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Differential effects of fatty acid chain length on the viability of two species of cactophilic Drosophila

Differential effects of fatty acid chain length on the viability of two species of cactophilic... Comp, Biochem. PhyJio/. Vol. 83A. No. 4, pp. 761-·764, 1986 0300-9629/86 $3.00 + 0.00 Printed in Great Britain ·(·· 1986 Pergamon Press Ltd DIFFERENTIAL EFFECTS OF FA TTY ACID CHAIN LENGTH ON THE VIABILITY OF TWO SPECIES OF CACTOPHILIC DROSOPHILA JAMES C FOGLEMAN* and HENRY W. KIRCHERt:j: *Department of Biological Sciences, University of Denver, Denver. CO 80208, USA. Telephone: (303) 871-366! and tDepartment of Nutrition and Food Science, University of Arizona, Tucson, AZ 85721. USA (Receit•ed 16 August 1985) Abstract-!. Two columnar cacti in the Sonoran Desert, agria and organpipe, contain medium chain (C ··C ) fatty acids. 8 12 2. Necrotic tissues of these cacti serve as feeding and breeding substrates for Drosophila mojavensis but not D. nigrospiracu/a. 3. Results show that capric and lauric acids are the predominant fatty acids of both cacti. 4. Fatty acid chain length exhibits a differential effect on larval viability with caprylic acid (C,) having the greatest and myristic acid (C ) having the least effect. 5. Drosophila mojavensis is more tolerant of free fatty acids than D. nigrospiracula, and this partly explains the ability of D. moja!iensis to utilize agria and organpipe cacti. Two of the giant columnar cacti of the Sonoran INTRODUCTION Desert, Stenocereus gummosus (agria) and S. thurberi The toxicity of fatty acids and their salts on insects (organpipe), have been reported as containing has a long history. In fact, they may be among the medium-chain fatty acids (C -Cd as principle oldest insecticidal agents (Shepard, 1951). Fatty acids constituents of the lipid fraction (Kircher, 1982). have been reported as effective contact insecticides Although the fatty acids are typically esterified to against aphids, mosquitoes, houseflies, Japanese neutral triterpenes and sterol diols (Kircher, !980), beetles and some forest insects (Binder et al., 1979; some free fatty acids are released during decay of the and references therein). tissue. One of the four cactophilic Drosophila species Not all fatty acid chain lengths are equally effective endemic to the Sonoran Desert, D. mojavensis, in causing insect mortality. Capric {C1 ) and lauric utilizes the necrotic tissue of agria and organ pipe as (Cd acids were the most lethal of those tested as feeding and breeding sites (Heed, 1978). Another contact insecticides against aphips (Tattersfield and sympatric drosophilid, D. nigrospiracula, apparently Gimingham, 1927; Dills and Menusan, 1935). In later cannot utilize these cacti as host plants due to tests, 1% lauric acid inhibited cricket growth extensive larval mortality (Fellows and Heed, 1972). (McFarlane and Henneberry, 1965) but was not very Instead, D. nigrospiracula uses the necrotic tissue of toxic to Pseudosarcophaga affinis; this insect was Carnegiea gigantea (saguaro) as feeding and breeding killed more readily by capric acid (House, 1967). substrates. Recently, Fogleman et al. (1985) reported Capric acid also killed all confused flour beetles in 8 that a mixture of free fatty acids extracted from weeks when 2.5% was added to a dry flour diet organpipe cactus and added to homogenized saguaro (House and Graham, 1967). More recently, l-5% of tissue at a concentration of 0.5% dry weight had a C,-C fatty acids in the diet of hide beetles sup­ 10 drastic effect on the larval viability of D. nigrospira­ pressed fertility (Cohen and Levinson, 1972) and cula but had no significant effect on D. mojavensis C -C fatty acid salts were most harmful to several 8 12 larvae. Although higher concentrations of organpipe forest insects (Puritch, 1978). fatty acids (I .0 and 2.0% dry wt) reduced the viability While it is apparent that the medium chain fatty of both species, the tolerance of D. mojaPensis was acids exhibit the greatest insecticidal properties, they greater than that of D. nigrospiracula at all concen­ are, in general only minor constituents of most plant trations. The ability of D. mojavensis to tolerate the lipid fractions. Fatty acids of most higher plants have fatty acids which are present in the necroses of agria of carbon atoms and chains that are an even number and organpipe cacti explains, in part, why it can use 14-22 carbon atoms long; with C and C M as the 16 1 these cacti as host plants and D. nigrospiracula most abundant forms. Although the major fatty acid cannot. in coconut oiL lauric acid is only present in small The experiments reported here attempt to extend quantities in many vegetable oils (Binder et al., 1979). our knowledge of the toxicity of medium chain fatty Capric acid is even more rare. acids by examining the effect of single fatty acids of different chain length on larval viability of these two species of Drosophila. The fatty acid compositions of agria and organpipe are also reported. tDied January, 1984. 