4.5 Oral sex in cichlid fishes

Cichlid (pronounced SICK-lid) fish are a diverse group of freshwater fish that occur throughout the tropics. They are small and not particularly tasty, so some of their unusual features may be overlooked. For example, some cichlid fish are “mouthbrooders,” whereby the females spawn and then suck their eggs into their mouths. This parenting strategy protects the eggs while imposing obvious costs: how do you forage for food without eating your own children?

Some cichlid mothers handle the feeding problem by spitting out the eggs, foraging, and then sucking the eggs back into her mouth for easy transport. One adaptation associated with this behavior is the mother’s tendency to pursue, and attempt to put in her mouth, anything that looks like an egg.

Enter the male cichlids that are characterized by egg-like shapes on their anal fins. A mouth-brooding female may respond to these “eggs” as though they were her own, and attempt to suck them into her mouth. And swimming past a mouth-brooding female, the male with the egg-shaped mimics puts himself in an excellent position to ejaculate into the female’s mouth.

Scientists have recently described the genetic basis of anal-fin egg mimicry in cichlid fishes. A single gene (csf1ra) that codes for a yellow pigment is active in fish with egg mimics, and appears to have evolved relatively recently.

• Did the female’s egg-sucking behavior exert selection pressure on this gene?
• Can you think of an evolutionary benefit to the male’s physiology (egg mimic) and behavior (ejaculation)?
• Can you think of any other organisms that have decorative devices that may have evolved to facilitate fertilization?

Describe, using either of the examples discussed above (cichlid fish, mouse), the general function of DNA.