MJIC"'- In. 101 (6), 69&-704 (l99n PriotltJ i.. em" s,;,.i.. 695

Aflatoxin-producing potential of communities of Aspergillussection Flavi from cotton producing areas in the United States

PETER J. COTTY

Sow/htm Rtgifll1lll ~UlI"h Ctnltr, Agrinll/UTill Rtstlml1 xrvirt, Uni/d 514/n Dtparlnlt1l/ of Agrinll/wTt. Nrw Or/tIlM,

LDuisilll1ll, 70J79. U.S.A.

Communities of AsputillU$ ~on F1l1Vi rnident in soils planted with collon were com~ among ~erll ueu in the KNthemUnited St..tes. lnddmce of A flauus .and A tllmtlrii differed .among uus. A f/arNS inddmce inl;rustd with ttmptr.ltuce .andd~eutd with latitude. Less thin 1" of lsol..18 were A I'IDmiUl or A JMIroaititwS. A f/arNS isolates wtte usigntcf to eithn Ihe S or LslniN on the b.lsis of K1ttoti.al morphology. S stnin isolales produced n\ltntrous sm.aII « 400 IMfI) sdttOtU.; L stmn isolalesproduced fev.·er, luger scIerotiL The S stnin of A j/Ruus was found in ..n urn. Afl.atoxin-proclucing potenti.al of A flaVI'S differedAmOng ueu .and w.., cOrTel..ttd with S slnin incideflCf:. L slr..in isol..les produced only JJ% .I, much .lll.aloxfn 81 .as S slninisolate.. No S strain isol..le produced both afl..toxin 81 .and afultoxin GI• COrTel..tions indicated that L strain toxigenicity bul not Sstrain toxigenicity varied geographically. While toxigenicities of most isolalrs were stable through single conidial tr.ansfer. 18% ofisolates expressed allered levels of toxigenicity after tr.ansfer. The obsrtvrd differences unong communities m..y refled geogTaphiclsolalion .and/or ..d.pt..Hon. .anc:I m..y cause different vulner..bilitin to .n..toxin contamin..tion unong crop' pl.anted in diverselocations.

Aflatoxins ue toxic,. carcinogmic compounds produced byseveral members of Aspt:rgillus sedion FI;lvl during wedionof aopt both prior to uu:I after harvest (Cotty tI a1.. 1994). Inmost industrialized countries. aflatoxin content of foods mdfteds is minimized through regulations (Stoloff, Van Egmond& Park. 1991). Aflatoxin-producing fungi can be· grouped bymorphological and g~netlc crit~ria into thre~ readily dis­tinguishable s~cies; AjUuJus, A pllrasiticus and A nflmius

<Kurtzmm. Hom & Hesseltine, 1987; Klich & Pitt, 1988). OnIhe bUis of sclerotial morphology. A jUU11tS can be dividedinto th~ 5 and L strains (Colty. 1989).5 strain isol.tes producenumerous small sclerotia « 400 ~) and fewer conidi;l thanL strain isolates which produce fewer, Larger sclerotia..Although most 5 strain isol;ltes produce relatively high levelsof aflatoxins (Cotty. 1989). some produce only .f1atoxins 81

and B" while others also produce aflatoxins G 1 and G z (EgeLCotty &; Eli.as, 1994). L strain isolates produce only aflatoxinsBI and 8, or are atoxigenic. A pilrilsilind and A nomius

produce aflatoxins B., B,. G. and G, (Kurtzman tI al., 1987;Egel If 111.. 199~).

AsptrgillllS flilfJHS is oHen the most frequently isolatedaflatoxin-producing spedes (Shroeder & Boller, 1973; Lisker,Michaeli & Frank. 1993). Gene flow within this species islimited by a vegetative compatibility system (Papa, 1984;Bayman &; Colly, 1991). As such, localities are occupied bycommunities of A flmms vegetative compatibility groups(VCGs) Ihat do not I:-elong to the same genetic population.Both the Sand L strains are composed of many VCGs(Bayrmn &; Cotty. 1993) and individual fields may containover 50 VCGs (Bayman & Cotty, 1991). Part of the

nri.bility in &toxin-producing ability among A. flatJItSisol.tes can be explJined by evolved differences amongvegttiltive comp.tibility groups (lb,yman & Cotty. 1993).