76! 762 JAMES C. FOGLEMAN and HENRY W. KIRCHER MATERIALS AND METHODS Table 1. Relative composition of the fatty acid fraction of agria and organpipe Both species of Drosophila were obtained from the labo­ Percentage of total* ratory of Dr William Heed, Department of Ecology and of Arizona, Tucson, AZ. Evolutionary Biology, University Fatty acid Agria Organ pipe Media for testing larval viability were prepared by dis­ 2.2 04 c6 free fatty acids (obtained from Sigma Chemical solving the c, 7.0 6.7 Co.) in ether, adding the mixture to dried saguaro powder 34.0 C,o 39.3 and allowing the ether to evaporate. The powder was c, 38.9 45.2 rehydrated and added to homogenized saguaro rot. The c,1, 1.8 0.5 resulting mixture was blended to insure homogeneity, 6.7 0.8 c", divided into six portions, and placed in half-pint milk c,. 0.6 0.8 1.7 1.0 bottles. The final concentration of the free fatty acids was c" 0.0 0.0 either 0.5 or 1.0% on a dry weight basis. Viability tests for c"' 0.0 lA each species and each fatty acid were set up in triplicate with C"' 0.9 0.2 c" 50 first instar larvae. This larval density was used because c,., 2.3 06 a preliminary experiment using 50, 100, 200 and 300 larvae 1.4 c,.2 1.5 per test showed that densities of 50 larvae per test yielded 0.3 0.6 CHi J a higher percentage adult emergence. Larvae were given Others 2.2 1.0 approx. 30 days to eclose into adults. The number of adults *Calculated as percent of total area under in each bottle after 30 days was used as a measure of larval all peaks. viability in the medium. The fatty acids that were tested include caproic acid (C ), caprylic acid (C ), capric acid (C ), lauric acid (C ) and myristic acid (C ). Control 10 12 variation in viability between treatments, i.e. different bottles contained saguaro rot without fatty acids. The data fatty acids at a particular concentration. These tests were analyzed using a one-way analysis of variance with do not compare the viabilities between the two replication (Sokal and Rohlf, 1969). Drosophila species for each treatment since com­ The fatty acid composition of agria and organpipe cacti was determined as follows: the cactus was skinned, blended parisons of this type are not as ecologically relevant with two volumes of MeOH and filtered. The residue was as the effect of the treatments within a species. It can extracted three times with 2: 1 CHC1 : MeOH. All filtrates be seen in Table 2 that, while all of the one-way were then combined and evaporated .. Lipids were removed analysis of variance tests were statistically significant by ether-water extraction followed by evaporation of the at the 0.05 level, D. mojavensis in 0.05% fatty acids ether fraction to dryness. Lipids were hydrolyzed by making were the least affected by the treatments and D. a mixture of lipids, 10% KOH and 95% ethanol (I: I: 10) nigrospiracula in 1.0% fatty acids were affected the and refluxing this mixture overnight on a steam bath. After most. saponification, the mixture was filtered and subjected to a Student-Newman-Keuls tests of differences be­ ether-water extraction. The aqueous phase was separated, made acidic with 1 N HCI and extracted with ether. The tween average viabilities of a species in media con­ ether fraction was evaporated to dryness and an excess taining fatty acids of different chain lengths at the (20 x) of 2% w/v H S0 in methanol was added to the 2 4 same concentration were also performed (Sokal and residue. After refluxing on a steam bath for several hours. 1969, pp. 235-242). The results of these tests Rohlf, H 0 was added and the fatty acid methyl esters were are indicated by superscripts in Table 2. For D. extracted with PE. mojavensis larvae in media containing 0.5% dry wt Gas chromatography of the fatty acid methyl esters of fatty acids, caprylic acid was the only one that agria and organpipe cacti was performed on a Varian produced a significant reduction in viability. The 3700 GC equipped with dual flame ionization detectors average viabilities in media containing the other fatty linked to an Apple lie computer through an analog/digital interface (All3, Interactive Structures). A 2m x 2 mm i.d. acids were not significantly different from the vi­ x l/4" o.d. glass column packed with 15% DEGS on 80/100 ability in the control medium. At 1.0% dry wt. both Chromosorb W AW (Supelco cat. No. l-1904) was used. caprylic and capric acids significantly reduced the Benzene was used as the solvent for the GC analysis. Peak