text-only page produced automatically by LIFT Text Transcoder Skip all navigation and go to page contentSkip top navigation and go to directorate navigationSkip top navigation and go to page navigation
National Science Foundation
Discoveries
design element
Discoveries
Search Discoveries
About Discoveries
Discoveries by Research Area
Arctic & Antarctic
Astronomy & Space
Biology
Chemistry & Materials
Computing
Earth & Environment
Education
Engineering
Mathematics
Nanoscience
People & Society
Physics
 

Email this pagePrint this page

Discovery
Worms Can Evolve to Survive Intersex Populations

Experimental evolution reveals a possible intermediate stage in the evolution of sex determination

Photo of a C. elegans worm tail belonging to a male, showing sensory rays.

A typical C. elegans worm tail belonging to a male.
Credit and Larger Version

December 5, 2011

Sexually reproducing species need at least two sexes in order to produce offspring, but there are many ways that nature produces different sexes. Many animals (including humans and other mammals) use a chromosomal sex determination system in which females have two X chromosomes, while males have one X and one Y.

On the other hand, for some reptiles, sex is temperature dependent--that is, incubating the eggs at different temperatures can determine whether the offspring develop as males or females.

Evolution in a trait can only happen when variation exists in that trait. However, gene mutations related to sex determination could be very risky if they result in animals that are unable to reproduce.

Evolution of sex

Researchers have long wondered how variation in such a trait could exist, since we might expect sex determination to be highly conserved--meaning that any variation in the genes would probably lead to trouble and thus be quickly eliminated by natural selection. To the contrary, there is evidence that these genes have evolved very rapidly and diverged greatly among species. But what happens in the intermediate evolutionary stages of sex determination?

Under normal circumstances, the worm-like nematode Caenorhabditis elegans has a chromosomal sex determination system: Most individuals are XX, which are actually hermaphrodites that can self-fertilize, but can also outcross with males, who only have a single X chromosome and develop male gonads. In a new paper in the journal Evolution by Michigan State University postdoctoral researcher Christopher Chandler and assistant professor Ian Dworkin--both members of the BEACON Center for the Study of Evolution in Action--along with collaborators at Iowa State and the University of Oregon used experimental evolution to test how C. elegans could overcome changes to that system.

The researchers used a mutant strain of C. elegans that has a temperature-dependent sex determination system: At cool temperatures, the animals typically develop as hermaphrodites, but if they are exposed to high temperatures during development, they become males. The researchers produced populations full of intersex individuals by exposing the larvae to intermediate temperatures. The intersex individuals had characteristics of both males and females, which made reproduction difficult for them.

Intersex reproduction

Chandler and colleagues then allowed these low-fertility, intersexed populations to survive and reproduce for 50 generations. At that point, they measured the later generations' sex ratio and fertility.

C. elegans sexes can be easily distinguished by tail shape: Males have a rounded tail tip, while females have a whip-like, pointed tip and the eggs can easily be seen near the tip of the tail. The intersex animals have both characteristics, with a rounded male-like tip but often carrying eggs like a female.

By the end of the experiment, all of the populations had returned to more typical sex ratios and higher fertility, despite being subjected to the intermediate temperature that originally produced intersex animals. The researchers found that the animals had not changed much in the expression of sex determination genes. Instead, Chandler said, "they appeared to be compensating through other genetic mechanisms, and likely in a number of different ways."

In other words, other genes were evolving to make up for changes in the sex determination genes, in a way that allowed individual animals to develop either as a male or a female, rather than intersex.

The findings have enormous implications for understanding how evolution works. As Dworkin points out, "even when the genes have a function as crucial as sex determination, the evolutionary process can overcome intermediate or less-fit types in sometimes surprising ways."

-- Danielle J. Whittaker, BEACON Center for the Study of Evolution in Action, Michigan State University, djwhitta@msu.edu

This Behind the Scenes article was provided to LiveScience in partnership with the National Science Foundation.

Investigators
Christopher Chandler
Ian Dworkin
Genna Chadderdon
Fredric Janzen
Patrick Phillips

Related Institutions/Organizations
Michigan State University
Iowa State University
University of Oregon

Locations
Michigan
Iowa
Oregon

Related Programs
Genetic Mechanisms
Evolutionary Processes Cluster
Long Term Research in Environmental Biology

Related Awards
#1120417 Mating systems and the origins of genetic conflict
#0922344 Swept under the rug: A systematic analysis of genetic background effects in Drosophila
#0641066 Partial Selfing and the Genetic Basis of Mating System Variation
#0939454 BEACON: An NSF Center for the Study of Evolution in Action

Related Websites
LiveScience.com: Behind the Scenes: Worms Can Evolve to Survive Intersex Populations: http://www.livescience.com/17189-worms-intersex-evolution-nsf-bts.html
Experimental Evolution of the Caenorhabditis elegans Sex Determination Pathway, Evolution, Sept. 8, 2011: http://onlinelibrary.wiley.com/doi/10.1111/j.1558-5646.2011.01420.x/abstract
BEACON Center for the Study of Evolution in Action: http://beacon-center.org/

Photo of a C. elegans worm tail belonging to a hermaphrodite.
A typical C. elegans worm tail belonging to a hermaphrodite.
Credit and Larger Version

Photo of an intersex C. elegans worm carrying eggs.
An intersex C. elegans worm carrying eggs.
Credit and Larger Version



Email this pagePrint this page
Back to Top of page