Cnrrlp. Biochrf~ P/~y.sio/.Vol. 75A. No. 3, pp. 313-324. 1983 0300-9629’83 $3.00 + 0.00 Printed in Great Britain 0 1983 Pergamon Press Ltd
MINIREVIEW
CELLULOSE DIGESTION IN INSECTS
bh’f~AEL M. MARTIN Division of Biological Sciences, University of Michigan, Ann Arbor. MI 48109, USA
Abstract- I. Cellulose digestion hns been demonstrated in the Thysanura (Lepismatidae), Orthoptera (Cryptocercidae, Blattidae). Isoptera (Mastotermitidae. Kalotermitidae. Hodotermitidae, Rhinotermit- dae. Termitidae). Coleoptera (Buprestidae. Anobiidae. Scarabaeidae, Cerambycidae), and Hymenoptera (Siricidne). 2. In all but the scarab beetles. cellulose digestion is brought about by a complex of three types of enzymes (C,-cellulases, C,-cellulases, and cellobiases). as in fungi. 3. Many insects are able to synthesize their own C,-cellulases and cellobiases, but few (if any) can synthesi7e C,-cellulases. 4. Insects compensate for their inability to synthesize C,-cellulases by exploiting the cellulolytlc potcntlal of protozoa. bacteria. or fungi. 5. The maintenance of permanent populations of hindgut protozoa. the maintenance of permanent populations of hindgut bacteria, and the ingestion of fungal cellulases are described as three distinct mechanisms by which insects have been shown to use the cellulolytic potential of microorganisms. 6. A process in which ingested cellulolytic bacteria proliferate in one region of the gut at the expense of ingested cellulose. only to be digested and assimilated in a more posterior section. is a fourth possible mechanism by which insects might accomplish the digestion of cellulose with the help of micro- organisms.
INSECTS THAT DIGEST CELLULOSE & Kasting, 1969; Burton et cd., 1977; Dixit & Mall. 1978; Mall rt N/., 1978). McGinnis & Kasting (1969) The ability of many insects to thrive on wood, foliage and detritus has naturally stimulated investi- conclusively demonstrated the inability of the pale gations of the extent to which such species are able to western cutworm, Ayrotis urthqmiu, to digest cellu- digest the structural polysaccharides in their food. lose by observing that less than 0.5”; of the carbon-14 from ingested labelled cellulose was respired in the Species that have been shown to possess a capacity to form of “%IO,. Although wood-feeders dominate the digest cellulose are listed in Table I. Based upon the list of cellulose digesters, there are many species of values of the approximate digestibilities or assimi- insects which feed upon bark, phloem tissue. and lation eficiencies that have been determined for a few wood that are unable to digest the cellulose they con- species, it appears that termites are more efficient sume. Beetles from the families Bostrychidae, Curcu- cellulose digesters than the wood-boring beetles and lionidae, Lyctidae and Scolytidae exemplify such spe- siricid wood wasps. The silverfish, Ctenolepisn~u cies (Parkin. 1940: Chararas, 1979). lineatrr. and the American cockroach, PtvQplmwtu mwricm~~, are omnivorous species that readily in- clude both cellulosic and non-cellulosic items in their THE CELLULOLYTIC ENZYMES OF diets. The rest of the species listed in Table I are INSECT GUT FLUIDS wood-feeders. Foliage and detritus-feeders are conspi- cuously absent. No evidence for significant digestion Cellulose digestion in insects is generally ac- of cellulose by foliage-feeding Orthoptera, Coleoptera complished by a collection of enzymes believed to or Lepidoptera or by detritus-feeding Plecoptera. be similar to the ones responsible for cellulolytic Coleoptera, Diptera or Trichoptera has been ability in fungi (Table 2). The “cellulase complex” in- reported. although low levels of hydrolytic activity cludes three major classes of hydrolytic enzymes: toward cellulose powder have been detected in the gut endoglucanases (C,-cellulases), cellobiohydrolases fluids of a few species of locusts (Evans & Payne, (C,-cellulases), and /3-glucosidases (cellobiases) (Reese 1964; Morgan, 1975a, 1976). The grasshopper, Me/u- & Mandels, 1971; Wood & McCrae. 1979; Chose er noplus hi~~ittutus. is able to degrade hypocotl cell walls (II., 198 I). The digestion of native cellulose is believed of bean seedlings, but there is no evidence to indicate to be initiated when endoglucanases attack isolated that cellulose is among the cell wall constituents amorphous regions of the predominantly crystalline digested (Talmadge & Albersheim. 1969). The absence cellulose matrix, creating nick sites in the linear cellu- of any enzymatic activity toward cellulose powder in lose chains. The cellobiohydrolases attack at the nick the digestive fluids of a number of Lepidopteran lar- sites liberating cellobiose, exposing additional poten- vae has been noted (Shinoda. 1930; Babers & Woke, tial sites for attack by the endoglucanase, and gener- 1937; Mathur. 1966: Chattoraj & Mall, 1969; Khan ally disrupting the highly ordered structure of the 314 MKHAEL M. MARTIN
