Getting My Bait

My second week at NIH has begun and I am starting to feel much more comfortable with the environment and projects I've been confronting. My wonderful Monday morning and afternoon were focused on sexing and crossing 92 zebrafish pairs of F2 generations. The first skill I had to master was how to distinguish males from females - not an easy task.

                                                                                                       
The picture above shows a female fish at the top and a male zebrafish below it. The main characteristics to look for in each sex are the following:

Females:

• Have a larger stomach (usually because of the eggs they carry)
• Have clear tails with yellow tips
• Have very yellow dorsal fins
• Are generally larger than male fish   

Males:

• Are usually torpedo shaped
• Retain a pinkish color
• Have clear dorsal fins
• Have very yellow tails 

The process of sexing seems easy when the differences are described so clearly, but in reality, many males and females are difficult to distinguish - especially if a male fish is full and looks like it is carrying eggs. Nevertheless, after sexing the fish, it was time to cross all 92 pairs.
           
Crossing is done by placing one male (sometimes two) and one female fish in a crossing tank like the one shown above. The male fish then swims on the side or behind the female, stimulating her to release eggs while the male externally fertilized them. Spawning usually ends 10-30 minutes from the time it starts and will ideally leave eggs at the bottom of the tank to be collected!