Boy or Girl? Genetic Testing

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Saticoy's PCR products in an agarose gel

On March 10, thousands of Condor Cam viewers watched as an endangered California condor chick emerged from its shell. The chick, named Saticoy by an online vote, is still being followed by thousands of people on Wildlife Conservancy’s Condor Cam. As viewers watched the downy chick grow, many wondered if they should call Saticoy “he” or “she.” That’s where the San Diego Zoo Institute for Conservation Research’s Genetics Division comes in.

California condors are sexually monomorphic, and it’s not possible to tell the male and female birds apart. It is important for management purposes, however, to know early on the gender of a newly hatched condor. Initially, determination of bird gender involved a highly invasive examination, but genetics techniques have allowed for less invasive sexing since the California condor recovery program began in the early 1980s, and with current methods, we can determine a condor’s sex using only a small drop of blood, a feather, or even a piece of eggshell membrane.

Our Genetics Division has genetically determined the sex of over 170 condors hatched at the San Diego Zoo Safari Park, as well as all other condors hatched at collaborating institutions or in the wild. But we don’t stop there. In addition to condors, we determine the sex of several other bird species for San Diego Zoo Global collaborative recovery programs, including San Clemente loggerhead shrikes and four species of native Hawaiian birds. This important practice supports animal care managers and field biologists in their efforts to develop sustainable populations of endangered species.

The process of sexing Saticoy began when we received a small blood sample from the chick’s first health exam performed by veterinarians at the Safari Park (see post Condor Chick: First Health Exam). Only a single drop of blood (about 10 microliters) is needed to perform gender determination using genetic techniques. First, the DNA is extracted from the blood cells by placing the cells into a tube, exposing them to enzymes, and incubating them in a water bath to release the DNA contained within.

After several steps of removing contaminants and washing the sample, pure DNA is available for use in the next step, the polymerase chain reaction (PCR). During PCR amplification, only a small amount of the DNA is required to obtain millions of copies of a particular DNA region of interest, which can then be used for further analyses. Birds have a sex-determination system like the XY system of humans, but in birds, the females are the heterogametic sex with chromosomes ZW, while the males are the homogametic sex with chromosomes ZZ. In the case of sexing condors, we amplify a gene that is found in both males and females and is able to show variation in sequence length between the two female sex chromosomes.

The final step is to run gel electrophoresis to analyze the PCR samples. Here, DNA separates within an agarose gel depending on the size of the DNA fragment produced by PCR. The gene amplified from a female’s W chromosome is longer and produces a different size fragment compared to the DNA fragment produced by the Z chromosome. The males, therefore, show only one band while females show two distinct bands in a gel. As the gel picture shows, Saticoy’s DNA sample produced one band by PCR, and we can then say…it’s a boy!

Heidi Davis is a research coordinator and Asako Yamamoto is a research technician for the San Diego Zoo Institute for Conservation Research.