Genetics DN: Why so blue? The determinants of color pattern in killifish

The bluefin killifish is a freshwater species that is found mostly in Florida. They are found in two main habitats, springs and swamps. An intriguing aspect of this species is that male bluefin killifish are brightly colored with many different color patterns. The brightest part of the fish is the anal fin, which is found on the bottom of the fish by the tail. Some males have red anal fins, some have yellow anal fins, and others have blue anal fins. This variation in color is called a polymorphism, meaning that in a species there are multiple forms of a single trait. In a single spring or swamp you may see all three colors!

Becky is a biologist studying bluefin killifish. One day, while out snorkeling for her research, she noticed an interesting pattern. She observed that there were differences in the polymorphism depending on whether she was in a spring or a swamp. Springs have crystal clear water that can appear blue-tinted. Becky noticed that most of the males in springs had either red or yellow anal fins. Swamps have brown water, the color of iced tea, due to the dissolved organic materials in the water. Becky noticed that most of the males in swamps had blue anal fins. After noticing this pattern she wanted to understand what might be causing it. Becky came up with two possible explanations. She thought males in swamps might be more likely to be blue (1) because of the genes they inherit from their parents, or (2) because individual color is responding to environmental conditions. This second explanation, when the expression of a trait is directly influenced by the environment that an organism experiences, is known as phenotypic plasticity.

Becky had to design an experiment that could tease apart whether genes, plasticity, or both were responsible for male anal fin color. She did this by collecting male and female fish from the two habitat types, breeding them, and raising their offspring in clear or brown water. If a father’s genes are responsible for anal fin color in their sons, then fathers from swamps would be more likely to leave behind blue sons. If environmental conditions determine the color of sons, then sons raised in brown water will be blue, regardless of the population origin of their father.

Becky and her colleagues collected fish from two populations in the wild - Wakulla Spring, and 26 Mile Bend Swamp - and brought them into the lab. These two populations represent the genetic stocks for the experiment. Fish from Wakulla are more closely related to each other than they are to fish from 26 Mile Bend. In the lab, they spawned female fish with male fish from the same population: females from Wakulla spawned with males from Wakulla, and females from 26 Mile Bend spawned with males from 26 Mile Bend. After the offspring hatched from their eggs, half were put into tanks with clear water (which mimics spring conditions) and half in tanks with brown water (which mimics swamp conditions). For the brown water treatment, Becky colored the water using ‘Instant, De-caffeinated, No-Sugar, No-Lemon’ tea. They raised the fish to adulthood (3-6 months) so they could determine their sex and the color of the son’s anal fins. Becky then counted the total number of male offspring, and the number of male offspring that had blue anal fins. She used these numbers to calculate the proportion of sons that had blue anal fins in each treatment.

HYPOTHESIS: Becky thought males in swamps might be more likely to be blue (1) because of the genes they inherit from their parents, or (2) because individual color is responding to environmental conditions. If a father’s genes are responsible for anal fin color in their sons, then fathers from swamps would be more likely to leave behind blue sons. If environmental conditions determine the color of sons, then sons raised in brown water will be blue, regardless of the population origin of their father.