Communities of AspagiflltS sedion F1avi vary in bothspedes composition and average afI;1toxin-producing ability(Hill tI ill., 1985; Usbr tf Ill.. 1993). Cropping sequences andculHv.ation pracHces may alter communitieS (Angle. Dunn &Wilgner. 1982; Schoeder & Boller. 1973). However, studiesare 1..eking which compare the composition of geographically~puate communities resident on similar substnt~s_ The Sslt.in occurs in tile United States (Cotty, 1989; Doster &

Miduilides. 1994). Thailand. (Saito tI al.. 1986). and Arnca(Hesseltine tI 111.. 1980) but little is known about theproportion of jsolates within A j1RfJItS communities belongingto this strain. A better understanding of geographicaldivergence and adaptation wilhin AsptTgillllS seelion Flavimay be useful in developing methods to reduce ;lflatoxinconl.miniltion by illtering the composition of A. flaVl1S

communities (Cotty, 1994 b). In the current study, communitiesof Asptrgillus section Flavi resident in agricultural soils plantedto cotton .....ere tx.mined in order to asseSS the extent towhich geographical divergence hu occuITt'd. Aflatoxin­producing polmtials of A. j7Rvws communities were comparedilnd rel.Jted to S slrilin incidence.

MATERIALS AND METHODS

Col/ution and ~u1ture 0/ isolates

Members of AsptrgilltlS seelion Flavi were isolated from soilcollected in agricultural fields planted 10 colton. Multiple

P. J. Cotty 699

+

Louisiana

0+0MR Pinal County

PimaCOWIty

MaricopaC~'Y

PR.J'

'00 Ion

'00 Ion

Fig_ t. kus in the United StOiles of Amtria wnpkd in lhe currmt study. YV. Y\lffIOl V..ney; GV. Gila Valley; DV. Dome V.Iley;MY. Mohawk V..ney; PRo Painted Rod:; MR. Maricopa Ra.d; WO. Wm Ddt..; CD. Cmtnl ~ltOl; At. A1OlbOlnu..

fields (2-5) within multiple areas (3-5) within two regions ofthe southern United States were sampled (fig. 1). The westernregion consisted of six desert areas (annual precipitation< 25 an) where temperature maxima frequently exceed40°C and cotton production is totally dependent on irrigation.typicaUy by furrow. Corrunercial cottonseed grown in thisrtgion is frequently contaminated withgreatertlwl 20 pg leg-I&toxin 81, The three most western ueas (Yuma Valley. Gil..V.J1Iey and Dome Valley) have the greatest inddence ofcontamination. The eastern region consisted of three areas(two in the Mississippi delta and one in coastal Alabama) withcooler temperature and annual precipitation in excess of100 em. These eastern areas have very low incidence ofafbtoxin contamination of cottonseed. Annual temperatureminima for each area were taken from the' United Sl"atesDepOirtment of Agriculture plant hardiness zone map (Cathey.1990).

For the initial sampling in 1989. a single composite samplewas taken from each field. The sample consisted of subsamplestaken from 10 locations distributed along a diagonal section ofthe field and spaced at least 10 m apart. Two to three scoopsUQ-20 g each) were taken from the top 5 an of soil at eachloc.ation.

A s«ond sampling was performed during 1993 in ordermore thoroughly to examine the incidence of the S stTain ofA. flavus within western region areas. Fi"e replicate compositesamples were taken at 20 m intervals along a diagonal sectionof each of 10 fields. Each replicate sample consisted of 5-10subsamples taken from the top 5 an of soil along a 3 m row.

Soil samples were mixed and isolates were cultured on amodified rose Bengal medium by dilution plate tWmique(Cotty. 199411'). Discrete colonies were rransferred from theisolation medium to 5/2 agu (5% V-8 vegetable juice. 2%

agar. pH 5'2; Cotty. 1989) J d after plating. After 1-10 dgrowth at 30°. agar plugs (3 mm diam.) of sporulating cultureswere submerged in sterile distilled water (5 mI in 20 mI Vial)and maintained at 80

• Three to seven separate dilution platingswere performed for each soil To remove bias from colonyselection. aU discrete colonies from a given dilution plate were<U!hH«!.