ccll~tiose aggregates. The continued combined action cedures, gut fluids from a number of cellulose digest- of the C,- and C,-cellulases results in the eventual ing insects have been found to exhibit activity attribu- complete degr~ld~~tion of the original cellulose and the table to C,-ceilulases, C,-cellulases. and cellobiases. production of cellobiose and a mixture of soluble One notable exception is the scarab beetle, Or!r.fc.s linear oligosaccharides of varying chain lengths. The nusicornis. in which no soluble cellulases of any kind cellobiose. which is a potential inhibitor of the C,- have been detected. In this insect, cellulose digestion and C,-cellulases. is hydrolyzed to glucose by cello- is thought to be brought about by hindgut bacteria biase. and the various oligosaccharides are further (Bayon & Mathelin. 1980). The cellulase complex of degraded. ultimately to glucose. by the action of both bacteria consists of only two principal components. the C,-cellulases and the cellobiases. Thus. the utiliz- endoglucdnases or C,-cellulases and /I-glucosidases or ation of cellulose is dependent upon the concerted cellobiases. Ceilobiohydro~ases or CT,-cellulascs have and syi~erg~sti~ action of three types of enzymes. If been demonstrated in only a few species of bacteria any one of the three is missing. cellulose digestion (Ghose ct trl.. 1981). Furthermore. bacteria produce c’mnot‘ occur. cell-bound as well as extracellular enzymes. The fail- The presence of a complete cellulase complex in an ure to detect any cellulases in the gut fluids of 0. insect’s digestive juices is indicated by the capacity of nasicorrlis could be explained if cellulose digestion is the ~LI( fluid or a cell-free extract of gut contents to accomplished by the cell-bound enzymes of bacteria liberate reducing sugars from. or to solubilize such that are attached to the fragments of plant tissue forms of crystalline cellulose as, Avicell@. filter paper present in the insect’s hindgut, and not by soluble or cotton, The presence of the entire cellulase enzymes presented in the gut fluids. This species pro- complex is generally taken as evidence for the pres- vides an emphatic reminder that the Murc to detect ence of enzymes comparable to the C,-cellulases of celluiolytic activity in an insect’s digestive fluids is not fungi, that is enzymes specific for crystalline cellulose. compelling evidence against the digestion of dietary It is now common practice to refer to such enzyme cellulose. especially in species with abundant popula- from insect gut iluids as C,-cellulases, although to tions of bacteria housed in enlarged segments of the date none has actually been shown to be a cellobio- gut. hydrolase. Digestive C,-cellulases are conveniently detected in insects by assaying gut fluids for the ca- THE SECRETION OF THE ENZYMES pacity to degrade carboxyrnethylceiluiose (CMC) or OF THE CELLULASE COMPLEX some form of swollen. dimorphous. or reprecipit~~ted BY INSECTS ccllulosc. Cellobiasc activity is readily assayed by measuring the capacity of gut fluid to hydrolyze cello- Table 3 is a comprehensive tabulation of insect spe- biose to glucose. cies in which C,-cellulase activity has been demon- When subjected to the appropriate assay pro- strated in the gut fluids by assays using CMC or a
Table I. Cellulose digesting insects
Order Evidence for Family capacity to Species digest cellulose References
AD U-871, “C. EN, NU Lasker & Giese (1956)
EN. NU Cleveland clt aI. (1934)
14C Bignell (1977)
EN Veivers et d. ( 198I )
AD (74-91) Selfert & Becker (1965) EN Mishra (1979)
AD (X2), EN Trager (1932);Hungate (I 938)
14C Mauldin et ai. (1972) AD (78-89) Seiferf & Becker (1965) AD (9L), ‘%I. EN, NU Trager (1932);Esenther & Kirk (1W4); Mauldin (1977) AD (96-993, EN, NU Seifcrt & Becker (1965); Orlova (1974) EN, NU Orlova (1974); Yamaoka & Nngatani (1975)
EN Martin 81 Martin (1978. 1979) EN Abo-Khatwa (1978) AD (91-97) Selfert & Becker (I 965) EN Potts & Hewitt (1973, 1974a,b) Cellulose digestion in insects 315
Table l-conrinurd
Order Evidence for Family capacity to Species digest cellulose References
EN Rivnay ( 1945) EN Schlottke (1945)
AD (33), EN Parkin (1940); Spiller (1951) AD (31) Miller ( 1934) EN Parkin (1940) Parkin (1940) :“D (49). EN Norman ( 1936); Parkin (I 940)
