Copyright 0 1988 by the Genetics Society of America Nuclear Migration and Mitochondrial Inheritance in the Mushroom Agaricus bitorquis William E. A. Hintz, James B. Anderson and Paul A. Horgen Mushroom Research Group, Center for Plant Biotechnology, Department of Botany, University of Toronto, Erindale Campus, Mississauga, Ontario, Canada L5L I C6 Manuscript received October 21, 1987 Revised copy accepted January 28, 1988 ABSTRACT Mitochondrial (mt) DNA restriction fragment length polymorphisms (RFLPs) were used as genetic markers for following mitochondrialinheritance in themushroom Agaricus bitorquis. In many basidiomycetes, bilateral nuclear migration between paired homokaryotic mycelia gives rise to two discrete dikaryons which have identical nuclei but different cytoplasms. Although nuclear migration is rare in A. bitorquis, unidirectional nuclear migration occurred when a nuclear donating strain (8- l), was paired with anuclear recipient strain (34-2). The dikaryonrecovered over the nuclear recipient mate (Dik D) contained nuclei from both parents but only mitochondria from the recipient mate; thus nuclei of 8-1, but not mitochondria, migrated through the resident hyphae of 34-2 following hyphal anastomosis. The two mitochondrial types present in a dikaryon recovered at the junction of the two cultures (Dik A) segregated during vegetative growth. Dikaryotic cells having the 34-2 mitochondrial type grew faster than cellswith the 8-1 mitochondrial type. Fruitbodies, derived from a mixed population of cells having the same nuclearcomponents but different cytoplasms, were chimeric for mitochondrial type. The transmission of mitochondria was biased in favor of the 8-1 type in the spore progeny of the chimeric fruitbody. Protoplasts of dikaryon (Dik D), which contained both nuclear types but only the 34-2 mitochondrial type, were regenerated and homokaryons containing the 8-1 nuclear type and the 34-2 mitochondrial type were recovered. HE field mushroom, Agaricus bitorquis, is heter- tion of nuclei occurs independently of any movement T othallic with mating types controlled by a single of organelles (CASSELTONand CONDIT 1972). Al- multiallelic incompatibility locus (RAPER1976). The though nuclear migration is rare in A. bitorpis, one fruitbodies produce four-spored basidia and single- stock (8-1) has been identified, which when paired spore isolates are homokaryotic with haploid nuclei. with a compatible mate (34-2), donates nuclei which A compatible mating of two homokaryotic isolates migrate unilaterally through the resident mycelium results in the presence of a raised line of dikaryotic of the recipient mate (ANDERSONet al. 1984). mycelium at the confluence of the paired cultures Extensive mt DNA restriction pattern heteroge- (RAPER and KAYE1978; ANDERSONet al. 1984). Clamp neity exists between isolates of A. bitorquis (HINTZet connections, often a feature of dikaryons within the al. 1985). The main purpose of this study was to Hymenomycetes, are absent in A. bitorpis. Dikaryosis develop RFLP markers to follow the transmission of can be confirmed visually with a microscope or by specific mitochondrial and nuclear types after hyphal fruitbody formation. Central to any study of breedinganastomosis, nuclear migration, fruitbody formation biology is the transmission of organellar genomes as and sporulation. We have shown thatthere is no well as the transmission of nuclear traits. We were movement of mitochondria associated with the mi- interested in determining the mitochondrial inherit- gration of nuclei following hyphal anastomosis. A ance pattern in crosses of A. bitorquis and assessing second purpose of this study was to resolve a dikar- the mitochondrial contribution to the phenotype of yon, of known nuclear and mitochondrial genotypes, vegetative cultures. into homokaryons, of predicted genotypes, by regen- In another basidiomycete, Coprinus cinereus, bipar- erating protoplasts. This allowed the reassociation of ental inheritance of mitochondria results from events the 34-2 mitochondrial genotype with the 8-1 nuclear directly following hyphal anastomosis in a pairing of genotype which resulted in a new homokaryon, with- homokaryons.Donor nuclei migrate bilaterally out sexual fusion of the nuclei in the dikaryon. through the established cells of each recipient mate, resulting in two discrete dikaryons, having identical MATERIALS AND METHODS pairs of nuclei but different cytoplasms (BAPTISTA- Growth and maintenance of strains: Working cultures FERREIRA,ECONOMOU and CASSELTON1983). Migra- of A. bitorquis (8-1, 34-2) were maintained on CYM agar Genetics 119: 35-41 (May, 1988). 