viability of D. mojavensis larvae. Average viability in identification was based on retention time compared to medium containing caprylic acid was reduced to near retention times of known compounds. zero. On the other hand, D. nigrospiracula was greatly affected by all fatty acids except myristic acid. The RESULTS addition of caprylic or capric acids to the saguaro The fatty acid compositions of agria and organpipe homogenate at a concentration of 0.5% dry wt are given in Table \. Although both cacti contain a prevented any D. nigrospiracula from eclosing. diverse set of fatty acid chain lengths, with some Caproic and lauric acids also significantly reduced unsaturated forms, two chain lengths are predom­ larval viability. At 1.0% dry wt, viability was zero on inant, capric acid (C ) and lauric acid (Cd. These caproic, caprylic and capric acids and near zero two, together with caprylic acid (C ), make up about (6.0%) on lauric acid. Only myristic acid had no 80% of the fatty acids in agria and over 90% of the significant effect on viability. fatty acids in organpipe. Statistical analysis indicates that there is no significant difference between the DISCUSSION cactus species with respect to fatty acid composition. The results of the viability studies for the two The fatty acid composition of agria and organpipe different concentrations of free fatty acids are cacti appears highly unusual in that typical plant presented in Table 2. The one-way analysis of vari­ fatty acids are, essentially, not present. The fact that ance tests demonstrate whether there is significant agria and organpipe contain medium chain fatty Cactophilic Drosophila and fatty acids 763 Table 2. Average percent viability± standard deviation on saguaro homo­ genate plus free fatty acids. Results of one-way ANOV A (F) and probability (P) are also given Substrate A'erage viability• (Saguaro plus ... ) D. mojavensis D. nigrospiracula Concentration= 0.5% dry wt Control 83.3 ± 15.0t 86.0 ± 10.4t c, 88.0± 13.lt 20.0 ± 8.7t§ c, 56.7 ± 4.6 0.0 ± O.Ot 76.0 ± 6.9t 0.0 ± O.Ot C,o 64.7±6.lt 33.3 ± 15.5§ c" c,. 82.7 ± 7.6t 84.7 ± 10.3t F(df=5.12) 4.751 52.324 <0.05 «0.001 Concentration = 1.0% dry wt Control 83.3 ± 15.0t 86.0 ± 10.4t c, 94.0 ± 6.0t 0.0 ± O.Ot c, 0.7 ± I.2t 0.0 ± O.Ot 9.3 ± 6.Jt 0.0 ± O.Ot Clo c, 85.3 ± 14.0t 6.0 ± 4.0t 86.7 ± 6.4t 88.7 ± 4.2t c,. F(df=5,12) 61.090 250.991 «0.001 «0.001 •superscripts relate averages which are not significantly different at the 0.05 level. acids had been previously established (Kircher, 1982), The concentration of free sterol diols has been ob­ but the relative composition had never been deter­ served to increase during rotting, but free fatty acids mined. The presence of medium chain fatty acids do not increase in concentration. Possible expla­ may, in fact, be a characteristic of the genus, Steno­ nations that have been offered for this lack of increase include the metabolism of free fatty acids by lipolytic cereus. Two related species, S. alamosensis (cina) and S. hystrix also contain medium chain fatty acids yeasts which grow in the necrosis and the chemical (Fogleman and Kircher, unpublished), although the reactivity of free fatty acids leading to their being relative composition is somewhat different. The fatty recomplexed with other components of the lipid acids in other columnar cacti, such as saguaro, have fraction (Fogleman et a/., 1985). The concentration the more typical chain lengths, C -C • of free fatty acids in necrotic organpipe tissue has 16 18 There are two general conclusions that can be been measured at around 0.5% dry wt. made based on the data in Table 2. First, although From the discussion above and the relative fatty both species are significantly affected by the addition acid composition given in Table I, it appears that of medium chain fatty acids to the larval substrate, capric acid (C ) is the most biologically important D. mojavensis are much less affected than D. nigro­ fatty acid in this model system. Although caprylic spiracula. The £-values derived from the one-way acid has a greater effect on larval viability, its concen­ analysis of variance tests reflect this trend clearly. At tration is not sufficient to cause a major reduction in either concentration, the value for D. mojavensis is viability. Lauric acid is in a relatively high concen­ considerably less than that for D. nigrospiracula. The tration but does not match capric acid in its effect on second conclusion is that, with respect to chain viability. Caproic and myristic acids are relatively minor components with little biological impact. Al­ length, caprylic and capric acids have the greatest effect on the larval viability of either species. though the fatty acids used in this study differ in their The primary