Sexual reproduction only second choice for powdery mildew

Jul 14, 2013
This shows powdery mildew on young wheat plants. Credit: UZH

Powdery mildew is one of the most dreaded plant diseases: The parasitic fungus afflicts crops such as wheat and barley and is responsible for large harvest shortfalls every year. Beat Keller and Thomas Wicker, both plant biologists from the University of Zurich, and their team have been analyzing the genetic material of wheat mildew varieties from Switzerland, England and Israel while the team headed by Paul Schulze-Lefert at the Max Planck Institute for Plant Breeding Research in Cologne studies the genetic material of barley mildew.

The results recently published in Nature Genetics and PNAS respectively unveil a long shared history of co-evolution between the host and the pest and the unexpected success of asexually produced mildew offspring. Moreover, the data provides fresh insights into the crop history of wheat and barley and their interaction with the mildew pathogen.

Asexually produced offspring more successful

Like other fungi, mildew reproduces in two ways: Sexually, where the is recombined, and asexually, where the offspring and the mother fungus are genetically identical. The researchers now demonstrate that the success of the two reproduction methods could not be more different: "Mildew fungi detected on afflicted host plants have only successfully reproduced sexually every few centuries, primarily reproducing asexually instead," explains Wicker.

This baffling fact has more deep-rooted causes: In order to infect the , the mildew fungus needs to be able to successfully disable the plant's defense mechanisms – the parasite has to be perfectly adapted to its host. "In a parasite-host situation, new combinations of genetic material are a disadvantage for the parasite as the adaptation to the host and its defense mechanisms deteriorates as a result." Genetically identical offspring of successful mildew fungi that have already been able to infect the host plant, however, have the ideal genetic prerequisites to be able to attack a host themselves. According to Schulze-Lefert, wheat and barley mildew offspring from asexual reproduction are normally more successful than their sexually reproduced counterparts. Asexual reproduction as a success model seems to be characteristic of many parasitic fungi, including those that afflict humans, such as athlete's foot.

Sex still worthwhile

Based on the gene analyses, the scientists were also able to prove that mildew already lived parasitically on the ancestral form of wheat 10,000 years ago, before wheat were actually domesticated as crops. None of the subsequent genetic changes in the crops due to breeding or spontaneous mutations was ever able to keep the mildew fungus away from wheat in the longer term. And this is precisely where the advantage of sexual reproduction lies and why the usually unsuccessful sexual reproduction cycle is still worthwhile for the mildew fungus: Wheat and mildew are embroiled in a permanent evolutionary arms race. "If wheat improves its defense mechanisms against the parasites, the fungus has to be able to follow suit or it has lost," explains Wicker. "That's only possible by recombining the genetic material; in other words, sexual reproduction."

Evidently, a sexual exchange and mixtures of the genetic material of different mildew varieties have occurred several times in the course of the millennia, giving rise to new mildew varieties that were able to attack new sorts of wheat. The scientists suspect that the grain trade in the ancient world was partly responsible for the emergence of new mildew varieties.

Explore further: Parasitic worm genomes: largest-ever dataset released

More information: Thomas Wicker et al. The wheat powdery mildew genome shows the unique evolution of an obligate biotroph. Nature Genetics. July 14, 2013. doi:10.1038/ng.2704

Stéphane Hacquard et al. Mosaic genome structure of the barley powdery mildew pathogen and conservation of transcriptional programs in divergent hosts. PNAS. May 21, 2013. doi:10.1073/pnas130607110

Related Stories

Powdery mildew at an evolutionary dead end

Dec 09, 2010

The size of a genome tells us nothing about the comprehensiveness of the genetic information it contains. The genome of powdery mildew, which can destroy entire harvests with its fine fungal threads, is a ...

Fertility or powdery mildew resistance?

Nov 12, 2010

Powdery mildew is a fungus that infects both crop and ornamental plants. Each year, powdery mildew and other plant pathogens cause immense crop loss. Despite decades of intense research, little is known of the plant molecules ...

Sex: it's a good thing, evolutionarily speaking

May 30, 2012

(Phys.org) -- Sure, sex may be fun, but it’s a lot of work, and the payoff is by no means certain. Scientists have speculated for a long time on why all living things don’t simply make like amoebas ...

Recommended for you

Parasitic worm genomes: largest-ever dataset released

6 hours ago

The largest collection of helminth genomic data ever assembled has been published in the new, open-access WormBase-ParaSite. Developed jointly by EMBL-EBI and the Wellcome Trust Sanger Institute, this new ...

Bitter food but good medicine from cucumber genetics

Nov 27, 2014

High-tech genomics and traditional Chinese medicine come together as researchers identify the genes responsible for the intense bitter taste of wild cucumbers. Taming this bitterness made cucumber, pumpkin ...

New button mushroom varieties need better protection

Nov 27, 2014

A working group has recently been formed to work on a better protection of button mushroom varieties. It's activities are firstly directed to generate consensus among the spawn/breeding companies to consider ...

Cataloguing 10 million human gut microbial genes

Nov 25, 2014

Over the past several years, research on bacteria in the digestive tract (gut microbiome) has confirmed the major role they play in our health. An international consortium, in which INRA participates, has developed the most ...

User comments : 0

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.