Fish Breeding, Introducing Recessive Gene

Photo is a male silver lyretail molly displaying a nice sailfin. This is the fish you want to create starting with a male silver sailfin molly and female green lyretail sailfin mollies

Fish Breeding, Introducing Recessive Gene

Recently I began a blog series on introducing new genes into a livebearer population. Technically the proper usage would be introducing alleles; and I’ll use “allele” properly in this blog instead of the popular usage of gene to avoid confusion, so you need to get used to it. An allele is an alternate version of a gene. For example, in humans there is a gene that controls eye color and it comes in two versions (alleles), blue and brown. There are modifier genes that change basic brown to hazel and blue to green, but there are only two basic eye color alleles. Some genes have more than two alleles.

In the first blog of the series (http://goliadfarms.com/introducing-a-dominant-gene/) I discussed the mechanics of introducing a single dominant allele into a swordtail population. This blog I’ll do the same for a recessive allele using mollies as an example. By the way, this process of introducing a recessive allele can be used in any breeding program of animals and plants.

First, let’s review what a molly is. A molly is any fish once placed in the genus Mollienesia (from whence its common name “molly” came). This genus has been combined with guppies into the genus Poecilia, which today contains 31 species, many of which aren’t common in the hobby. Some of these are available from clubs like the American Livebearer Association (www.livebearers.org). We raise some of these less common species as well.

With that background out of the way, let’s get started. Imagine you have a breeding colony of green lyretail mollies and you’ve acquired a male silver molly. And, you want to create a strain of silver lyretail mollies. How do you go about doing this?

First, you have probably correctly assumed the silver allele is recessive simply due to this blog’s title. The normal (often called “wild”) color of mollies is green or gray. Green is dominant to silver. If you mate a silver molly to a homozygous green molly, all the first generation fry will be green. Let’s discuss what is meant by homozygous and heterozygous. When the same allele is inherited from both parents an offspring is called homozygous for that gene. If two different alleles are inherited, then an offspring is heterozygous for that gene. “Homo” means same and “hetero” means different.  With that out of the way, what happens if you mate a pair of the first generation fish (geneticists call these fish F1 for first filial generation)? Since each F1 fish is heterozygous and carries the green allele and the silver allele (silver is recessive to green so these fish are green), when you mate F1s together you’ll get three kinds of fish in the F2 (second filial) generation. There will be:

• homozygous greens, having two copies of the dominant green,
• heterozygous greens having one green and one silver allele, and
• homozygous silvers, having two silver alleles.

In general, three-fourths of the fish will be green and one-fourth silver. This is the traditional 3:1 ratio you get when mating organisms that are heterozygous for a dominant and its related recessive allele. We’ll come back to these matings and their results shortly.

First to make this a bit clearer let’s use the geneticist’s Punnett square and some graphics to illustrate the mating of silver molly male to a green molly female. The Punnett square (named after its inventor) is a grid used to predict the results of a cross. A Punnett square sets forth the gametes (eggs and sperm) produced by the two breeders and the offspring types resulting from the sperm fertilizing the eggs. In this mating the Punnett square is simple. The male’s gametes (sperm) are on the vertical, left side of the square and the female’s gametes (eggs) are across the top. The combination of alleles in the offspring are represented in the single cell since all the fry end up with a recessive silver (s) allele and a dominant green (G) allele since the parents are homozygous and produce only one type of gamete. Since green (G) is dominant, despite having a silver (s) allele, each fry is green.

Now let’s mate the heterozygous F1 green mollies together. In this case, the Punnett square will have four cells since each fish produces two types of gametes, one for silver and one for green. Again the male’s gametes are on the right side and the female’s gametes across the top. If you count the squares, you’ll see that on average you’d get one silver fish for three green fish. I say “on average” since the two different types of sperm fertilize the two types of eggs randomly.

