it comes to genetics) that they will produce up to 90% males.

The reason that you can't get predominantly-male offspring by crossing a Wami and a Blue tilapia (Oreochromis aureus), is because they both have the same chromosome pairs for determining gender. WZ for the female and ZZ for the male. And while the chromosomes themselves don't automatically guarantee anything, the sites that they affect just happen to produce about the same levels of testosterone and estrogen in both species. Not exactly the same levels, but close enough so that the offspring develop as both male and female. That's not to say that you couldn't hybridize a Wami to a Blue to achieve some other mutation, like a different color. Although, I'm not really sure why you'd want to.

Other Tilapia Genetic Crosses

It is a betrayal of trust for otherwise professional operations to make generalized statements about crossing tilapia strains, or oversimplifying the process, to customers who have come to them seeking knowledge. These statements are self-serving, almost always false, and the benefits of the reported cross is highly subjective. Not to mention the fact that many of their suggested or implied practices, can ultimately result in the release of impure species, and failed aquaculture. For example, one well-meaning and reputable producer of grow towers, makes the statement that their tilapia are a cross of Nile and Blue, and that "their" (since you won't get the same result) hybrid inherits the fast growth common to Nile, as well as the temperature tolerance found in Blue tilapia. This statement may or may not be true for each individual tilapia (in fact it's completely random), however the inference is that anyone who crosses a Nile with a Blue will get a fast-growing and temperature tolerant hybrid, which is irresponsible and false.

All tilapia have 44 chromosomes as 22 pairs packed full of DNA, the instructions for building a tilapia. When tilapia sperm meets tilapia egg, the number of possible combinations is astronomical. While some traits are dominant, every aspect of the resulting tilapia is up to chance. Any attempt to predict the outcome of a tilapia cross would yield better odds playing the lottery. Even if someone was successful at achieving one desired trait, such as temperature tolerance, it is impossible to determine what other traits were also passed on, including the undesirable ones.

Critical Point: It takes a scientific instrument called a DNA Sequencer to even begin to understand the traits of a single fish, let alone an entire hybrid line. Many traits, such as disease resistance, or metabolic rates, are unobservable. Never assume that a cross will result in the best of both worlds, it never will.

Our own Wami/Mozambique hybrids have been DNA sequenced in an effort to protect certain rights to the Hybrid lines, however this only serves as a kind of identification card. It would take hundreds, if not thousands, of these snapshots to develop any patterns for accurate determination of dominant traits. The only things that we know for sure are that they are predominantly male, have a more evenly distributed bulk from head to tail, and that they metabolize food very efficiently. We also know some of  their downside: they appear to have less tolerance to parasites, and of poor water quality. Other than that, other traits appear to be distributed an individual basis.

Crossing a Blue tilapia with a Nile tilapia can result in a fast growing hybrid for one person, and a slow growing hybrid for another. It can result in a fish that ages to a large size, or one that dies when it reaches six inches in length. It can result in a fish that can survive a pH range of 3 to 11, or one that dies below 4, or in a fish that can tolerate oxygen down to 3 ppm, or struggles below 5 ppm. Only carefully recorded observations, over the span of many years, can determine the suitability of any tilapia cross, and the parameters can change with each new set of parents.

In every case, it is always better for the tilapia farmer to stick to pure strain Blue or Nile tilapia, or well-documented and proven hybrids, rather than dabble in random genetic experimentations. Remember that tilapia farming success depends on predictability. Being able to forecast a harvest date, predict the cost per pound of production, or to understand the limits of environmental parameters, all require reliable baseline statistics. This is why recent crosses like the so-called white Nile and Hawaiian gold are not, and will probably never be, commercially viable.