AflAtoxin production

AAatoxin production was assessed in the medium of Adye &:Matales (1964) with 3 g 1-1 of NH.SO. as the sole nitrogensource as previously described (Cotty &: Bayman. 1993).&Ienmeyer flasks (250 mI) containing 10 mI of medium wereinoculated with approximately 3'5 x lW spores. Flasks wereincubated in the dark for 5 d after which 10 mI ~tone wasadded to each flask to lyse fungal cells and extract aflatoxinsfrom the mycelium. After 1-4 h. solutions were passedthrough Whatman No. 4 paper and extracted twice with25 mI of methylene chloride. Extracts were filtered through abed of anhydrous sodium sulphate (about 40 g) to removewater. evaporated 10 dryness. dissolved in methylene chloride.and separated along with &toxin standards by tic (Cotty &:

&lyman. 1993). Extracts were either diluted or concentratedto permit accurate densitometry and aflatoxin B) was quantifiedon tic plates by scanning densitometry (Pons. Robertson &:

Goldblatt, 1966) and expressed as IJg per fermentation; thelimit of detection was 10 ng.

All isolates were initially assessed for aflatoxin productionand some were assessed again if they had been both negativefor aflatoxin production and had displayed the colouredcolony reverse typical of A. pll'rasih·cus and A. flavus onthe MPA <AsP"Si/l1U f/4truS and pll'rasilicus agar) medium

Communiti6 of AspngmW$ section Flavi 700

T~ble 1. Contributioflll of 1M S...d L ,traint to I..... a&toxin produdng pottnlw of AJi1rr1i'/"'!"'uw COlTlmUItiliQ rnicltnt in dilJrmlt uu, in 19&9

Field, lJOl,ln A !"'pus toxigmidty Incidence Str,in toxigll\icity Contribution toNO' "'" (~, wg Ubtom B,)" "'"'" by Ifn wg I&toxin B,Y commwVty loxig~

YV ,'" 10JAB L ,.~ .. ,,~

S ..~ m ,,~

GV , n. 196A L ,.~ " ,~

S 16~ "" ,,~

DV , 156 &9A8 L ,,~ ., JO~

S ..~ '" ,..PR 1 " HAB L ..~ " '"S ,,~ " ,,~

MR , 110 ". L .,~ " 16~

S ,o~ '01 ,.~

wo , 11) 142A8 L ..~ " ,,~

s ,,~ H' ..~CD 1 " '''A L ,,~ sa ,,~

S ,,~ '" ,,~

Be , 170 16AS L 97~ ,. 16~

S ,~ '" H~

• Sft Fig. I.• AVlf2le a&loxin B, productd by III A !"'_IsolataVllues Iftlvengn olAdd muns: those fo1Iowed by I common letta-Ift not Jignifiantiy diKerml

kCOrdin&: 10 Tuby', HSD tnt. In other c:oiurMs. difkrmcn IlTlOnIIfIU 1ft not slgnifiant by lNIyJis ol vlrilnce (P _ 0"05).

• AVCTiIgt a&loxin B, produced by oo1Itn usigned 10 lither man S or dnin L• Proportion of A j1IJws loxigenidty (footnotl b) Iltribuled to either t..... S or L 'Inin. - ($wn IIUtoxin B:"""'l!(Sum IIUtoxin 81"-"" x 100.

L!.!...--,,,,-_"DOVClI LJ:!.J I MR II DC WD II WD_1",2 .... "'_1_7"'..... _,,,,_7 -1210>'"

Area and mean minimum lempera~

fig. 1. frtqumcies of A flaUIIS (solid bars) And A tlf_rii (hitchedbars) among Asptrfillws section FllVi isolates from sevtral cottonproducing areu in the United States. X·axis nwnbel'1 indicate thelMuaI minimum temperature rmge (uthey. 1990) in degl"fft:

ulsius. Aru &bbrevtInons are expWned in fig. 1. V.Iun for aruCD differ significMltly (P - 0-05 by Tukey's HSO tnl) from vil1uesfor the other areas which do not differ from each other. WO had oneReid with minimum temperature - 12<1 to -9<1 And two fields-9" to -7".

RESULTS

were evaluated. Percentages were transformed (arcsin of th~

square root) prior to s!atistical analyses as recommended bySakal & Rohlf (1981).