36 W. E. A. Hintz, J. B. Anderson and P. A. Horgen (STEVENS1981) at 23". Mating was done by pairing two 1 mm' plugs of homokaryotic cultures at a distance of 1.0 cm on CYM agar plates (ANDERSONet al. 1984). Plugs of dikaryotic mycelium were removed from the interface and also distal to the interface of the two homokaryons and transferred to CYM agar plates. For fruiting trials sterile grain (1.0 g CaC0'; 100 g rye seed; 120 ml H20) (CHANG and HAYES1978) was inoculated with dikaryotic mycelium and incubated for 21 days at 23". The "spawn" was cased with water-saturated calcined earth (Turface, International Minerals and Chemical Corp. Mundelein, Illinois) mixed with, 5.0% (w/w) CaCOJ and 1.0% (w/w) Hydrogel (Viterra Planta-gel, Nepera Chemical Co., Harriman, Sew York) (SANA~TONIO 1971) and transferred to an incubator with 85% relative humidity at 21". Spore prints were prepared frommature fruiting bodies and single germlings were isolated following dilution plating of spores on CYM agar. Protoplasts were isolated from dikarvotic mycelium and homokaryons were recovered as described by CASTLE,HOR- GEN and ANDERSON(1987). Mycelium for DNA isolations was grown in CYM liquid medium in petri dishes with no agitation. FIGLKE1 .-Suclear migration in Agaricus. An A. bitorpi.\ nu- Source of genetic markers: For the generation of nuclear clear donating strain (X- l, Ipf)was paired with a sexually compatible DNA markers, A. bitorpis (8- 1) nuclear DNA was separated nuclear accepting strain (34-2, right) on CYM agar. Following from mt DKA by centrifugation in bis-benzimide (Hoescht hyphal anastomosis and nuclear migration, plugs of dikaryotic 33258, Sigma Chemical Co.) cesium chloridegradients mycelium were recovered from the junction of the two parental (HUDSPETHet al. 1980). Purified nuclear DNA was digested cultures (Dik A, open circle) and from the recipient mate (Dik D, with the restriction enzyme EcoRI (Pharmacia, Uppsala, closed circle). Sweden) and ligated to similarly digested bacterial plasmid vector pBR 325 (MANIATIS,FRITSCH and SAMBROOK1982). Several clones, containing 2.0-7.0 kilo-basepair (kbp) in- serts, were chosen at random for use as nuclear markers. r! Mitochondrial DNA markers were generated by cloning BamHI digested A. brunnescens mt DNA into the polylinker region of the bacterial plasmid vector pUC 18. The cloned mt DNAs used in this study were between 6.0 and 10.0 kbp in length. Restriction analysis: Whole-cell DNA was isolated from freeze-dried mycelium according tothe procedureof ZOLAN and PUKKILA(1986). DNA samples (ca. 5 pg) were digested with BamHI and subjected to electrophoresis on 15 X 18 cm, 0.7% agarose gels in TBE (89 mM Tris-HCI; 89 mM boric acid; 2.0 mM EDTA, pH 8.0) at 30 V for 16 hr. Bacteriophage lambda DNA, digested with both EcoRI and HindIII, was used as a size standard. The digested DNA I was transferred from the gel onto Genescreen Plus mem- brane(Dupont NEN Research Products, Boston, Massa- chusetts) by capillary blotting according to manufacturer's directions. Plasmid DNAs (ca. 1 kg) were labeled with [a- "PIdCTP (3000 Ci/mrnol; Dupont NEN) using the Be- thesda Research Laboratories (Gaithersburg, Maryland) nick-translation kit. On average, specific activity of 10' cpm/kg DNA was obtained. Hybridization and wash con- ditions were as described by HINTZet al. (1985). Autora- diography was performed using a Dupont NEN intensify- ing screen and Kodak XAR-5 film. ~I~LKCr.--~eg~egauor~ UI IIULIC~Idllu lllllucllulluIldl IIMI~.FIJ during fruiting. EamHI digested DSAs from A. biforguk parental homokaryotic strains 8-1, 34-2 (lanes 1 and2 respectively), the RESULTS dikaryon recovered at the junction of the homokaryons (Dik A, Two bitorpis lane 3), the dikaryon recovered over the nuclear recipient mate homokaryotic isolates of A. (8-1, 34- (Dik D, lane 4). homokaryotic spore progeny of Dik A (AISI, 2), having unique mt DNA restriction patterns, were AlS6, A2S5, A2S6; lanes 54,and homokaryotic spore progeny paired on CY M agar at a distance of 1 cm. After 2 1 of Dik D (D2S6, D2S7, D3S6,D3S7; lanes 9-12) were electropho- days of growth, fluffy mycelium, distinctive of dikar- resed on a 0.74 agarose gel, transferred to a Genescreen Plus yon formation, appeared at the junction of the two filter, and hybridized with radiolabeled nuclear DNA marker p8- In3 (A) or mt DNA marker p5Om9 (B). The sizes of strongly homokaryotic strains. During the next 14 days, the hybridizing fragments are indicated at the right of the figure zone ofdikaryotization migrated unilaterally over the (kbp). Weakly hybridizing fragments or partial digestion products resident mycelium of strain 34-2 (Figure 1). Plugs of ,L'PVP nnt ;ncl>nA~Ain the ~n~lwcic Mitochondrial Inheritance in Agaricus 37 TABLE 1 Genotypes of A. bitorquis strains Nuclear probes Mitochondrial probes Mating Strain p8- I n3 p8- 1n7 p8- 1 n33 p50m9 p5OrnlO p50m4b 8-1 a( A1 a(13.5)a(15.6)a(l1.813.7) a(5.0/4.4/4.1)a(17.1) 1 0.4)a 34-2 A2 b(17.6/13.2) b(5.5/4.7/3.8) b(8.4l1.7) b(4.6) b(19.2) b(18.2) b(19.2) b(4.6) b(8.4l1.7) b(5.5/4.7/3.8)b(17.6/13.2) A2 34-2 Dik A A1/A2 ab ab ab ab ab ab b a AlSl A2 b a a a a b b AIS6 AI b b a a a a a A2S5 A2 a a a a a A2S6 AI a a b b a a Dik D A l/A2 ab ab ab b b b b b D2S6 A2 b b b b b D2S7 AI b b a b b b D3S6 A2 b b b b b b D3S7 A1 a a b b b b Sizes of restriction fragment(s) (kbp) hybridizing to the probe in parentheses.