Photo: A female Veiltail Swordtail.
A few months ago while processing our Blushing Swordtails I discovered an unusual female. She appeared to be a veiltail, but I wasn’t sure.
Before continuing discussing that particular female, let me provide some background. Our breeding colony of Blushing Swordtails consists of lyretail females and normal males. Why is this so? Well, we must use regular, meaning non-lyretail, males because lyretail males are functionally fertile. Allow me explain. Lyretail males produce viable sperm, but their intermittent organ, the gonopodium, which functions like a mammalian penis, is too long. There are cases when bigger isn’t better and this is one. The lyretail males are unable to deliver sperm to the female. Fortunately, the lyretail gene is dominant. Technically, the lyretail allele (allele is the term for a version of a gene) is dominant to the normal fin type allele. Any fish carrying at least one lyretail allele will be a lyretail. Furthermore, absent the use of artificial insemination (AI) (for a discussion of AI in swordtails see http://www.ctsa.org/files/publications/Lyretail6319619779998111681.pdf), it is not possible to get a homozygous (having both lyretail alleles, one from each parent) lyretail. As a result all lyretail swords (again absent AI) are heterozygous lyretails. Since lyretail males are functionally sterile it is necessary to mate lyretail females to non-lyretail males. This mating will produce on average only 50% lyretail offspring. To illustrate this here is a pair of Black Swordtails; the female is a lyretail and the male isn’t.
Below, a Punnett square showing this results of the mating of a lyretail female to a non-lyretail male. The lyretail female’s gametes are listed across the top and the male’s gametes to the left. The female produces two types, lyretail (Lt) and non-lyretail (n). The male produces only one type of gamete, non-lyretail (n). The resulting offspring are either heterozygous lyretail (Lt-n) or homozygous non-lyretail (n-n).
Okay, so the function sterility of lyretail males is why our Blushing Swordtail breeding colony has only non-lyretail males. We use only lyretail females to maximize the percentage of lyretails the colony will yield. Lyretails bring more money than non-lyretails and Susie, my wife and business partner and a pragmatic business person, insists on maximizing profits.
Now, let’s get back to the strange female. She didn’t really look like a lyretail, but more like a veiltail. Veiltail is another trait found in swordtails. It is also a dominant allele, I think. I’ve diligently searched online and in my references (someone has misplaced a couple of my swordtail genetics books, I blame Susie, but it could have been me), but I haven’t been able to find anything about veiltail swordtails. So, I’m having to work from memory. I don’t recall veiltail (if it even exists) affecting the male’s gonopodium, but maybe it does. Below is a photo of one the female’s offspring that looks much as she did (she’s since gone to fish heaven).
When I found the original female, I thought she might not be a lyretail but maybe a veiltail. I placed her in her own vat with a couple of male Blushing Swordtails. Then, I got busy and pretty much forgot about her. Recently Susie was going through our database and asked about the breeding colony in vat 2A02 (greenhouse 2, row A, second vat). She wanted to know why it hadn’t been processed and why I was tying up valuable greenhouse real estate for just three fish. She insisted we process the vat with her aim of doing away with that breeding colony. But once in the vat, we found some interesting adults. About half the fish had normal tails, neither lyretail nor veiltail. The other half looked more like veiltails than lyretails. This confirmed my suspicion that this veiltail trait is likely dominant to normal tails and that it was likely the female was heterozygous for the trait. Here’s a Punnett square showing what I think happened when the original weird female was mated to a normal male. “Vt” is dominant veiltail and “n” is recessive normal, I think.
Okay, so why didn’t I think these were just strange lyretails? The genetics for lyretail would work as well, producing 50% lyretail. First of all, they just didn’t look like lyretails. Secondly, while the males had somewhat elongated gonopodia, they weren’t the extremely long ones of lyretails. And third, the males’ ventral fins weren’t long like they are in lyretails. Fourth the males’ dorsals didn’t look like lyretail dorsals. See the male in the photo below to see if you agree.
To further explore this trait (veiltail or not), I set up two new breeding colonies. One has veiltail females and their normal tailed brothers (or half-brothers depending upon which of the two males sired them). The other has normal tailed females from the original female with veiltail males. What might these two matings tell me?
In the first mating, the veiltail females and normal males, I expect to get 50% veiltail offspring since I expect the females are heterozygous for veiltail. This is essentially repeating the mating done with their mother except with more fish. In the second mating, normal tailed females with veiltail males, I’m hoping to get 50% veiltails after the first batch of fry. Even if the veiltail males are functionally fertile, the first batches of fry are likely to be from some of the normal tailed males too. This would skew the ratio of veiltails to normal tails toward the normal tails. Subsequent batches should be primarily from the veiltail male if they are fertile since even though female store sperm from earlier matings new sperm seem to out compete the older sperm. If I get no veiltails then I’d assume the male veiltails are functionally sterile.
I probably should have set up a third breeding colony, but Susie takes a dim view of too many “experimental” vats. This colony would have had only veiltails of both sexes.
By the way, if anyone has information about the veiltail trait in swordtails please let me know.