question here is whether the presence individual effects on larval viability, the toxicity of of medium chain fatty acids in organpipe and agria necrotic agria and organpipe tissue is a phenomenon cacti is responsible for the inability of D. nigro­ to which all fatty acids (except myristic acid) con­ spiracula to utilize these cacti as host plants. The total tribute to some extent. concentration of fatty acids (all chain lengths) in The actual physiological basis for the toxic effect of fresh tissue has been estimated at about 3% dry wt medium chain fatty acids on cactophilic Drosophila is for organpipe and 1.6% dry wt for agria (Fogleman unknown. General speculation on this subject has eta/., 1985; Kircher, unpublished). Most of the fatty been reviewed by Puritch (1978). He suggested that acids in the fresh tissue of these cacti, however, are the toxic effect may be the result of inhibition of monoesterified to triterpenes and sterol diols. Fatty oxidative phosphorylation. This idea is supported by acids which are complexed in this fashion are appar­ a number of animal studies cited in the review. The tolerance of D. mojavensis to medium chain ently not toxic since the addition of crude lipids fatty acids, and the intolerance of D. nigrospiracula, (organpipe) to saguaro homogenate at concen­ supports the statement that these fatty acids are, in trations up to 10% dry wt had no significant effect on the larval viability of D. mojavensis or D. nigro­ part, responsible for the ability of the former but not spiracula (Fogleman eta/., 1985). Free fatty acids, as the latter species to utilize necrotic sections of agria well as sterol diols and triterpenes, are released and organpipe cacti as breeding substrates. The free sterol diols which are present in these cacti are also during the rotting process by microbial hydrolysis. JAMES C, FOGLEMAN and HENRY W. KIRCHER known to reduce the viability of D. nigrospiracula but Drosophila. In Ecological Genetics: The Interface (Edited by Brussard P. F.} pp. 109-126. Springer, New York. not D. mojavensis (Fogleman eta!., 1985). Together, House H. L. {!967) The nutritional status and larvicidal they make up a set of natural plant products which activities of C -C saturated fatty acids in Pseudo­ 6 14 are involved in determining insect-host plant re­ sarcophaga affinis (Diptera: Sarcophagidae). Can. Ento­ lationships in the Sonoran Desert. mol. 99, 384--392. House H. L. and Graham A. R. ( 1967) Capric acid blended Acknowledgements-The authors would like to thank into foodstuff for control of an insect pest. Tribolium Susann Duperret for technical assistance. This work was confusum (Coleoptera: Tenebrionidae). Can. Ent. 99, supported by a grant (BSR-8207056) from the National 994-999. Science Foundation. Kircher H. W. (1980) Triterpenes in organpipe cactus. Phytochemistry 19, 2707-2712. Kircher H. W. (1982) Chemical composition of cacti and its relationship to Sonoran Desert Drosophila. In Ecological REFERENCES Genetics and Evolution: The Cactus-Yeast-Drosophila Binder R. G., Chan B. G. and Elliger C. A. (1979) Model System (Edited by Barker J. S. F. and Starmer Antibiotic effects of C -C fatty acid esters on pink W. T.), pp. !43-!58. Academic Press, New York. 10 12 bollworm, bollworm, and tobacco budworm. Agric. Bioi. McFarlane J. E. and Henneberry G. 0. (1965) Inhibition of Chern. 43, 2467-2471. the growth of an insect by fatty acids. J. Insect Physio/, Cohen E. and Levinson Z. H. (!972) The effect of fatty acids 11, 1247--1252. on reproduction of the hide beetle Dermestes maculatus Puritch G. S. ( 1978) Biocidal effects of fatty acid salts on (Coleoptera: Dermestidae). Life Sci. Part II ll, 293-299. various forest insect pests. In The Pharmacological Effect Dills L. E. and Mensuan H. Jr (1935) A study of some fatty of Lipids (Edited by Kabara J. J. ), pp. 105-112. American acids as contact insecticides. Contr. Boyce- Thompson Oil Chemists Society, Champaign, IL Inst. 1, 63-82. Shepard H. H. (1951) The Chemistry and Action of Fellows D.P. and Heed W. B. (1972) Factors affecting host Insecticides. McGraw-Hill, New York. plant selection in desert-adapted cactiphilic Drosophila. Sokal R. R. and Rohlf F. J. (1969) Biometry. W. H. Ecology 53, 850-858. Freeman, San Francisco. Fogleman J. C., Duperret S. M. and Kircher H. W. (1985) Tattersfield F. and Gimingham C. T. (1927) Studies on The role of phytosterols in host plant selection by cacto­ contact insecticides. Part VI. The insecticidal action of the philic Drosophila. Lipid5 (in press). fatty acids, the methyl esters and ammonium salts. Ann. Heed W. B. (1978) Ecology and Genetics of Sonoran Desert Appl. Bioi. 14, 331-358. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Comparative Biochemistry and Physiology Part A Physiology Unpaywall