­QI

of Pitt d Ql. (1983). Certain isolates were also .assessed foraflatoxin production after being transferred by single conidiwn.

Sped" tmd strain identijiCiltion

Isolates producing aflatoxin 81 but not Mlatoxin GI , mddisplaying a colony reverse colour reildion typical of A.. ftaDW$link on MPA were identified as Asptrgillus jlavus. Isolatesproducing sclerotia typical of the S stfilin (average diam..< 400 IIDV of A..f1atlW$ on 5/2 agar (Cotty, 1989) were as-­signed to the S strain. aJl other A ftaDW$ isolates were assignedto the L or typical strain. Isolates which produced both afIa·toxins 81 and GI in culture were grown on Cz.apek-Oox agarfor 30 d al 310 md examined for conidial morphology,scIerotiaJ shape, and colony colour. Isola!es with elongatesclerotia and relatively smooth conidia were consideredA. ltOt7Iius Kurtzman. 8. W. Hom & Hesselt (Kurtzman tt af..1987; Egel d aI.. 1994). Isolates with dark grem colonies mdrough conidia were considered A. parasih'cus Speare (Klich &Pitt. 1988). Assignment of several isolates 10 either A. nomiusor A. pamsiticus was confirmed with molecular criteria asreported elsewhtre (Egel tt af.. 1994). Isolates which apparentlydid not produce aflatoxins were grown on AfPA for 10 d at31<1; those which produced rough brown conidia and a browncolony reverse were considered A. tamarii Kita (Colty, 1994a).Those which produced smoolh conidia on 5/2 agar andcolonies typical of A. ftaDUS on AfPA wer~ consideredA. jlaDUS.

The most frequently isolated member of Asptrgillus sectionFlavi was A.. ftarnu. However, significant differences occurredamong the areas in the frequencies of both A. jlalJUS and A.tQmarii. A. tamari; occurred more frequenlly and A. jlalJUS less

Stati$tkal Qnalysnfrequently in the central delta (CO) than in any othtr area (Fig.

Statistical analyses were performed with Statistica\w version 2). Less than 2% of the examined Aspnrillus section F1avi4.5 and Excel version 4.0. Analysis of variance was used to isolates produced aflatoxin GI. No producer of aflatoxin GIlest differences among Ir~atments prior to application of was isolated from the five desert areas in Arizona (Yumilmultiple comparison techniques. For comparisons among VaJley, Gila Valley, Dome Valley, Painted Rode and Milricopillareas ITable I, Fig. 2), fields (2-5 per area) served as replicates Valley). However, aflatoxin GI producers comprised 3-5 % of

_____••nndd..>w.,.,.OJonlv..Jnc!llded jf more than 10 iso~I••'",.,"f",o~m",-'"h"o"m'--C,""d";~o~n"F~I.~v~;-,;",~1.~I~,~'~f<-,o~m~,,~,~.~,~;n~lh~'Cm~;d:d~I'~"'~U~I~h:,~m~U:::n~;l:'~d _

P. J. Cotty

T.Me 1.F~ oi AfJlt7li!l"f jJ.uw and the 5 main oi A flmtwswithin AspnJi/lwf Mdion fhvi COIM'UIitin tflldent n .griaI!NrII soils inwntan AriwN In 199J

Fwld

I MR 61 65 1C1. MR 64 69 6CJ MR S1 69 9C4 YV 66 91 4685 YV 66 96 6686 YV 15 99 1981 CV 69 100 6986 CV 58 61 6689 GV 59 96 698

10 MY 61 100 99A

• MR. MuKop.I ROM!; YV, YwnI V&1Iey; GV. Gila Vllley: MY. MobIwkValley.

• NWTlber oi Aspt"iIIW$ ~ion Flavi bolates: 10-15 isol.les for each offive replicates per field Were examined.

• Per«nt of Aspnrillws sedion Flavi ilOlates assigned 10 Af/AOWf:signiIiant IP - 0-(5) differ~ among fieldl were not dl'tectf'd _ding 10Tukey'l HSD tnt.

• Perunt oiA~ isolatltl belong\rIs 10 strain S. Vilun are .WI"lIge:s oifive replic:aln: thoR followed by • common Jetm are not IignificIntJydiffennt (P - 0<)5) attording 10 Tuhy'l HSD let.