AD (68). 14C Riissler (1961); Bayon & Mathelin (1980) EN Soo Hoo & Dudrinski (1967)
EN Schlottke (1945) EN Ivanovic & Barbie (1966) EN Ripper (1930); Miiller (1934) EN Schlottke (1945); Chararas & Libois (1976) AD (33) Miller (1934) AD (11-21) Falck ( 1930): Becker ( 1942) AD (33) Miiller (1934) AD (14-47) Mansour & Mansour-Bek (1934a) lvanovic & Barbie (1966) :“D (49). EN Miiller (1934) EN Parkm (1940) EN Schlottke (I 945); Ivanovic & Barbie (1966) EN Ripper (1930); Miller (1934) EN Deschamps (1944); Schlottke (1945) EN Parkin ( 1940); Schlottke ( 1945) EN Schlottke (1945) EN Parkin ( 1940) AD (30-S7), EN Mishra & Singh (197X) EN Mansour & Mansour-Bek (1937) EN Parkin (1940)
EN Kukor & Martin (1983) AD (22) Miiller (1934) AD (31) Miller (1934)
Abbreviations: AD. cellulose digestion demonstrated by comparing the cellulose contents of food and frass; the number in the parenthesis is the approximate digestibility of cellulose; 14C. cellulose digestion demonstrated by noting the production of 14C0, or the incorporation of 14C into tissues following ingestion of U-‘4C-cellulose; EN, gut fluid demonstrated to possess the enrymatx capacity to degrade filter paper, cotton, Avice@ or some other form of crystalline cellulose: NU. ability to digest cellulose inferred from capacity to survive on a diet of pure cellulose.
suitable form of amorphous cellulose as the test sub- and that the rather restricted occurrence of the ability strate. It is a long list that includes not only familiar to digest cellulose in insects is not due to the restric- cellulose-digesting species, such as termites, wood ted distribution of this class of enzymes. roaches and cerambycid beetles, but also many spe- Cellobiase activity has also been detected in the gut cles from groups which are not thought to be capable fluids of a diverse array of insect species, both diges- of assimilating cellulose. In a number of the investiga- ters and non-digesters of cellulose, and has been dem- tions summarized in Table 3, C,-cellulase activity was onstrated in extracts of salivary glands or midgut found to be present in extracts of salivary glands and tissues from 2 species of locusts, 6 termites. one pyr- midgut tissues, indicating that the enzymes were pro- rhocorid bug. the larvae of one sciarid fly, and even duced by the insects and not by microbial symbionts from silkworms (Table 4). P-Glucosidase activity has residing in the gut. C,-cellulases of insect origin have been detected in the gut fluids, gut tissues, and sali- been demonstrated in 17 species of roaches, 8 ter- vary glands of many additional species, and it is very mites. and 31 aphids. To be sure, in many of the probable that cellobiose would be hydrolyzed by the species listed, C,-cellulase activity is low may be due digestive fluids of many of these species as well. Thus, to enzymes which normally exert their catalytic action the presence of enzymes able to hydrolyze cellobiose on non-cellulosic poly- or oligosaccharides. Nonethe- are of common occurrence in insects, and the inability less, it seems clear that the presence of enzymes with of most insects to digest cellulose cannot be attributed C,-cellulase activity is not unusual in insect gut fluids, to the narrow distribution of the requisite cellobiases. 316 MICHAEL M. MARTIN
Table 2. Enzymes of celiulose digestion in fungi and probably also in insects
Enzyme (Alternate designations) Mode of action and products Substrates
The cellulase complex A combination of the three categories Microcrystalline cellulose of enzymes designated below, which powder, Avicell@, cotton, brings about the complete digestion and filter paper. of native cellulose to glucose. 1.4@+Glucan 4-glucanohydrolase Random attack on & 1,4-glucosidic CMC and other soluble (EC 3.2.1.4) bonds, generating transient derivatives of cellulose, (Endo-b-I.4-Glucanase) cellodextrins, cellobiose, and phosphoric acid swollen (Endoglucanase) glucose. cellulose. and cellodextrins (Carboxymethylcellulase) (increasing activity with (CMCase) increasing chain length). lC,-Cellulase) No activity toward crystalline cellulose. Hardly any activity toward cellobiose. 1.4-~-l~-Glu~an cellobiohydrol~~ Removal of cellobiose units from Microcrystalline cellulose (EC 3.1.1.91) the non-reducing end of a linear powder, Avicell@, cotton, (Cellobiohydrolase) chain by attack on penultimate swollen cellulose, and cellodex- (C,-cellulase) P-I ,4-glucosidic bonds. trins (increasing activity with increasing chain length). Limited activity toward CMC.
