Originally published as Genetics Published Articles Ahead of Print on June 4, 2006.

Genetics, Vol. 173, 1991-2004, August 2006, Copyright © 2006
doi:10.1534/genetics.106.057562

A Nonself Recognition Gene Complex in Neurospora crassa

Cristina O. Micali1 and Myron L. Smith2

Biology Department, Carleton University, Ottawa, Ontario K1S 5B6, Canada

2 Corresponding author: Biology Department, Carleton University, 1125 Colonel By Dr., Ottawa, Ontario K1S 5B6, Canada.
E-mail: mysmith{at}ccs.carleton.ca

Nonself recognition is exemplified in the fungal kingdom by the regulation of cell fusion events between genetically different individuals (heterokaryosis). The het-6 locus is one of ~10 loci that control heterokaryon incompatibility during vegetative growth of N. crassa. Previously, it was found that het-6-associated incompatibility in Oak Ridge (OR) strains involves two contiguous genes, het-6 and un-24. The OR allele of either gene causes "strong" incompatibility (cell death) when transformed into Panama (PA)-background strains. Several remarkable features of the locus include the nature of these incompatibility genes (het-6 is a member of a repetitive gene family and un-24 also encodes the large subunit of ribonucleotide reductase) and the observation that un-24 and het-6 are in severe linkage disequilibrium. Here, we identify "weak" (slow, aberrant growth) incompatibility activities by un-24PA and het-6PA when transformed separately into OR strains, whereas together they exhibit an additive, strong effect. We synthesized strains with the new allelic combinations un-24PA het-6OR and un-24OR het-6PA, which are not found in nature. These strains grow normally and have distinct nonself recognition capabilities but may have reduced fitness. Comparing the Oak Ridge and Panama het-6 regions revealed a paracentric inversion, the architecture of which provides insights into the evolution of the un-24–het-6 gene complex.




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