In our particular set of matings, creating a silver lyretail sailfin molly, we are also dealing with another gene, one for tail type. The lyretail allele is dominant to non-lyretail, which is the normal (wild) tail type. Unlike in swordtails and platies, lyretail in mollies does not prevent males from sexually functioning. In swordtails the lyretail allele also causes the anal fin and pelvic fins to elongate. That causes the male’s gonopodium to be too long to insert sperm into the female. This is not the case in mollies and the gonopodium of a lyretail male is normal. So, a lyretail molly can be either homozygous or heterozygous for lyretail.

Okay, so what do we do with our male silver and female green lyretail mollies? First, we simplify things by raising virgin female green lyretails. That way we know their offspring will be from the silver male. Why raise virgins? We raise virgins because female Poeciliidae, which includes mollies, swords, platies, etc., can store sperm from earlier matings and have multiple batches of fry from an earlier mating. So to be sure the fry we get from the lyretail females belong to the silver male, we raise virgins. Raising virgin female Poeciliidae is a pain. Someday I’ll blog about techniques that can make it easier. By the way, if we were talking about ANY other fish than Poeciliidae we wouldn’t have worry about raising virgins because no other fish store sperm.

Things get more complicated when you are dealing with two characteristics that are inherited through entirely different genes. So, what do we know about your fish at this point? We know the silver male is homozygous  and has two copies of the allele for the recessive silver trait. We know he is homozygous also for non-lyretail since he doesn’t have a lyretail. Let’s assume you’ve had the green lyretail molly strain for enough generations without seeing any silvers to assume that all of those fish are homozygous for green color. By the way, since the lyretail allele in mollies is a dominant, you could have some heterozygous lyretail females. But, since you’ve had these fish for lots of generations and aren’t producing any non-lyretails, we’ll assume they are also homozygous lyretail.  In your first mating, the Punnett square will have only one cell because both parents are homozygous. The male will produce only one type of gamete (s/n) while the female also only produces one kind of gamete (G/Lt). The symbol “G” is for the dominant green allele, “s” is for the recessive silver color, “Lt” is for the dominant lyretail, and “n” is for the recessive normal tail.

As you can see from above, when we mate your silver male to the female green lyretails all the F1 fish will be green lyretails. Why? This is because each fry will be heterozygous for the dominant lyretail and heterozygous for the recessive silver alleles.

When mating the F1s things get more interesting and your Punnett square will have 16 cells because each parent will produce four types of gametes as indicated in the next square. These gametes will randomly combine to form a number of genetic types (genotypes) and four types of fish (phenotypes) in a 9:3:3:1 ratio. On average out of 16 F2s nine will be green lyretails, three will be green non-lyretails, three will be silver lyretails (the desired fish), and one will be silver non-lyretail.

With the genetics behind us, let’s recap. You placed the male silver molly with the virgin female green lyretails. After about a month the first batches of fry were born. They were all green lyretails, but were heterozygous for both color and tail type. They are carrying the desired silver allele. With good food and warm temperatures, these fish will be sexable and ready to reproduce at about 3.5 months. There are two ways to go from here, I’d recommend doing both assuming your male silver molly is still alive and well. First, let’s assume you raised virgin F1 green lyretail females. It will simplify things. Pick the best 20 or so lyretail females. Ten of these will be placed with their father (by the way inbreeding does not deserve its bad reputation, so don’t hesitate to use it). The other ten are placed with the best F1 male green lyretail, their brother of half-brother.

From the daughters mated to their father you’ll yield 50% silvers and half of those will be lyretails. From the other mating you’ll yield 25% silvers and half of those silvers will be lyretails. It’s the silver lyretails you want.  When you mate these fish you’ll get only silver fish. At first, you’ll also get some non-lyretails. As each generation passes the percentage of non-lyretails will decline since by chance some of your breeders will be homozygous for lyretail. Time and chance will reduce the percentage of non-lyretails if you diligently select only lyretails for breeding. There is a method to speed this up by raising virgin females and doing test crosses to determine if they are homozygous, but it’s not really necessary.

Through this set of matings you either introduced silver, a recessive gene, to your green lyretail sailfin molly population; or you could look at is as having introduced the dominant lyretail gene to your silver sailfin molly population.

In a future blog we’ll look at additive or multiple genes.

Good fishkeeping!