On Scripps College Week: Why do some chromosomes act selfishly?
Patrick Ferree, professor in the W. M. Keck Science Department, finds out.
Patrick Ferree, a developmental geneticist at the Claremont Colleges, is studying
how certain chromosomes selfishly hijack reproductive development to gain a
transmission advantage.
Dr. Patrick Ferree is a professor in the W. M. Keck Science Department, which is
affiliated with Scripps College and Pitzer College, both located in Claremont, CA.
Dr. Ferree received a bachelor’s degree in biology from the University of North
Carolina Chapel Hill and a Ph.D. from the Department of Molecular, Cellular, and
Developmental Biology at the University of California, Santa Cruz. He did
postdoctoral studies at Cornell University. His current research focuses on
reproductive development and understanding mechanistically how selfish genetic
elements cause disharmony within the genome, using insects as experimental
model organisms.
Unexpected Conflict in the Nucleus
Generally, chromosomes are humble servants of the nucleus. As carriers of the
genes, they are passively partitioned to daughter cells during cell division.
However, some chromosomes behave selfishly – even aggressively – so that they
are transmitted to offspring at levels much higher than the other chromosomes.
And while this kind of genetic cheating is good for the inheritance of a selfish
chromosome, there can be harmful effects to the other chromosomes and the
organism. This type of disharmony is referred to as genome conflict.
Our group is studying a selfish chromosome known as PSR, which is found in
different insect wasp species. PSR stands for Paternal Sex Ratio because it
destroys half the wasp’s other chromosomes at the very beginning of embryonic
development; this behavior causes female-destined embryos to become male,
resulting in wasp populations that are extremely male-biased.
Why does PSR do this? This selfish chromosome is transmitted only by the male
parent to offspring, so the more males carry PSR, the more this selfish
chromosome can spread in wasp populations.
A real mystery has been how PSR destroys the other chromosomes. Using
genome sequencing technologies, we identified several genes expressed by PSR.
When we experimentally reduced expression of one of these genes, the
chromosome-destroying effect was lost, and female progeny reappeared. This
result shows that this gene acts like a toxin that targets the other chromosomes
for destruction and is responsible for the sex ratio biasing effect.
Our work raises many new questions. How do such toxin genes work? What are
their evolutionary origins? Broadly, what are the conditions in which a
chromosome can become selfish, and could such a thing occur in the human
genome? Stay tuned.