1.4~[j-u-Glucoside 4-glucohydrolase Hydrolysis of the p-1,4- Cellobiose, other p-linked (EC 3.2.1.21) glucosidic bond of cellobiose disaccharides of glucose, (~-r~-Glucosidase) to generate glucose. and cellodextrins. No (Cellohiase) activity toward cellulose.
Table 3. Insects with digestive C,-cellulases
Insect Order Capacity Identified sources Family to digest of C,-cellulases in Species cellulose digestive fluids References
High SG, HGP Trager (1932); Wharton & Wharton (1965)
IJnknown SG Wharton & Wharton (1965) Moderate SG, GB Wharton & Wharton (1965); Cruden & Markovetz (1979) Unknown SG Wharton & Wharton (f965) Unknown SC Wharton & Wharton (1965)
Unknown SC Wharton & Wharton (1965) Unknown SC Wharton & Wharton (1965) Unknown SG Wharton & Wharton (1965) Unknown SG Wharton & Wharton (1965) Unknown SC Wharton & Wharton (1965) Unknown SC Wharton & Wharton (1965) Unknown SG, GB Wharton & Wharton (1965); Cruden & Markovetz (1979) Unknown SG Wharton & Wharton (1965) Unknown SG Wharton & Wharton (1965) Unknown SG Wharton & Wharton (I 965) Unknown SC Wharton & Wharton (1965) Unknown SG Wharton & Wharton (1965)
Limited or none Unknown (WA) Morgan (I 976) Limited or none Unknown (WA) Talmadge & Albersheim (1969) Limited or none Unknown (WA) Evans & Payne (1964)
Presumably high SG, MGT. HGP Veivers et al. (I 98 I)
High SG, MGT Mishra (19X0) Cellulose digestion in insects 317
Table 3-continued
Insect Order Capacity Identified sources Family to digest of C,-cellulases in Species celluiose digestive fluids References
Hodotermjtidae ~(~dat~r~n~s tt~ossa~i~icus High MGT Botha & Hewitt (1979) Rhinotermitidae Coptotertnes iucteus Presumably high MGT, HGP O’Brien et rtl. (1979) Reticulitermes speratus High SG, HGP Yokoe (1964): Yamaoka & Nagatani (1975) R. hesperus Presumably high HGB Thayer (1978) Zooterrnopsis sp, High HGP Yamin Br Trager (1979) Termitidae Mucroterrnes natalensis Presumably high SC, MGT, IF?- Martin & Martin (1978, 1979) M. suhhyulinus Presumably high MGT, IFT Abo-Khatwa (1978) Microcrmtermes edentatus Presumably high Unknown Kovoor (1970) Nusutitermrs exitiosus High MGT O’Brien et ul. (1979) Terrws ohesus Presumably high Unknown Misra & Ranganathan (1954) Trirtewitermes trineruoides Presumably high MGT Potts & Hewitt (1973, 1974a,b) PLECOPTERA Pteronarcyidae Alforxrrcys proteus Limited or none Unknown (WA) Sinsahaugh et al. (1981) Pteronarcps cali@72ica Limited or none Unknown (WA) Martin et rtf. (1981b) P. pictetei Limited or none Unknown (WA) Martin et ut. (198lb) PSOCOPTERA Pseudocaeciliidae Ps~l~daca~cilius e~l~tus Unknown Unknown Sinha & Srivastava (1970) H EMIPTERA Pentatomidae Puiomrna angulosa Unknown Unknown Hori (1975) Eurydema ruyosum Unknown Unknown Hori (1975) Coreidae Coreus murgimzus Unknown Unknown Hori (1975) HOMOPTERA Aphididae Acyrthosiphon carayenae Unknown SC Adams & Drew (1965) A. pisum Unknown Adams & Drew (1965) Aphis ,ftihtrr Unknown :: Adams & Drew (1965) A. hrliarlthi Unknown Adams & Drew i1965j A. poali Unknown :: Adams & Drew (1965) Aultrwrthum solani Unknown SG Adams & Drew (1965) Betuluphis quadrituherculata Unknown SG Adams & Drew (1965) Calaphis (‘?) hetu[uecolens Unknown SC Adams & Drew (1965) Ductwotus cirsii Unknown SG Adams & Drew (196.5) D. russeilae Unknown SG Adams & Drew (1965) D. rarclsuci Unknown SG Adams & Drew (1965) D. sp. Unknown SG Adams & Drew (1965) ~ri(~so~~u ~u~I~~~~I~~I Unknown SG Adams & Drew (1965) ~ydap~ljs.