Abstract
Verticillium dahliae is a cosmopolitan, soilborne fungus that causes a significant wilt disease on a wide variety of plant hosts including economically important crops, ornamentals, and timber species. Clonal expansion through asexual reproduction plays a vital role in recurring plant epidemics caused by this pathogen. The recent discovery of recombination between clonal lineages and preliminary investigations of the meiotic gene inventory of V. dahliae suggest that cryptic sex appears to be rare in this species. Here we expanded on previous findings on the sexual nature of V. dahliae. Only 1% of isolates in a global collection of 1120 phytopathogenic V. dahliae isolates contained the MAT1-1 idiomorph, whereas 99% contained MAT1-2. Nine unique multilocus microsatellite types comprised isolates of both mating types, eight of which were collected from the same substrate at the same time. Orthologs of 88 previously characterized sex-related genes from fungal model systems in the Ascoymycota were identified in the genome of V. dahliae, out of 93 genes investigated. Results of RT-PCR experiments using both mating types revealed that 10 arbitrarily chosen sex-related genes, including MAT1-1-1 and MAT1-2-1, were constitutively expressed in V. dahliae cultures grown under laboratory conditions. Ratios of non-synonymous (amino-acid altering) to synonymous (silent) substitutions in V. dahliae MAT1-1-1 and MAT1-2-1 sequences were indistinguishable from the ratios observed in the MAT genes of sexual fungi in the Pezizomycotina. Patterns consistent with strong purifying selection were also observed in 18 other arbitrarily chosen V. dahliae sex-related genes, relative to the patterns in orthologs from fungi with known sexual stages. This study builds upon recent findings from other laboratories and mounts further evidence for an ancestral or cryptic sexual stage in V. dahliae.
Highlights
Sexual reproduction is thought [1] to act as a mechanism to combine fit alleles from different individuals, and to break apart locally disadvantageous allele combinations under dynamic selection pressures [2]
Complete multilocus microsatellite types were generated for 941 isolates; all 12 MAT1-1 isolates had different MLMTs, whereas 410 different MLMTs were observed for MAT1-2 isolates
This phenomenon may be partly explained by clonal expansion of certain successful, highly fit genotypes which do not require sexual reproduction to complete the disease cycle [23,31], unlike some other plant pathogens
Summary
Sexual reproduction is thought [1] to act as a mechanism to combine fit alleles from different individuals, and to break apart locally disadvantageous allele combinations under dynamic selection pressures [2]. While sexual reproduction may in theory be costly and disrupt favorable gene combinations, experimental evidence has suggested that sex in fungi increases the rate of adaptation to new environments [3]. The formation of sexual structures and spores was the primary evidence of sex in fungi. The only documented sexual structures are formed on certain media and/or growth conditions in vitro [4,5]. Some putatively asexual plant pathogens have been found to sexually reproduce in nature only in specific ecological conditions and geographic locales, such as near the center of origin of the species [6]
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