T.ble J. Com'lation coeffidmll (r) and probacbiIines (I') fot rtlationshiplamons selected variab~1

Variable 1 Variable 1. , P

'lloAP.ows l..Iotitude -o-J1 0""'lloAP.uw MinimWTI lem~ralure .OS ....~AJlo- %It btmMii -.., ....~AJlo- A fLtow loxigmieity -." ...A fI-- loxigenlcily 'llo Af/llows in linin S ... ....A fl-- loxigenicity Shain 5 loxigmic:ily ." ....A fLrows toxigmiOly Shain l toxigmicily .,. ....A f/nwI toxigenicily Minimum lempmlNre -0-11 .OSAP.r1ld toxigmicity ulitude -00' ...Strain 5 toxigmicity Minimum temperalure -0-02 ..,Strain S loxigmidly l..Ioliiu<k .Il ."Strain l toxigmidly MinimWTI temperalure -0-26 .UStr.1n L toxigmicily Laliiu<k -0-61. 000''lloAf/ll11WSInStrainS Latitude 00' 0"179'llo A p.- in Strain S Minimum lemperaiw"e .JO 0"101'lloAJIor-inStninS Strain S toxigmiOly 00' .,,,....... Minimum ImIflUalure ... ............. Latitude ... ...,_....

MininMn lemper.hau ... ....States (Baldwin County. Western Delta and Central Delta).This difference between the two regions was significant(P = 0·05 by analysis of variance).

The S strain of A. flauus was found in all areas studied(Table 1) and. during 1989. there were no significant differences(P = 0'093) among areas in the incidence of such isolates.However, within Arizona. there was a significant correlation(r = 0'64; P = 0'013) between longitude andS strain incidencewhich was attributable to the low incidena of S stnin isolateswithin the Maricopa Road area and the high incidena inwestern Arizona. More rigorous sampling in 1993 confirmedthis difference (Table 2).

The average amount of aflatoxins produced by A. flavus

701

isolates (average toxigenicity) differed significantly (P = 0'05)among areu with less aflatoxin being produced by isolates!Tom the Mmcopa Road area than !Tom both the CentralDelta and Gila VaUey areu (Table 1). Average toxigenicitywas significantly correlated with both S strain incidence and 5strain toxigenicity, and, with a lower level of confidence (P =0·09), with L strain toxigenicity (Table 3). Toxigenicity wasnot correlated with either minimum temperature, latitude. orthe percent of Asptrgillus st'ction Flavi isolates aSSigned to thespecies A. flat/14S (% A. flDuws).

The two stnins of A.. flat/14S diR"ered markedly in aRatoxi­genicity. Forty per ant (239 of 597) of strain L isolatesproduced less than 0'5 IJg aflatoxin BI (Fig. 3). A similarnumber (237) of L strain isolates produced over 10 IJgaflatoxin BI . In contrast. very few S strain isolates (7 of 355)produced. less than O'S IJg aflatoxin BI • and 90% producedover 10 IJg aflatoxin B). Individual strain toxigenicities did notdiffer among areas (Table 1) and were not correlated withminimum temperature (Table 3). However, L strain toxi­genidty, but not S strain toxigenidty, was negativelycorrelated with latitude. Incidence of A. flat/14S was inverselycorrelated with latitude and incidence of A. tarruuii, anddirectly correlated with minimum temperature (Table 3).

Although toxigenicities of 72 % of isolates were stablethrough single conidial transfer, aflatoxin.producing ability ofmany isolates changed (Table 4). Eight of 84 isolates thatproduced less than 0'5 IJg aflatoxin BI in two independentfermentations prior to single conidial transfer produced morethan 10 IJg aflatoxin Bl after transfer. In other tests, twoindependently dmved single conidial transfers were comparedfrom each of 18 isolates but none differed in aflatoxin­producing category (Table .). Five isolates were alsotnnsferred by Single conidium serially three to six timeswithout alteration in aflatoxin-producing category.

