Alternation of nuclear phase in the filamentous basidiomycete, Helicobasidium mompa

Abstract

Variation in the number of nuclei and cellular ploidy were observed in eight strains of Helicobasidium mompa. The basidiospores, single-spore isolates and field-isolated strains were all dikaryons. The cellular ploidy, which was assessed by analyzing the fluorescence emitted by DAPI-stained nuclei, was unstable: monokaryotic strains derived from the original dikaryotic strains by successive subcultures were mainly tetraploid, although the original dikaryon was in most cases diploid. On the other hand, a dikaryotic strain derived by treatment with benomyl was haploid. These results suggest that diploid dikaryon is a normal nuclear phase of H. mompa in nature, and the alternation of ploidy may be due to a feature of the mating system of this fungus.Key Words--cellular ploidy; cytology; fluorescent microscopy; haploidaization; Helicobasidium.Mating between compatible monokaryons of the tetrapolar filamentous basidiomycete Coprinus cinereus (Schaeffer ex Fries) S. F. Gray involves dikaryotization and clamp-cell formation, followed by development of fruiting bodies, meiosis and basidiospore formation. The number of nuclei and their ploidy are generally stable during mycelial growth, except during development of fruiting bodies and meiosis (Wong and Gruen, 1977; Casselton and Olesnicky; 1998; KQes, 2000), though subculturing the bipolar filamentous basidiomycete Pholiota nameko (T. Ito) S. Ito et Imai frequently induces dedikaryotization of the dikaryotic mycelium (Masuda et al., 1995; Kumata et al., 1995). Moreover, strains containing both diploid and haploid nuclei have been reported for Cryptococcus neoformans (Sanfelice) Vuillemin, a bipolar basidiomycete yeast, and shifts in cellular ploidy occur widely (White and Tacobson., 1985;Tanaka et al 1999; Hata et al., 2000). These phenomena suggest that the number of nuclei of dikariotic mycelium in bipolar basidiomycetes is less stable than in tetrapolar basidiomycetes, regardless of whether the mating type locus is a multiple allele. This may be because certain mating-type genes associated with nuclear migration, present in tetrapolar basidiomycetes, are lacking in bipolar basidiomycetes (Casselton and Olesnicky 1998; KLies, 2000).The filamentous basidiomycete Helicobasidium mompa Tanaka is a commercially important, soil-borne, root pathogen affecting a wide range of plant species, tt is the causal agent of purple root rot disease, and host plants infected by the fungus quickly wither and die. In Japan, this fungal disease, which spreads rapidly and is ~Corresponding author. very difficult to prevent, has done great damage to commercially grown grape vines and apple and pear trees. Unfortunately, despite its importance, little is known about the cytology and genetics of this fungus.Investigation of the cellular ploidy of H. mompa is very important for understanding its pathogenesis and mating system. Pulsed-field electrophoresis is a good tool for differentiating chromosomes among strains, especially with respect to molecular size (Zolan and Pukkila, 1995), but it r...

Transmissibility of viral double-stranded RNA between strains of the violet root rot fungus Helicobasidium mompa and the potential for viral dsRNA infection to this fungus using monokaryotic strains

“…Because the RDRP gene is indispensable in viral replication (Ghabrial 1998), we used strain V70 as a dsRNA donor and investigated the transmission of dsRNA-1 between strains of H. mompa. Karyotypes of two strains, V1M and V3M, were already reported as monokaryotic (Aimi et al 2001), and we reconfirmed that these strains were monokaryotic before the experiment.…”

“…We speculate this is due to the monokaryotic state of H. mompa. Aimi et al (2001) reported that the diploid dikaryon seems to be a normal nuclear phase of H. mompa in nature. They also attributed monokaryotization in H. mompa to the fusion of two nuclei and the unstableness of cellular ploidy in this fungus to either a lack of or a defect in mating-type factors.…”

“…Although not all the strains of H. mompa have unstable cellular ploidy (Aimi et al 2001), we expect unstable strains to be able to infect different individuals of H. mompa with dsRNA elements via hyphal anastomosis. Although combi- nations of fungal strains may be limited, we consider that monokaryotic strains of H. mompa will be utilized as an effective tool for dsRNA infection.…”

“…Although viral dsRNA is transmitted within the same fungal strain or between closely related strains, for the present we consider that the viral dsRNA infection method using hyphal anastomosis is likely the most effective for H. mompa. Thus far, mycelial incompatibility (Katsumata et al 1996;Akahira et al 2000) and alterations in karyotype (Aimi et al 2001) have been reported as cytological characteristics in H. mompa. In this report, we first conducted pairing experiments to determine whether the mycelial incompatibility of H. mompa would prevent dsRNA transmission between strains.…”

Horizontal transmission and host-virulence attenuation of totivirus in violet root rot fungus Helicobasidium mompa

“…Although the two monokaryotic strains were thought to be the same or closely related (Suzaki et al 2003), HmTV1-17 virus seemed to be transmitted from these monokaryotic strains into different MCGs randomly (Table 2). Aimi et al (2001) reported that the karyotype of H. mompa was variable and that monokaryotic strains of H. mompa resulted from nuclear fusion in dikaryotic strains. Mating-type factors of basidiomycetous fungi were associated with maintenance of the dikaryotic phase (Casselton and Olesnicky 1998), and Raper and Raper (1973) reported that defects in mating-type factors in the basidiomycete Schizophyllum commune affected self-nonself recognition between the fungal isolates.…”