~~efliculj Unknown SC Adams & Drew (1965) ~~~~cr~~sip~~u~rca~i~rnicun~ Unknown SC Adams & Drew (1965) M. euphorhiu Unknown SC Adams & Drew (1965) M. ptericoteus Unknown SG Adams & Drew (1965) Metopoiophium dirhodum Unknown SC Adams & Drew (1965) Myzocuflis waishii Unknown SG Adams & Drew (1965) Myrus cerusi Unknown SC Adams & Drew (I 965) M. persicae Unknown SG Adams & Drew (I 965) Nearctupkis hakeri Unknown SG Adams & Drew (I 965) Neomyzus circurnjfe.ws Unknown SG Adams & Drew (1965) Pentutrichopus thomasi Unknown SC Adams & Drew (I 965) Periphylus lyropictus Unknown SG Adams & Drew (1965) P. wyundinus Unknown SC Adams & Drew (I 965) Prociphiltrs Ieselata Unknown SG Adams & Drew (I 965) Pterocnmma hicolor Unknown SG Adams & Drew (1965) Rhopaiosiphum sp. Unknown SC Adams & Drew (1965) R. padi Unknown SC Adams & Drew (1965) R. cerusifoliae Unknown SC Adams & Drew (1965) COLEOPTERA Scarahaeidae S~~ric~,~t~lis~e~iIj~?afa Unknown Unknown Soo Hoo & Dudzinski (1967) Cer~lmbycidae Ergutes fahrr Presumably moderate Unknown Chararas ( 1979) 31x MICHAEL M. MARTIN
Insect Order Capacity Identified sources Filmily to digest of C,-cellulases in Species cellulose digestive Ruids References
Presumably moderate Unknown Chararas (1979) Presumably moderate Unknown Chararas (1979) Moderate Unknown Mishm & Sin& (1978)
Limited or none Unknown Chararas (1979) Limited or none Unknown Chnraras (1979) Limited or none Unknown I hararas (1979) Limited or none Unknown Charxras ( 1979)
Limited or none Unknown Ctiarar;ts (1979) Limited or none Ilnknown C’h;traras (1979) Limited or none Unknown Chararas ( 1979) Limlted or none Unknown Chararas (1979) Limited or none Unknown Chararas (1979) Limited or none Unknown Churaras ( 1979) Limited or none iinknown Chararas ((979) Limited or none Unknown Chararas ( 1979) Limited or none Unknown Chxtras (1979)
Limited or none Unknown Monk (1976)
Limited or none Unknown (WA) Bjxnov (1972) Limited or none Unknown Monk (I 976) Limited or none [Jnknown Monk (1976) Limited or none Unknown (WA) B.iarnov ( 1972)
Limited or none Unknown (WA) Bjxrnov (i977)
Limited or none Unknown (WA) Bjiuxov (1972)
Limited or none Unknown Bjarnov [ 1972) Limited or none Unknown Monk (1976) Limited or none Unknown Monk (1976)
Limited or none Unknown Bjxnov ( 1972)
Limited or none Unknown Martin e[ rri. ( 1%I a) Limited or none Unknown Martin rr al. ( I% I a)
Limited or none Unknown (WA)
Limited or none Unknown
Limited or none Unknown
Modemte IFT Kukor & Martin (19X3)
Unknown Unknown Ishaaya & Plant (1974)
Abhrcviations: WA. wc~tk activity; GB, gut bacteria; HGB. hindgut bacterin; HGP. hindgut proto- ~0;); IFT. ingested fungal tissue: MGT. midgut tissue; SC. salivary glands.
