A paper suggesting ancient asexual fungi have more than one genome

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January 14, 2005 Previous | Archive | Next

Fungus report stirs debate A paper suggesting ancient asexual fungi

have more than one genome reignites controversy | By Q Choi

An ancient group of fungi might possess more than one genome, Swiss

researchers report in the January 13 issue of Nature.

Arbuscular mycorrhizal fungi colonize the roots of most land plants.

Their cells contain hundreds of nuclei, leading Mohamed Hijri and

Ian R. of the University of Lausanne in Switzerland to

suggest multiple genomes could evolve within single individuals.

" These fungi have no sexual reproduction, which means if deleterious

mutations accumulated, they might lead to extinction. But these

fungi are extremely old, dated at 450 million years, " Hijri

said. " We think they developed this strategy of multiple genomes

against deleterious mutations, such that some genes can get knocked

out, but other genomes might have functional versions. "

" This means Mendelian genetics and classical evolution cannot apply

to these organisms. They are completely unique, " Hijri told The

Scientist.

The report has reignited a longstanding debate. " I think this data

is not particularly believable. I'm a little worried they might have

presented an experimental artifact or misinterpretation of results, "

said Pawlowska of Cornell University in Ithaca, NY, who did

not participate in this study.

In 2001, Hijri and colleagues reported polymorphism of ribosomal DNA

in spores of fungi as evidence for their idea. But in 2004,

Pawlowska and at the University of California at

Berkeley suggested the variation was due to polyploidy. They focused

on a POL1-like sequence (PLS1) in the arbuscular mycorrhizal fungus

Glomus etunicatum, whose spores contain 13 variants of PLS1.

Pawlowska and 's mathematical model, based on the random

inheritance of nuclei to each clonally produced offspring, predicted

that if each variant existed in genetically different nuclei, then

the loss of some variants would almost certainly occur after one

generation. Instead, they found clonally produced spores possessed

all 13 variants, suggesting polyploidy was behind the variation.

In the latest paper, Hijri and colleagues investigated the

polyploidy question. Using flow cytometry, they measured the nuclear

DNA content of G. etunicatum as 37.45 megabases (Mb). If G.

etunicatum nuclei were 13 N, the genome size of this fungus would be

2.88 Mb, much smaller than any other eukaryote and smaller than that

of Escherichia coli and most other bacteria, the researchers argue.

Using real-time polymerase chain reaction, the investigators then

estimated the number of copies of PLS1 was 1.88 per nucleus. Given a

maximum of two copies per nucleus, and colleagues conclude

the 13 variants must be spread out in different nuclei, meaning

there is heterokaryosis, or genetic differences among the nuclei.

Pawlowska told The Scientist she disagrees with the new paper,

citing her prior work, which, among other data, in the supplementary

section noted that two isolates of the fungi from different

locations, " one from Minnesota and the other from California, had

both a complement of 13 variants of a genetic marker. According to

their argument, if you expect all the nuclei to be different, you

would expect to find different numbers of variants in different

isolates due to loss and acquisition of different types of nuclei. "

Soren Rosendahl of the University of Copenhagen, who did not

participate in either study, said both opposing groups did excellent

work. " It's clear they are using different methods, " he told The

Scientist. In his team's investigation of arbuscular mycorrhizal

fungi G. mosseae, G. geosporum, and G. caledonium, which analyzed

genetic variations based on expressed DNA instead of genomic DNA as

Hijri and colleagues did, " we never saw this heterokaryosis. But

then again, we used a different method and different organisms, so

that doesn't resolve the problem. "

Pawlowska plans on repeating Hijri and colleagues' experiment in her

own lab soon. " The fact this paper is published makes the whole

question more visible and hopefully will draw more researchers to

apply different methods and tools to address this problem, " she said.

Links for this article

M. Hijri, I.R. , " Low gene copy number shows that arbuscular

mycorrhizal fungi inherit genetically different nuclei, " Nature,

433:160-3, January 13, 2005.

http://www.nature.com

Mohamed Hijri

http://www.unil.ch/dee/page7251_fr.html

Pawlowska

http://ppathw3.cals.cornell.edu/People/labs/Pawlowska/Index.html

G. Kuhn et al., " Evidence for the evolution of multiple genomes in

arbuscular mycorrhizal fungi. " Nature, 414:745-8, December 13, 2001.

[PubMed Abstract]

T.E. Pawlowska, J.W. , " Organization of genetic variation in

individuals of arbuscular mycorrhizal fungi, " Nature, 427:733-7,

February 19, 2004.

[PubMed Abstract]

Soren Rosendahl

http://www.bi.ku.dk/staff/staff-vip-details.asp?ID=33

E.H. Stukenbrock, S. Rosendahl, " Development and amplification of

multiple co-dominant genetic markers from single spores of

arbuscular mycorrhizal fungi by nested multiplex PCR, " Fungal Genet

Biol, 42:73-80, January 2004.

[PubMed Abstract]