Differential effects of fatty acid chain length on the viability of two species of cactophilic Drosophila

Fogleman, James C; Kircher, Henry W
Comparative Biochemistry and Physiology Part A Physiology – Jan 1, 1986
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Abstract

Comp, Biochem. PhyJio/. Vol. 83A. No. 4, pp. 761-·764, 1986 0300-9629/86 $3.00 + 0.00 Printed in Great Britain ·(·· 1986 Pergamon Press Ltd DIFFERENTIAL EFFECTS OF FA TTY ACID CHAIN LENGTH ON THE VIABILITY OF TWO SPECIES OF CACTOPHILIC DROSOPHILA JAMES C FOGLEMAN* and HENRY W. KIRCHERt:j: *Department of Biological Sciences, University of Denver, Denver. CO 80208, USA. Telephone: (303) 871-366! and tDepartment of Nutrition and Food Science, University of Arizona, Tucson, AZ 85721. USA (Receit•ed 16 August 1985) Abstract-!. Two columnar cacti in the Sonoran Desert, agria and organpipe, contain medium chain (C ··C ) fatty acids. 8 12 2. Necrotic tissues of these cacti serve as feeding and breeding substrates for Drosophila mojavensis but not D. nigrospiracu/a. 3. Results show that capric and lauric acids are the predominant fatty acids of both cacti. 4. Fatty acid chain length exhibits a differential effect on larval viability with caprylic acid (C,) having the greatest and myristic acid (C ) having the least effect. 5. Drosophila mojavensis is more tolerant of free fatty acids than D. nigrospiracula, and this partly explains the ability of D. moja!iensis to utilize agria and organpipe cacti. Two of the giant columnar cacti of the Sonoran INTRODUCTION Desert, Stenocereus gummosus (agria) and S. thurberi The toxicity of fatty acids and their salts on insects (organpipe), have been reported as containing has a long history. In fact, they may be among the medium-chain fatty acids (C -Cd as principle oldest insecticidal agents (Shepard, 1951). Fatty acids constituents of the lipid fraction (Kircher, 1982). have been reported as effective contact insecticides Although the fatty acids are typically esterified to against aphids, mosquitoes, houseflies, Japanese neutral triterpenes and sterol diols (Kircher, !980), beetles and some forest insects (Binder et al., 1979; some free fatty acids are released during decay of the and references therein). tissue. One of the four cactophilic Drosophila species Not all fatty acid chain lengths are equally effective endemic to the Sonoran Desert, D. mojavensis, in causing insect mortality. Capric {C1 ) and lauric utilizes the necrotic tissue of agria and organ pipe as (Cd acids were the most lethal of those tested as feeding and breeding sites (Heed, 1978). Another contact insecticides against aphips (Tattersfield and sympatric drosophilid, D. nigrospiracula, apparently Gimingham, 1927; Dills and Menusan, 1935). In later cannot utilize these cacti as host plants due to tests, 1% lauric acid inhibited cricket growth extensive larval mortality (Fellows and Heed, 1972). (McFarlane and Henneberry, 1965) but was not very Instead, D. nigrospiracula uses the necrotic tissue of toxic to Pseudosarcophaga affinis; this insect was Carnegiea gigantea (saguaro) as feeding and breeding killed more readily by capric acid (House, 1967). substrates. Recently, Fogleman et al. (1985) reported Capric acid also killed all confused flour beetles in 8 that a mixture of free fatty acids extracted from weeks when 2.5% was added to a dry flour diet organpipe cactus and added to homogenized saguaro (House and Graham, 1967). More recently, l-5% of tissue at a concentration of 0.5% dry weight had a C,-C fatty acids in the diet of hide beetles sup­ 10 drastic effect on the larval viability of D. nigrospira­ pressed fertility (Cohen and Levinson, 1972) and cula but had no significant effect on D. mojavensis C -C fatty acid salts were most harmful to several 8 12 larvae. Although higher concentrations of organpipe forest insects (Puritch, 1978). fatty acids (I .0 and 2.0% dry wt) reduced the viability While it is apparent that the medium chain fatty of both species, the tolerance of D. mojaPensis was acids exhibit the greatest insecticidal properties, they greater than that of D. nigrospiracula at all concen­ are, in general only minor constituents of most plant trations. The ability of D. mojavensis to tolerate the lipid fractions. Fatty acids of most higher plants have fatty acids which are present in the necroses of agria of carbon atoms and chains that are an even number and organpipe cacti explains, in part, why it can use 14-22 carbon atoms long; with C and C M as the 16 1 these cacti as host plants and D. nigrospiracula most abundant forms. Although the major fatty acid cannot. in coconut oiL lauric acid is only present in small The experiments reported here attempt to extend quantities in many vegetable oils (Binder et al., 1979). our knowledge of the toxicity of medium chain fatty Capric acid is even more rare. acids by examining the effect of single fatty acids of different chain length on larval viability of these two species of Drosophila. The fatty acid compositions of agria and organpipe are also reported. tDied January, 1984. 