As indicated in Table 1, the gut tluids of a number be present. In contrast to the numerous reports of of insects. including the silverfish, the wood roach, C,-cellulase and cellobiose production by insects. several termites, and quite a few beetles. are able to data suggesting that insects are able to secrete their effect the degradation of microcrystafline cellulose. own C,-cellulases exist for only three species. and Thcrcfore, C,-cellulases or comparable enzymes must even in these the evidence is not completely unam- Table 4. Insects with digestive cellobiases
Insect Capacity Identified sources Order to digest of cellobiases in Family cellulose digestive fluids References -
Moderate Unknown Lasker & Giese (I 956)
Unknown Unknown Fisk & Rao ( 1964)
Limited or none MGT Morgan (f975b) Limited or none SG. NGT Evans & Payne ( 1964)
Presumably high SG Veivers PI (il. ( I98 I )
High MGT Mishra (1980)
Presumably high HGP McEwen er trl. ( 1980)
Presumably high MGT Abo-Khatwa (197X) Presumably high Unknown Kovoor (1970) High Probably MGT McEwen ef tti. ( 1980) Presumably high Probably MGT McEwen et ul. ( 1980) Presumably high MGT Potts & Hewitt (1973)
Unknown Unknown Sinha & Srivastava (1970)
Unknown Unknown Takanona & Hori (1974)
Unknown SG Khan & Ford (1967)
Unknown Unknown Nielsen ( 1962) Unknown Unknown Nielsen ( 1962)
Unknown Unknown Nielsen ( 1962)
Unknown Unknown Soo Hoo & Dudzinski (1967)
Limited or none Unknown Bjarnov (I 972) Limited or none tinknown Bjarnov ( 1972)
Limited or none Unknown Bjarnov (1972) Limited or none Unknown Bjarnov (1972) Limited or none Unknown Bjarbov (I 972) Limited or none Unknown Bjarnov (I 972)
Limited or none Unknown Bjarnov ( 1972)
Limited or none Unknown Bjarnov 1972)
Limited or none Unknown Bjarnov 1972)
Limited or none Unknown Bjarnov 1972)
Limited or none SC Mukaiyama (1961)
Limited or none Unknown Bjarnov (1972) Limited or none Unknown Bjarnov (1972)
Limited or none MGT Ferreira & Terra (1980)
Unknown Unknown Schulze & Ehrhardt (1963)
Abbreviations: HGP, hindgut protozoa; MGT. midgut tissue: SG. salivary glands.
319 320 MICHAEL M. MARTIN biguous and compelling. The silverfish, Ctermlepi.sw~ produce soluble C,-cellulases or of bacteria which lillcwtrr. is the best candidate for a genuine digest cellulose without the necessary secretion of C,-cellulase producer. Silverfish have simple guts with such enzymes. no enlarged segments or blind sacs which might func- tion as fermentation chambers. Lasker & Giese (1956) CONTRIBLITIONS OF PROTOZOA. BACTERIA were unable to culture any cellulolytic bacteria from AND FUNGI TO CELL1 LOSE DIGESTION these insects. and were also able to demonstrate effi- IN IhSECTS cient cellulose digestion in nymphs which were pre- sumed to be symbiont free, having been reared from Three distinctly different types of insect-microbial surface sterilized eggs on a diet of sterile, dried rolled interactions have been shown to serve as mechanisms oats. Lasker and Giese were unable to culture any by which insects digest cellulose using the cellulolytic bacteria from the guts of their presumed symbiont- potential of microorganisms. Termites and wood free nymphs, but they did not perform total direct roaches maintain permanent populations of celluloly- counts of bacteria in the gut. Thus the possibility is tic protozoa in their hindguts. Scarab beetles and the not absolutely ruled out that the presumed symbiont- American cockroach house permanent populations of free specimens still contained cellulolytic microbes of bacteria in their hindguts. presumably including cellu- ;I type which could not be isolated as viable colonies lolvtic strains. The fungus-growing termites and the using the culturing methods employed. Also. the diet slrlcid wood wasps culture cellulolytic fungi and of sterile. rolled oats was assumed to be free of cellu- ingest cellulolytic enzymes when they consume their lolytic microbes and enzymes, although that assump- symbiont along with their food. A fourth mechanism tion appears not to have been tested experimentally. which has not yet been demonstrated in insects, but T~LIS, while the case for C,-cellulase secretion by the which may prove to be important in certain species, is midgut of Crer~olrpis~nu linecctcc is quite convincing, it the rapid proliferation in the gut of cellulolytic bac- falls just short of being absolutely air-tight. teria ingested along with a cellulosic substrate. fol- Potts & Hewitt (1973. 