Male Red Lyretail Swordtail

Red Lyretail Swordtail Experiment

One problem with raising lyretail swordtails is that the males are functionally sterile. While they produce viable sperm, they have an elongated gonopodium (a penis-like modified anal fin used to inject sperm into the female), which prevents them from mating. Lyretail in xiphophorines (members of the genus Xiphophorus containing platies, variatus, and swords) is a dominant gene. So, any fish inheriting a copy of the gene will grow a lyretail. Since the males are functionally sterile, the females must be mated to non-lyretail males. As a result all of the offspring of these matings will be heterozygous for lyretail or homozygous for non-lyretail. Why is this? A fish can only inherit the allele (a variant of a gene) for normal fins from its non-lyretail father. From its lyretail mother it will inherit either the non-lyretail allele or the lyretail allele. In the former case the fish will be non-lyretail; in the latter it will be lyretail. Now, using artificial insemination (AI), homozygous lyretails can be produced, but AI is a labor intensive procedure (I’ve done it and it’s a pain) and not really feasible for regular production of lyretails. So, breeders use non-lyretail males with lyretail females and cull out the 50% of offspring which are non-lyretails. Typically, the non-lyretail breeder males are selected from offspring of the best female lyretails in order to accumulate genes that code for improved lyretails, such as those for straight lyres and wide tails.

It occurred to me that possibly a lyretail male could be successful at mating if his gonopodium, and the two ventral fins that are used with the gonopodium to insert sperm and which are also elongated by the lyretail gene, were shortened to normal length. So, I set up an experimental vat with 20 non-lyretail females and six lyretail males with their gonopodia and ventral fins shortened.

How did I shorten the fins? Simple I placed the fish on a damp towel and using a sharp, sterile razor blade, I cut them short. Surprisingly, the  females around cringed at this more than I did.

Why did I put the six surgically modified males with non-lyretail females? Well, just to remind you, female Poeciliidae (livebearers such as swords, mollies, and guppies) can store sperm and have multiple spawns from single mating. If one wants to be sure of male parentage of Poeciliidae one typically raises virgin females. This process is time-consuming and I rarely bother. Instead I set up matings that make it certain the desired offspring can be identified. Often I use genetic markers such as albinism. In this case, its done by using non-lyretail females who don’t have to be virgin because they could only have been bred to non-lyretail males. So, after being placed with my modified lyretail males, any future lyretail offspring could only come from the modified males. We’ll see what happens in a couple of months.

Interestingly enough, while processing the red lyretails, we found a male with a short gonopodium although his ventral fins were long. On the off chance he is functional, I gave him five non-lyretail females in his own breeding vat. Like the experimental vat fish, if any lyretail offspring are produced, they must be from the lyretail male.