by Grae O’Toole, Director, Center for Wild Bird Rehabilitation & Emily Curd Guswa, Assistant Professor of Biology at Landmark College
This semester two sections of Principals of Biology 2 learned about cells, sex determination, and DNA technology while helping the Vermont Institute for Natural Science (VINS) learn more about their raptor ambassadors. At first blush, this may seem like an unlikely partnership. Principals of Biology 2 focuses on life below the cellular level, and VINS is an organization that specialized in environmental education and research and avian rehabilitation. However, a lot of environmental and organismal research relies upon genetic tools to solve mysteries hidden in DNA.
Landmark students set out to uncover the mystery of how many sex chromosomes individual birds contain in their cells. Not all birds have external traits that vary by sex. For those species with similar-looking males and females, an easy way to tell them apart is to count sex chromosomes. In Humans, genetic males carry two different sex chromosomes (XY) and genetic females carry two copies of the same sex chromosome (XX). Birds have a different system where males have two copies of the W sex chromosome and females have a single copy of two different chromosomes (WZ). The W and Z chromosome share some of the same genes, and there is variation in those genes between the W and Z chromosome. We can apply standard genetic techniques to differentiate between sex chromosomes.
DNA from a few cells in the quill tip of a flight feather is sufficient to determine sex chromosomes. Over the last month, Landmark students have learned three commonly used techniques to acquire feather DNA and count sex chromosomes: DNA extraction, PCR, and gel electrophoresis. Students learned the chemistry required to successfully crack open cells, capture DNA, and clean it. Students learned PCR theory and technique to successfully generate billions of copies of W and Z chromosome-specific DNA. Finally, students learned how to visualize their amplified DNA with gel-electrophoresis, which separates different-sized DNA and illuminates it with UV light. The end result is a clear image of one or two DNA bands (see the Figure below).
Revealing the sex chromosome number was really exciting! Students were responsible for an individual bird, and could easily identify the sex chromosomes of their bird on the gel. Some had positive controls birds with known sex and others had birds with unknown sex. The students found out if their bird’s sex was known after reading the gel. The truly amazing result is that the genetic sex for all positive control birds matched the results from the gel! The DNA extraction for a few samples failed and that is ok. From this failure, we learned that flight feathers work best for bird sex determination. We tested flight and body feathers for sex chromosomes, and only one body feather sample produced a band. The good news is we can get new material and try again. Science after all is a process, and from this small experiment, we learned a lot!
Gel electrophoresis results for bird sex chromosome determination. The gel images show the number of different sex chromosomes for birds samples SP22_B2_02(02)- SP22_B2_13 (03) (Bird ambassador identities not provided). Positive controls with known sex are indicated by PC-M (male) PC-F (female). 100 base pairs (bp) DNA ladder was used as a DNA size standard and indicates the relative length of DNA in 100 nucleotide bp increments. Samples 05 and 06 were body feather extracts and did not produce bands. The No DNA sample is a negative control where water was added instead of DNA to ensure our reagents were not contaminated.