76! 762 JAMES C. FOGLEMAN and HENRY W. KIRCHER MATERIALS AND METHODS Table 1. Relative composition of the fatty acid fraction of agria and organpipe Both species of Drosophila were obtained from the labo­ Percentage of total* ratory of Dr William Heed, Department of Ecology and of Arizona, Tucson, AZ. Evolutionary Biology, University Fatty acid Agria Organ pipe Media for testing larval viability were prepared by dis­ 2.2 04 c6 free fatty acids (obtained from Sigma Chemical solving the c, 7.0 6.7 Co.) in ether, adding the mixture to dried saguaro powder 34.0 C,o 39.3 and allowing the ether to evaporate. The powder was c, 38.9 45.2 rehydrated and added to homogenized saguaro rot. The c,1, 1.8 0.5 resulting mixture was blended to insure homogeneity, 6.7 0.8 c", divided into six portions, and placed in half-pint milk c,. 0.6 0.8 1.7 1.0 bottles. The final concentration of the free fatty acids was c" 0.0 0.0 either 0.5 or 1.0% on a dry weight basis. Viability tests for c"' 0.0 lA each species and each fatty acid were set up in triplicate with C"' 0.9 0.2 c" 50 first instar larvae. This larval density was used because c,., 2.3 06 a preliminary experiment using 50, 100, 200 and 300 larvae 1.4 c,.2 1.5 per test showed that densities of 50 larvae per test yielded 0.3 0.6 CHi J a higher percentage adult emergence. Larvae were given Others 2.2 1.0 approx. 30 days to eclose into adults. The number of adults *Calculated as percent of total area under in each bottle after 30 days was used as a measure of larval all peaks. viability in the medium. The fatty acids that were tested include caproic acid (C ), caprylic acid (C ), capric acid (C ), lauric acid (C ) and myristic acid (C ). Control 10 12 variation in viability between treatments, i.e. different bottles contained saguaro rot without fatty acids. The data fatty acids at a particular concentration. These tests were analyzed using a one-way analysis of variance with do not compare the viabilities between the two replication (Sokal and Rohlf, 1969). Drosophila species for each treatment since com­ The fatty acid composition of agria and organpipe cacti was determined as follows: the cactus was skinned, blended parisons of this type are not as ecologically relevant with two volumes of MeOH and filtered. The residue was as the effect of the treatments within a species. It can extracted three times with 2: 1 CHC1 : MeOH. All filtrates be seen in Table 2 that, while all of the one-way were then combined and evaporated .. Lipids were removed analysis of variance tests were statistically significant by ether-water extraction followed by evaporation of the at the 0.05 level, D. mojavensis in 0.05% fatty acids ether fraction to dryness. Lipids were hydrolyzed by making were the least affected by the treatments and D. a mixture of lipids, 10% KOH and 95% ethanol (I: I: 10) nigrospiracula in 1.0% fatty acids were affected the and refluxing this mixture overnight on a steam bath. After most. saponification, the mixture was filtered and subjected to a Student-Newman-Keuls tests of differences be­ ether-water extraction. The aqueous phase was separated, made acidic with 1 N HCI and extracted with ether. The tween average viabilities of a species in media con­ ether fraction was evaporated to dryness and an excess taining fatty acids of different chain lengths at the (20 x) of 2% w/v H S0 in methanol was added to the 2 4 same concentration were also performed (Sokal and residue. After refluxing on a steam bath for several hours. 1969, pp. 235-242). The results of these tests Rohlf, H 0 was added and the fatty acid methyl esters were are indicated by superscripts in Table 2. For D. extracted with PE. mojavensis larvae in media containing 0.5% dry wt Gas chromatography of the fatty acid methyl esters of fatty acids, caprylic acid was the only one that agria and organpipe cacti was performed on a Varian produced a significant reduction in viability. The 3700 GC equipped with dual flame ionization detectors average viabilities in media containing the other fatty linked to an Apple lie computer through an analog/digital interface (All3, Interactive Structures). A 2m x 2 mm i.d. acids were not significantly different from the vi­ x l/4" o.d. glass column packed with 15% DEGS on 80/100 ability in the control medium. At 1.0% dry wt. both Chromosorb W AW (Supelco cat. No. l-1904) was used. caprylic and capric acids significantly reduced the Benzene was