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Eukaryotic Cell, November 2009, p. 1732-1738, Vol. 8, No. 11
1535-9778/09/$08.00+0     doi:10.1128/EC.00135-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Horizontal Chromosome Transfer, a Mechanism for the Evolution and Differentiation of a Plant-Pathogenic Fungus{triangledown} ,{dagger}

Yasunori Akagi,1 Hajime Akamatsu,1,{ddagger} Hiroshi Otani,1 and Motoichiro Kodama1,2*

Laboratory of Plant Pathology,1 Fungus/Mushroom Resource and Research Center, Faculty of Agriculture, Tottori University, 4-101 Koyama-Minami, Tottori 680-8553, Japan2

Received 12 May 2009/ Accepted 3 September 2009

The tomato pathotype of Alternaria alternata produces host-specific AAL toxin and causes Alternaria stem canker on tomato. A polyketide synthetase (PKS) gene, ALT1, which is involved in AAL toxin biosynthesis, resides on a 1.0-Mb conditionally dispensable chromosome (CDC) found only in the pathogenic and AAL toxin-producing strains. Genomic sequences of ALT1 and another PKS gene, both of which reside on the CDC in the tomato pathotype strains, were compared to those of tomato pathotype strains collected worldwide. This revealed that the sequences of both CDC genes were identical among five A. alternata tomato pathotype strains having different geographical origins. On the other hand, the sequences of other genes located on chromosomes other than the CDC are not identical in each strain, indicating that the origin of the CDC might be different from that of other chromosomes in the tomato pathotype. Telomere fingerprinting and restriction fragment length polymorphism analyses of the A. alternata strains also indicated that the CDCs in the tomato pathotype strains were identical, although the genetic backgrounds of the strains differed. A hybrid strain between two different pathotypes was shown to harbor the CDCs derived from both parental strains with an expanded range of pathogenicity, indicating that CDCs can be transmitted from one strain to another and stably maintained in the new genome. We propose a hypothesis whereby the ability to produce AAL toxin and to infect a plant could potentially be distributed among A. alternata strains by horizontal transfer of an entire pathogenicity chromosome. This could provide a possible mechanism by which new pathogens arise in nature.

* Corresponding author. Mailing address: Laboratory of Plant Pathology, Faculty of Agriculture, Tottori University, 4-101 Koyama-Minami, Tottori 680-8553, Japan. Phone and fax: 81-857-31-5364. E-mail: mk{at}muses.tottori-u.ac.jp

{triangledown} Published ahead of print on 11 September 2009.

{dagger} Supplemental material for this article may be found at http://ec.asm.org/.

{ddagger} Present address: Biological Resources Division, Japan International Research Center for Agricultural Sciences, 1-1 Ohwashi, Tsukuba, Ibaraki, 305-8686 Japan.

Eukaryotic Cell, November 2009, p. 1732-1738, Vol. 8, No. 11
1535-9778/09/$08.00+0     doi:10.1128/EC.00135-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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