DISCUSSION

Over 95 % of isolates within Asptrgi/lus section Flavi wereassigned to A. f/R'Ows and A. tamDrii in the current study. Inprevious studies, section Flavi communities in Texas (Schroeder&; Boller. 1973) and California (Doster &; Michailides. 1994)were dominated by A. {lAvus and A.. lamDrii with A. lamRrii aminor component (0-11 %), similar to that observed through­out Arizona and Alabama in the current study. In contrast.A. lamari; was dominant in the Central Delta area; this wasthe coolest area examined and temperature may confer aselective advantage to A. tamDrii.

Aflatoxin-producing potentials of fungal communities maysignificantly influence risk of aflatoxin contamination(Schroeder &; Boller, 1973; Cotty II al.• 1994). Averagetoxigenidty, a measure of community potential to produceaflatoxins, dilfen.d significantly among areas (Table 1). even in

Arizona. the region with the greatest frequency and severity ofcottonseed contamination (Simpson ri at 1913). Industryobservations indicate western Arizona has an incidence andseverity of aflatoxin contamination much higher than theMaricopa Road area. Thus, anecdotal evidence suggestsassociation between toxigenicity of A. {lavlu communities andcrop contamination. However, A. j1Rvus communities in the

Communities of A5JNrgillus section Flavi 702

BCS

CDS

WDS

MR

S

PRS

DV

S

o+-!:::!-+~~:::+-f-l::::'-+-~~~-':::!--+-';:!-+-':::!-+-n ~ ~ M ~ ~ rn OC n ~

LLLLLLLLSS

20

100

Arta o(ori,m and strain

Fig. 3. Incidence of 5 and l strm isolates of AspngiUus {laUN$ that producm various quanHHn of U1atoxin 81 in culhue, 0, less tNno-S LIS; a O'S-10 LIS; • 10-100 LIS; ., ov~ 100 LIS. YV. Yuma V.alIey; GV, Gill. V.alIey; DV, Dome V.alIey; MY, MohawkValley; PRo PJ.inted Rock; MR. Maricopl. Road; WO, West Delil.; CO. Centr;aJ Deltl.; Al Alabama.

Table 4. InAuma: 01 ""ale conidial tTamfn- Gel th.t qu;antity 01 aB..toxin 8, produced by iso4tn 01 Asprr,iIlws /1Ifl1N$ with vl.lYin3 InitiIJ aB..toxin­-- Presmt i50llotn prodlKing v&rious l{\Wltities ofaB..!oxin 8, .after 5in$le spore mmfer f".4Jlniti;aJ qu;antity 01

;a&toxin B.

"'..... O'SII«""IOI11 101lg-1001II >loolIg

<O'SlJg &4 &2 1 & 2o-S lIg-l.0 iii 14 64 IS 21 010 lIg-lllO IJg & 0 lS 6J U1001lg< 39 0 10 21 69

Albloxin qIlMltilin are the totall.fT\O\1lll produced in 10 mI of the medium of AlIre" MI.taln (J964) durintl • s d ftrmMtllion.1 32-. All memben of theinitial < o-s 11& group failed 10 produce detect.ble levels of afl.,toJdN (limit cl dritdion 0-5 Pg) in two independent tntl prior to single spore lranIl1:r.

central south~ United States, where aflatoxin contaminationof cottonseed is low. were not necessarily less toxigenic thancommunities in Arizona. The low incidence of contaminationin the emtr&! south6Tl United States probl.bly results from acooler, more humid environment.

The current study found the S strain of A. flalJus widelydistributed in cotlon producing areas. Average toxigenicity ofS strain isolates and the percenta.ge of A flJn1US isolatesbelonging to strain S were correlated with average toxigenicityof the A. flaIJus community. These correlations reflect the highaflatoxin·producing potential of most S strain isolates (Fig. 3)which,. contrasting with frequent low aflatoxin production byl strain isolates. may indicate differential adaptation toselective inAuence of ecological niches. Aflatoxin researcherstend to overlook the common highly toxigenic S strainisolates (Cotty tI lit 1994).