1974a.b) have suggested that lowed by the digestion of the bacteria in a more pos- the midgut of the higher termite, Trirwraitermes tri- terior section of the alimentary tract. Endosymbiotic mw~~idrs. secretes a single enzyme with both C,- and bacteria or yeasts, housed in specialized cells (myceto- C,-cellulase activity. This proposal rests upon the cytes) or organs (mycetomes). represents a fifth type in claim that the C,- and C,-cellulolytic activity present insect-microbial interaction which is quite wide- in ;I chromatographic fraction obtained during the spread. It was once thought that such symbionts purification of a homogenate of worker abdomens might also be involved in cellulose digestion, but no was due to a single enzyme, the same C,-enzyme that evidence in support of that idea has ever been had previously been detected in an extract of midgut presented. The significance of such intracellular sym- tissue. bionts lies instead in their provision of B-vitamins. Veivcrs c’r ~1. (1981) detected low levels of sterols. and essential amino acids. C,-cellulase activity in an extract of the salivary The obligatory dependence of the lower termites glands of the primitive termite, Mu.stoterrnrs dtrrwi- and of the bvood roach upon hindgut protozoa for r1ic~~5i.s.Only IO”,, of the total C,-activity assayed in cellulose digestion has been widely recognized since this termite was present in the salivary extract, how- the classic investigations of Cleveland (1924). Cleve- ever. Most of the activity (73:“) was in the hindgut, land CJ~u/. (1934), Trager ( I931) and Hungate ( 1938. which houses an abundant protozoan population. 1943) and has been thoroughly reviewed (Honigberg. The proposal that the C,-cellulase activity in the sali- 1970; O’Brien & Slaytor 1982: Breznak. 1982). The vary extract is due to enzymes secreted by the ter- protozoan symbionts are anaerobic species from mitts rests upon the assumption that no contami- unique genera of oxymonad. trichomonad and hyper- nation of the salivary gland preparations occurred mastigote flagellates restricted to the four families of during dissection. lower termites and to the wood roach family. Trager It is often stated that the wood-boring anobiid and (1932) demonstrated that cellulolytic enzymes. includ- cerambycid beetles secrete all of their own cellulases. ing C,-cellulases. were produced by intestinal flagel- While there is no evidence to preclude this possibility, lates present in the roach. C. p~rnc~t~/rrr~r.\.and in two neither is there any to support it. The assumption is species of termites, R. /Itrripc~.\ and T. cl,lc/lr.sric,ol/i.\. based entirely upon the lack of any correlation For many years it \\as not certain whether the bctucen a capacity to digest cellulose and the pres- enzymes were actually produced by the protozoa or ence of intracellular symbionts (Mansour & Man- by bacteria invariably present in the protoplasm of sour-Bek. 1934b; Parkin. 1940). The origins of the the protozoa. That uncertainty has recently been C’,-cellulascs of these insects remain completely un- resolved by the successful cultivation of bacteria-free known at the present time. protozoa (Yamin & Trager. 1979), and the clear dcm- In summary then, while additional research may onstration that cellylolytic enrvmcs, including confirm the ability of a few insects to produce C,-cellulases. are produced by the- protozoa them- C,-cellulases. this ability is certainly not widespread, selves and not by intracellular bacterial symbionts. and should be regarded as the exception rather than Considering the imprcssivc cltic~cncq with which the the rule. Thus it seems evident that it is the inability exploitation of protozoan C,-ccllulascs has allowed to stnthesire and secrete C,-cellulases that explains the lower termites to digest and assimilate ccll~~losc. it the Inability of most insects to digest cellulose. With seems surprising that this mechanism for acquiring the possible exception of the silverfish, insects which cellulolytic capacity is not more widespread among are able to assimilate cellulose do so by exploiting the wood-feeders. Perhaps a better understanding of the ccllulolytic potential of protozoa and fungi which biochemical requirements of these unique protoroa. Cellulose digestion in insects 321
