Structure, Function, and Phylogeny of the Mating Locus in the Rhizopus oryzae Complex

Department of Biology, Duke University, Durham, North Carolina, United States of America.
PLoS ONE (Impact Factor: 3.23). 12/2010; 5(12):e15273. DOI: 10.1371/journal.pone.0015273
Source: PubMed


The Rhizopus oryzae species complex is a group of zygomycete fungi that are common, cosmopolitan saprotrophs. Some strains are used beneficially for production of Asian fermented foods but they can also act as opportunistic human pathogens. Although R. oryzae reportedly has a heterothallic (+/-) mating system, most strains have not been observed to undergo sexual reproduction and the genetic structure of its mating locus has not been characterized. Here we report on the mating behavior and genetic structure of the mating locus for 54 isolates of the R. oryzae complex. All 54 strains have a mating locus similar in overall organization to Phycomyces blakesleeanus and Mucor circinelloides (Mucoromycotina, Zygomycota). In all of these fungi, the minus (-) allele features the SexM high mobility group (HMG) gene flanked by an RNA helicase gene and a TP transporter gene (TPT). Within the R. oryzae complex, the plus (+) mating allele includes an inserted region that codes for a BTB/POZ domain gene and the SexP HMG gene. Phylogenetic analyses of multiple genes, including the mating loci (HMG, TPT, RNA helicase), ITS1-5.8S-ITS2 rDNA, RPB2, and LDH genes, identified two distinct groups of strains. These correspond to previously described sibling species R. oryzae sensu stricto and R. delemar. Within each species, discordant gene phylogenies among multiple loci suggest an outcrossing population structure. The hypothesis of random-mating is also supported by a 50:50 ratio of plus and minus mating types in both cryptic species. When crossed with tester strains of the opposite mating type, most isolates of R. delemar failed to produce zygospores, while isolates of R. oryzae produced sterile zygospores. In spite of the reluctance of most strains to mate in vitro, the conserved sex locus structure and evidence for outcrossing suggest that a normal sexual cycle occurs in both species.

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    • "It is worth mentioning that an ANK-containing gene has been also identified in the MAT idiomorph of Rhizopus oryzae, a species belonging to a basal group of fungi, the Mucorales [63]. However, this R. oryzae gene contains two additional domains (BTB and RCC1) that are not present in T. indicum Ti_orf3, suggesting that these two ANK-containing genes are not orthologs. "
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    • ") is currently hampered by the lack of appropriate single-copy markers. The biological species recognition concept (Mayr 1982) is difficult to apply in Mucorales because germination of zygospores has been observed in only a few species due to the lengthy period of dormancy and the particular conditions needed for every species (Gryganskyi et al. 2010). Therefore, the production of zygospores has been used instead as a single criterion (e.g. "
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    • "The numbers at branches correspond to bootstrap support values generated with 1,000 bootstrap replicates. As previously described [21,56], internal nodes are not statistically supported due to high evolutionary rates of mating type proteins [57], leading to poorly supported topologies. However, the R. irregularis sequences s12765 and mucoralean SexP proteins fall into the same clade, while other Rhizophagus spp. "
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