used as the solvent for the GC analysis. Peak viability of D. mojavensis larvae. Average viability in identification was based on retention time compared to medium containing caprylic acid was reduced to near retention times of known compounds. zero. On the other hand, D. nigrospiracula was greatly affected by all fatty acids except myristic acid. The RESULTS addition of caprylic or capric acids to the saguaro The fatty acid compositions of agria and organpipe homogenate at a concentration of 0.5% dry wt are given in Table \. Although both cacti contain a prevented any D. nigrospiracula from eclosing. diverse set of fatty acid chain lengths, with some Caproic and lauric acids also significantly reduced unsaturated forms, two chain lengths are predom­ larval viability. At 1.0% dry wt, viability was zero on inant, capric acid (C ) and lauric acid (Cd. These caproic, caprylic and capric acids and near zero two, together with caprylic acid (C ), make up about (6.0%) on lauric acid. Only myristic acid had no 80% of the fatty acids in agria and over 90% of the significant effect on viability. fatty acids in organpipe. Statistical analysis indicates that there is no significant difference between the DISCUSSION cactus species with respect to fatty acid composition. The results of the viability studies for the two The fatty acid composition of agria and organpipe different concentrations of free fatty acids are cacti appears highly unusual in that typical plant presented in Table 2. The one-way analysis of vari­ fatty acids are, essentially, not present. The fact that ance tests demonstrate whether there is significant agria and organpipe contain medium chain fatty Cactophilic Drosophila and fatty acids 763 Table 2. Average percent viability± standard deviation on saguaro homo­ genate plus free fatty acids. Results of one-way ANOV A (F) and probability (P) are also given Substrate A'erage viability• (Saguaro plus ... ) D. mojavensis D. nigrospiracula Concentration= 0.5% dry wt Control 83.3 ± 15.0t 86.0 ± 10.4t c, 88.0± 13.lt 20.0 ± 8.7t§ c, 56.7 ± 4.6 0.0 ± O.Ot 76.0 ± 6.9t 0.0 ± O.Ot C,o 64.7±6.lt 33.3 ± 15.5§ c" c,. 82.7 ± 7.6t 84.7 ± 10.3t F(df=5.12) 4.751 52.324 <0.05 «0.001 Concentration = 1.0% dry wt Control 83.3 ± 15.0t 86.0 ± 10.4t c, 94.0 ± 6.0t 0.0 ± O.Ot c, 0.7 ± I.2t 0.0 ± O.Ot 9.3 ± 6.Jt 0.0 ± O.Ot Clo c, 85.3 ± 14.0t 6.0 ± 4.0t 86.7 ± 6.4t 88.7 ± 4.2t c,. F(df=5,12) 61.090 250.991 «0.001 «0.001 •superscripts relate averages which are not significantly different at the 0.05 level. acids had been previously established (Kircher, 1982), The concentration of free sterol diols has been ob­ but the relative composition had never been deter­ served to increase during rotting, but free fatty acids mined. The presence of medium chain fatty acids do not increase in concentration. Possible expla­ may, in fact, be a characteristic of the genus, Steno­ nations that have been offered for this lack of increase include the metabolism of free fatty acids by lipolytic cereus. Two related species, S. alamosensis (cina) and S. hystrix also contain medium chain fatty acids yeasts which grow in the necrosis and the chemical (Fogleman and Kircher, unpublished), although the reactivity of free fatty acids leading to their being relative composition is somewhat different. The fatty recomplexed with other components of the lipid acids in other columnar cacti, such as saguaro, have fraction (Fogleman et a/., 1985). The concentration the more typical chain lengths, C -C • of free fatty acids in necrotic organpipe tissue has 16 18 There are two general conclusions that can be been measured at around 0.5% dry wt. made based on the data in Table 2. First, although From the discussion above and the relative fatty both species are significantly affected by the addition acid composition given in Table I, it appears that of medium chain fatty acids to the larval substrate, capric acid (C ) is the most biologically important D. mojavensis are much less affected than D. nigro­ fatty acid in this model system. Although caprylic spiracula. The £-values derived from the one-way acid has a greater effect on larval viability, its concen­ analysis of variance tests reflect this trend clearly. At tration is not sufficient to cause a major reduction in either concentration, the value for D. mojavensis is viability. Lauric acid is in a relatively high concen­ considerably less than that for D. nigrospiracula. The tration but does not match capric acid in its effect on second conclusion is that, with respect to chain viability. Caproic and myristic acids are relatively minor components with little biological impact. Al­ length, caprylic and capric acids have the greatest effect on the larval viability of either species. though the fatty acids