Evolution of Avian and Mammalian sex chromosomes.
Image: E.R.S. Roldan and Montserrat Gomendio “The Y chromosome as a battle ground for sexual selection” Trends in Ecology & Evolution 1999, 14:58-62.
You all are probably curious to know what is happening with my birds these days, so I shall tell you about how sex is identified in birds, such as parrots, where the sexes look the same. But wait a minute, you ask, what does an update about my birds have to do with identifying sex in parrots?
You might recall that my hawk-headed (red-fan) parrot is still formally unnamed because I don’t know what sex she is. I have chosen these names for the bird; Persephone if she’s a female, Orpheus (thanks to one of my readers) if she’s a male. Currently, I refer to her by the unisex name, “Pipsqueek”, which will probably remain one of her (his?) nicknames for life.
Anyway, I am solving the problem of identifying the bird’s sex today. About one hour ago, I clipped one of “Pipsqueek’s” toenails into the quick to collect some blood so I can mail it to Avian Biotech today. After they receive the specimen, they will isolate the DNA from the red blood cells (RBCs) — unlike mammalian RBCs, bird RBCs contain nuclei, which contain DNA. After isolating the DNA, they will perform a PCR test that selectively amplifies a specific region on the bird’s sex chromosomes by a million-fold. This PCR product is then run on an agarose or polyacrylamide gel which separates the amplified DNA fragments based on their sizes — the larger the DNA fragment, the slower it moves (when in grad school and working as a postdoc, I used to “sex” my own birds and anyone else’s who provided bird blood for me to use, so I am very familiar with this technique). The results look something like this (right).
These PCR results show two distinct DNA “bands” for female birds (F) and one for males (M). This is because the sex of birds is genetically determined oppositely from mammals. Female birds are the heterogametic sex, meaning that they possess two sex chromosomes, known as “ZW”, that are different from each other, whereas male birds are the homogametic sex, possessing two identical Z chromosomes. When a small portion of these sex chromosomes are PCR amplified, they provide either one or two differently-sized DNA fragments, which are easily detected on a gel, thus providing a quick and easy method to identify sex in birds.
“Pipsqueek” has probably not experienced much pain in her short life, so she sat very quietly (uncharacteristically so) in my lap while I wrote this to you. I feel bad for hurting her. However, now that I have finished writing this entry for you, she appears to have recovered from her upset because she is now busily trying to rip the pocket off the seat of my jeans after chewing a hole in my bathtowel.