Self-fertilizing worms lose thousands of active genes in reproduction process

Nov 06, 2012
Worms that Lose That Loving Feeling Also Lose Thousands of Active Genes
The female C. briggsae has evolved to a self sufficient reproducer. The arrow points to eggs this female self-fertilized.

(Phys.org)—A new University of Maryland study of worms that reproduce without a mate shows that not only have these creatures lost their mojo in the dating game of evolution, they've lost thousands of genes as a consequence.

In a paper appearing in the Nov. 12 issue of , a team from the University of Maryland reports that the self-fertilizing nematodes, or roundworms, they studied have only about two thirds as many active genes as their ancestors, which reproduced exclusively through male-female mating.

"Our study confirms that when females shift from mating with males to fertilizing their own eggs, the number of genes utilized by that shrinks," says the study's leader, Eric Haag, and associate professor of Biology at the University of Maryland.

"We also found that the genes activated only in males or females seem to be especially likely to be lost in the self-fertilizing species. Overall, our study forges a fascinating link between how a species reproduces and the size and content of its genome."

Cliffs Notes Version of the Genome

Haag's team looked at the genomes of two common species of roundworm, and the related C. briggsae. About the size of a dust speck, these worms have, over a few millions of years, evolved from animals with females and males into species where the female has become a hermaphrodite that creates her own sperm to fertilize her eggs. In these species, males are very rare, and no mating is needed for reproduction.

Haag has done other on these worms, but his idea to compare the genomes of hermaphrodites and their close relatives came from ongoing genome projects. "The genome sequences have ambiguities that make comparing their sizes tricky, but I noticed they were always bigger in the species that had typical male-female sexes," Haag says.

Applying cutting edge sequencing technology in collaboration with genomics researchers from the National Institutes of Health, Haag's team compared the complete sets of active genes (the transcriptomes) in males and hermaphrodites of C. elegans and C. briggsae to males and females of three related species that still reproduce exclusively by mating. This adds information that DNA sequencing alone cannot, such as differences in how each sex uses a gene.

"C. elegans and C. briggsae seem to have the 'Cliffs Notes' version of the genome," says Haag. They have the most critical genes of their ancestors, but have lost thousands of others. These include all sorts of genes, but those turned on only in males or only in females, and thus likely to be related to sex and reproduction, are especially likely to be lost.

"The lost male-active genes are likely used for aspects of sperm production or delivery, while many lost female-active genes encode proteins that regulate other genes. We speculate that many of the lost genes were needed for optimal mating, and may no longer be necessary."

What's Lost is Lost

Haag says that while the hermaphrodite life has worked well for C. elegans and C. briggsae to date, their self-sufficient sex lives could set them up for extinction.

"Others have shown that worms have a tendency to lose DNA in the lab, and our results indicate this is happening in nature, too. related to sex are especially likely to disappear. Once a sex-related gene is lost, it probably stays lost," Haag says. "Over time this reduces mating further, and each cycle the genome gets smaller. But if variation becomes important again, and they try to go back to mating, they can't do it well anymore. Self-fertilizing species go extinct faster than those that keep mating, and this may be why."

Haag says the study also shows that an organism's genome can change a great deal and not make a difference to its day-to-day survival. "It's often thought that the is like a machine that's been engineered, and that all the nuts and bolts are important and must stay in place or the organism stops working. This study is one of a number that show many of those nuts and bolts aren't necessary after all."

Explore further: Aging white lion euthanized at Ohio zoo

Related Stories

Mating that causes injuries

Feb 20, 2009

Researchers at Uppsala University can now show that what is good for one sex is not always good for the other sex. In fact, evolutionary conflicts between the two sexes cause characteristics and behaviors that are downright ...

Worms can evolve to survive intersex populations

Dec 06, 2011

(PhysOrg.com) -- 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) ...

Fortunately for men, size doesn't matter (much)

Jan 10, 2012

(PhysOrg.com) -- Researchers from The Australian National University have discovered that the male-specific Y-chromosome is shrinking – and it’s happening at different rates across species.

The sexual tug-of-war -- a genomic view

Mar 15, 2010

The genes that are most beneficial to males are the most disadvantageous for females, and vice versa. However, this genetic conflict between the sexes is important in maintaining genetic variation within a species, researchers ...

Sex chromosome evolution tracked in fruit fly

Jul 20, 2012

(Phys.org) -- Fruit flies are commonly used in genetics research because their lifespan is short, they are easy to breed in the laboratory, and mutants are widely available. There are about 1,500 known species. ...

Insect 'incest' signals an end to males

Jul 15, 2011

(PhysOrg.com) -- Evolution may lead to males disappearing as they are replaced by 'parasitic fathers' who infect their daughters at birth in order to mate with them.

Recommended for you

Ninety-eight new beetle species discovered in Indonesia

2 hours ago

Ninety-eight new species of the beetle genus Trigonopterus have been described from Java, Bali and other Indonesian islands. Museum scientists from Germany and their local counterparts used an innovative approa ...

A vegetarian carnivorous plant

Dec 19, 2014

Carnivorous plants catch and digest tiny animals in order and derive benefits for their nutrition. Interestingly the trend towards vegetarianism seems to overcome carnivorous plants as well. The aquatic carnivorous bladderwort, ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

Isaacsname
not rated yet Nov 06, 2012
That's a huge problem here in the Southern US.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.