Fungi in A5ptrgi11115 section Flavi are more abundant inwarmer regions (Wicklow. Wilson & Nelso!\- 1993; Manabe& Tsul'\lta. 1978) and in the current study, the percent ofsection Flavi isolates identified as A. f/aous was highlycorrelated with minimum temperature and inversely correlatedwith latitude. Although overall A. j1aOII5 toxigenicity was

correlated with neither temperature nor latitude, averagetoxigenicity of A. [la1JU5 strain L decreased with latitude andthis correlation indicates divergence among communities indifferent areas. However, forces driving geographical di­vergence are unclear. The temperature and latitudinalcorrelations both may primarily reflect divergence of thedesert region and Central Delta communities. Previous workerscorrelated A f/aous toxigenicity with minimum temperature<Wicklow & Cole, 1982; Manabe & Tsuruta. 1978). butstudied a far wider range of latitudes and minimumtemperatures than the current study.

Correlations bdw~ rainE&!1 and A5pugillus communityparameters were not evaluated in the CUJTent study due toconfounding influences of irrigation. The desert regions havevery low rainfall with the Yuma. Gila and Dome Valleys,receiving less than 7 an annually. Therefore, currently onlyfungal communities within agricultural fields were studiedwhere crops are flood irrigated at an annual rate that mayexceed 200 em.

Production of aflatoxins BI and C I in culture a.nd in cropsalso indicates geographical divergence among communities ofAspergillus section Flavi. Only 1'5% of the section Flavi

P. J. Cotty

isolates examined produced both aflatoxins B, and G, and inno area did more than 5% produce both toxins. Thus,A. parasificus and A. nomius occurred at low frequencies similarto those previously observed in Israel and Texas aoffe, 1969;Shroeder &. Boller, 1973; Lisker d al., 1993). Higher incidencesof A. parasilicus are occasionally observed (Angle d al., 1982;Doster &. Michailides, 1994). In studies spanning two decades,A. parasiUcus comprised 10-50% of section Flavi communitiesin several crops in south Georgia (Lillard, Hanlin &. Lillard,1970; Hill tI al., 1985; Hom tf al., 1994). Geographicaldivergence among section Flavi communities may explaindiscrepancies among studies on the relative importance ofA. parasificus to aflatoxin contamination (Lisker tI al., 1993).Indeed, incidences of aflatoxins B, and G1 in foods importedinto Japan (Maeda, 1990) and Taiwan (Tseng, 1994) stronglysuggest geographical divergence.

Differential distribution of S strain isolates with differenttoxigenicities also indicates geographical divergence. All Sstrain isolates from North America produce only aflatoxins B1

and B, (current study; Hesseltine tt ,1I., 1970; Cotty, 1989;Doster &. Michailedes, 1994), whereas isolates from Africa andThailand also produce aflatoxins G1 and G t (Hesseltine tf al.,1970; Saito tI al.. 1986). Geographical divergence withinAspergillus section Flavi may indicate limited migration and/orfungal adaptive differences.

AAatoxin.producing ability is often variable in culture(Kale, Bhatnagar &. Bennett 1994). In the current study mostisolates consistently produced either relatively high orrelatively low aflatoxin concentrations. However, eight (10%)isolates that produced less than 0'5 lJg in two successivefennentaHons subsequently produced over 10 lJg after singleconidial transfer. This change in phenoty-pe may have resultedfrom eliminating a cytoplasmic factor (Schmidt tI al., 1983), bydevelopment of a homokaryon from a heterokaryotic culture,by transposon-mediated inactivation of an aflatoxin bio­synthesis gene (Lemke, Davis &. Creech. 1989), or bypurifying mixed cultures (Lemke ef al., 1989). Conicliogenesisapparently influences isolate toxigenicity differentially andcertain isolates vary continuously through serial single conidialtransfers (Lemke tI al., 1989). Thus, although aflatoxin­producing potential is conserved within certain vegetativecompatibility groups (Bayman &. Cotty, 1993). this stability oftoxigenicity does not extend through all groups.

The author thanks Mrs Darlene Downey for technicalassistance.

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Angle. I. S. DuN'\, K. A. &: Wlgner. G. H. (1982). Effect 01 cultuul practices

on the wil populations of Asptrgill..s {IilVI<! &nd AsTN'Sifl..s pe",..ili"". S<>ilSci"'ct Soddy DI Amm'll jDH7'IUlf 46, 301-304,

Baymal\, P. &: Colly. P.I. (1991). Vegetative compatibility &Jld geneti(

diversity in the Aspcrgill..s {Iilt'l<! population 01 a single field. C~naai~n

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