which should be possible to attain now that successful to species involved in complex, highly coevolved sym- culturing methods have been developed, will explain biotic associations with fungi. It may prove to be very the narrow phylogenetic distribution of this type of common (Martin, in press). Indeed, it is possible that insect-microbial symbiosis. many of the cerambycid beetles, which have been It has often been presumed that cellulose digestion assumed to produce the entire complex of cetlulases by the higher termites is accomplished by hindgut present in their digestive juices, acquire the bacteria, since the Termitidae lack the xylophagous C,-cellulases in their midgut fluids by ingesting fungal protozoa typical of the lower termites. However. evi- associates. Cellulases of fungal origin have been dence in support of this presumption is meager. Bac- reported in the amphipod. Gff~?~~~~~~~.~,~ss(~~~~~. an terial isolates with cel~ulolytic activity have been aquatic detritLls-feeder (B~rlocher, 1982). obtained from a few termite species, but there are no The proiife~~tioti in the gut of ingested bacteria has data to suggest that such bacteria are of any quanti- been demonstrated in the rhinocerus beetle, Oryctrs tative importance to cellulose digestion irl ri~o (Lee & rrckerr~is (Bayon & Mathetin. 1980), and in several Wood, 197 I ; Breznak. 1975, 1982: O’Brien & Slaytor. soil-feeding invertebrates, including the termite, Pro- 1982). Cellulolysis by resident hindgut bacteria has cuhiterrws uhwiensis (Bignell et u/., 1980). the milli- been proposed in the rhinocerus beetle, 0. nasicornis pede. Glorneris rwyiwta (Anderson & Bignell, 1980), (Bayon, 1980; Bayon & Mathelin, 1980). and may and the isopod Truclmr~iscus rathkri (Reyes & Tiedje, occur in other scarabs as well (Wiedmann, 1930; Cou- 1976a). This proliferation of bacteria is accompanied turier, 1961). although cellulolytic bacteria have not by the degradation of hemicellulose and the assimi- actually been isolated from the intestinal tracts of lation of microbial cells in Truchrmiscus (Reyes & these species. Gut bacteria. both from the midgut and Tiedje. 1976b). Thus. the indigestible constituents of hindgut, have been implicated in cellulose digestion in the ingested detritus are tr~~nsformed into microbial two species of roaches, P. ~~?~~~~cu~I~(Bignell, 1977; biomass which is digestible by the isopod. Although C’ruden & Markovetz. 1979) and E. posficus (Cruden to date there has been no demonstr~~tion that the pro- 6i Markovetz. 1979). Many insects harbour abundant liferation of ingested bacteria occurs at the expense of gut floras. and it is quite possible that further research ingested cellulose, that possibility clearly exists, and will identify other species which exploit the celluloly- needs to be considered as an additional mechanism tic capacities of their gut bacteria. However, it should by which insects might accomplish the digestion of be borne in mind that hindgut bacteria have also been cellulose by exploiting the metabolic capabilities of shown to ferment sugars, fix nitrogen. degrade uric microorganisms. It is even possible that this process is acid, synthesize amino acids. and participate in a host responsible for the digestion of a portion of the of biochemical processes besides cellulose digestion. ingested cellulose in the midgut of the rhinocerus Thus, the mere presence of an abundant gut flora beetle. from which celluiolytic strains of bacteria can be iso- lated does not constitute sufficient evidence to con- CONCI,USION clude that an insect derives any benefit from the cellu- lolytic potential of the bacteria it harbors within its The investig~~tion of insect-microbi~~l interactions, alimentary tract. pioneered with such elegance and insight by Buchner Recently it has been demonstrated that the fungus- and Cleveland over a half a century ago, is experienc- growing termites ~fuctotrnnrs r~c~tctlrrrsis(Martin & ing something of a renaissance at the present time. Martin, 1978. 1979) and M. suhl~~alir~~~s(Aba-Khatwa, Recent studies continue to uncover an ever growing 1978). and the larvae of the sir&d wood wasp, Sires number of ways by which insects exploit the bio- cyanrus (Kukor & Martin, 1983) acquire a ca- chemical characteristics of microorganisms, including pacity for cellulose digestion by ingesting fun&at their capacity to digest cellulose. These studies not enzymes. The termites are able to produce some of only contribute to a greater understanding of the nu- their own cellobiases and C,-cellulases, but the tritional ecology of insects. but also promise to pro- C’,-cellulases and some of the C,-cellulases in their vide basic insights into the biochemical processes midgut fluids are derived from the conidiophores of a which mediate interactions between insects an d symbiotic fungus. ~~~~i~j~u~?~~c~.~sp.. which the ter- microorganisms, and perhaps to help identify some of mites culture in their nests and consume in small the factors which determine whether an interspecific qu~~ntities along with the wood and other cellulosic intersection evolves into one of mutu~~lism, commens~l- substrates which make up the bulk of their food. 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