used in this study differ in their The primary question here is whether the presence individual effects on larval viability, the toxicity of of medium chain fatty acids in organpipe and agria necrotic agria and organpipe tissue is a phenomenon cacti is responsible for the inability of D. nigro­ to which all fatty acids (except myristic acid) con­ spiracula to utilize these cacti as host plants. The total tribute to some extent. concentration of fatty acids (all chain lengths) in The actual physiological basis for the toxic effect of fresh tissue has been estimated at about 3% dry wt medium chain fatty acids on cactophilic Drosophila is for organpipe and 1.6% dry wt for agria (Fogleman unknown. General speculation on this subject has eta/., 1985; Kircher, unpublished). Most of the fatty been reviewed by Puritch (1978). He suggested that acids in the fresh tissue of these cacti, however, are the toxic effect may be the result of inhibition of monoesterified to triterpenes and sterol diols. Fatty oxidative phosphorylation. This idea is supported by acids which are complexed in this fashion are appar­ a number of animal studies cited in the review. The tolerance of D. mojavensis to medium chain ently not toxic since the addition of crude lipids fatty acids, and the intolerance of D. nigrospiracula, (organpipe) to saguaro homogenate at concen­ supports the statement that these fatty acids are, in trations up to 10% dry wt had no significant effect on the larval viability of D. mojavensis or D. nigro­ part, responsible for the ability of the former but not spiracula (Fogleman eta/., 1985). Free fatty acids, as the latter species to utilize necrotic sections of agria well as sterol diols and triterpenes, are released and organpipe cacti as breeding substrates. The free sterol diols which are present in these cacti are also during the rotting process by microbial hydrolysis. JAMES C, FOGLEMAN and HENRY W. KIRCHER known to reduce the viability of D. nigrospiracula but Drosophila. In Ecological Genetics: The Interface (Edited by Brussard P. F.} pp. 109-126. Springer, New York. not D. mojavensis (Fogleman eta!., 1985). Together, House H. L. {!967) The nutritional status and larvicidal they make up a set of natural plant products which activities of C -C saturated fatty acids in Pseudo­ 6 14 are involved in determining insect-host plant re­ sarcophaga affinis (Diptera: Sarcophagidae). Can. Ento­ lationships in the Sonoran Desert. mol. 99, 384--392. House H. L. and Graham A. R. ( 1967) Capric acid blended Acknowledgements-The authors would like to thank into foodstuff for control of an insect pest. Tribolium Susann Duperret for technical assistance. This work was confusum (Coleoptera: Tenebrionidae). Can. Ent. 99, supported by a grant (BSR-8207056) from the National 994-999. Science Foundation. Kircher H. W. (1980) Triterpenes in organpipe cactus. Phytochemistry 19, 2707-2712. Kircher H. W. (1982) Chemical composition of cacti and its relationship to Sonoran Desert Drosophila. In Ecological REFERENCES Genetics and Evolution: The Cactus-Yeast-Drosophila Binder R. G., Chan B. G. and Elliger C. A. (1979) Model System (Edited by Barker J. S. F. and Starmer Antibiotic effects of C -C fatty acid esters on pink W. T.), pp. !43-!58. Academic Press, New York. 10 12 bollworm, bollworm, and tobacco budworm. Agric. Bioi. McFarlane J. E. and Henneberry G. 0. (1965) Inhibition of Chern. 43, 2467-2471. the growth of an insect by fatty acids. J. Insect Physio/, Cohen E. and Levinson Z. H. (!972) The effect of fatty acids 11, 1247--1252. on reproduction of the hide beetle Dermestes maculatus Puritch G. S. ( 1978) Biocidal effects of fatty acid salts on (Coleoptera: Dermestidae). Life Sci. Part II ll, 293-299. various forest insect pests. In The Pharmacological Effect Dills L. E. and Mensuan H. Jr (1935) A study of some fatty of Lipids (Edited by Kabara J. J. ), pp. 105-112. American acids as contact insecticides. Contr. Boyce- Thompson Oil Chemists Society, Champaign, IL Inst. 1, 63-82. Shepard H. H. (1951) The Chemistry and Action of Fellows D.P. and Heed W. B. (1972) Factors affecting host Insecticides. McGraw-Hill, New York. plant selection in desert-adapted cactiphilic Drosophila. Sokal R. R. and Rohlf F. J. (1969) Biometry. W. H. Ecology 53, 850-858. Freeman, San Francisco. Fogleman J. C., Duperret S. M. and Kircher H. W. (1985) Tattersfield F. and Gimingham C. T. (1927) Studies on The role of phytosterols in host plant selection by cacto­ contact insecticides. Part VI. The insecticidal action of the philic Drosophila. Lipid5 (in press). fatty acids, the methyl esters and ammonium salts. Ann. Heed W. B. (1978) Ecology and Genetics of Sonoran Desert Appl. Bioi. 14, 331-358.

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