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Identification accumulation and biosynthesis of the cuticular hydrocarbons of the southern armyworm Spodoptera eridania cramer lepidopteraNoctuidae.

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Archives of Insect Biochemistry and Physiology 16:19-30 (1991)
Identification, Accumulation, and Biosynthesis
of the Cuticular Hydrocarbons of the
Southern Armyworm, Spodoptera eridania
(Cramer) (Lepidoptera: Noctuidae)
Lin Guo and Gary J. Blomquist
Department of Biochemistry, Uninersity of Nevada, Reno, Nevada
The accumulation and biosynthesis of cuticular and internal hydrocarbons in
the Southern armyworm, Spodoptera eridania, were examined at closely timed
intervals during larval and pupal development. Gas chromatography-massspectrometry (GC-MS) was used to identify n-alkanes, monomethylalkanes, and
dimethylalkanes ranging in chain length from 23 to 35 carbons. The amount
of cuticular hydrocarbon stayed relatively constant duringeach stadium, while
the amount of internal hydrocarbon increased dramatically during the first
half of each larval stadium, presumably to replace the cuticular hydrocarbon
lost o n the shed cast skin with each molt. The accumulation of internal hydrocarbon was mirrored by large increases in the rate of incorporation of labeled
acetate into the hydrocarbon fraction. Hydrocarbon production fell to very
low rates during the latter part of the fourth and fifth larval stadia. Relatively
high rates of hydrocarbon production were observed during the first and last
one-third of the pupal stage and essentially all of the hydrocarbons produced
during this stage remained internal. These data document large changes in
the rates of hydrocarbon production during development in S. eridania and
suggest that most of the hydrocarbon produced during each stage was stored
internally and then transported to the cuticle of the next stage.
Key words: hydrocarbon composition, hydrocarbon accumulation, hydrocarbon biosynthesis
INTRODUCTION
Hydrocarbons are important constituents of the cuticular lipids of insects,
where they serve essential roles in preventing water loss and are important in
many species in chemical communication [l-41. They have been studied exAcknowledgments: Supported in part by a grant (CAM 8900879) from the USDA-CRGO. We
thank Murray Hackett for CC-MS analysis and David Quilici and Linda Mead for rearing the
Southern armyworm colony.
Received July26,1990; accepted August 24,1990.
Address reprint requests to Lin Cuo, Department of Biochemistry, Howard Science Building,
Universityof Nevada, Reno, N V 89557-0014.
0 1991 Wiley-Liss, Inc.
20
Cuo and Blomquist
tensively in a number of insect groups and several recent reviews describe
these studies [2-41.
Cuticular hydrocarbons are often present as complex mixtures of straightchain, methyl-branched, and unsaturated components [4]. The proportion of
each class of hydrocarbon show large variations in different insect species [3].
The hydrocarbon composition can be affected by temperature [ 5 ] ,habitat [6],
and developmental stage [7]. The relationship between low cuticular permeability and the occurrence of long-chain methylalkanes in cuticular hydrocarbon mixtures has been demonstrated in several insect species [4].
Variations in hydrocarbon biosynthesis at different developmental stages were
observed in several insects. Newly molted adult American cockroaches, Periplaneta arnericana, have relatively low levels of hydrocarbon biosynthesis while
older ones have a higher rate of biosynthesis [8]. Two periods of rapid accumulation of cuticular hydrocarbon were observed in the development of
Snrcophagu bullata [9], the first occurring during pupation and the 3 day period
following pupation, the second occurring during the 4 day period preceding
the pupal-adult ecdysis. The biosynthesis of hydrocarbons was stimulated
by ecdysterone in S. bullata [lo]. Hydrocarbons were synthesized at specific
time periods during larval and pupal development in the cabbage looper,
Trichoplusia ni. The rate of hydrocarbon biosynthesis increased dramatically
after each molt [7,11] during the feeding stages and fell to very low levels
during the wandering stages.
Most insects are susceptible to dessication at certain times during development [12]. Thus, it was of interest to determine the identity of the cuticular
and internal hydrocarbons and to examine their accumulation and rates of
formation in the Southern armyworm. These studies are reported herein.
MATERIALS AND METHODS
Insects
Southern armyworms were reared according to the procedure reported in
detail elsewhere [ 131. Regimes for rearing insects were photoperiod, 14:lO (L:D);
temperature, 25°C k 1”C:22”C & 1°C(L:D); and relative humidity, ca 50-60%.
The larval diet was essentially the same as described by Rehr et al. [14], but
contained foliage of lima bean, rather than red kidney bean.
Radioactive Substrate
Sodium [ l-I4C]acetate (45 mCi/mmol) was obtained from ICN Biomedicals
Inc., Costa Mesa, CA.
In Vivo Studies
Larvae of known ages were injected with 1.0 pl each of [1-14C]acetate(0.5
pCUp.1) just under the last pair of prolegs. Pupae of known ages were injected
through the last two abdominal segments. A microcapillary injection technique,
as described in Dwyer et al. [ll], was used to minimize trauma to the insect.
Following a 2 h incubation at about 27T, insects were killed by immersion in
hexane. Three groups of five insects each were used for each time point. Values reported represent the mean ? standard deviation.
HydrocarbonProduction in the Southern Armyworm
21
Extraction, Separation, and Identification
Cuticular lipids were extracted by rinsing insects in redistilled hexane (5 ml
per group of five insects) for 5 min, removing the solvent, and adding 2 ml of
hexane for 1 min. The extracts were combined and reduced in volume under
a stream of N2. Hydrocarbons were isolated by chromatography on minicolumns
(6 x 0.5 cm) of Biosil-A (Bio-RadLabs, Richmond, CA)eluted with 8 ml of hexane.
Internal lipids were extracted by the method of Bligh and Dyer [15] and the
various lipid classes were separated by thin-layer chromatography (Silica gel
type H). The plates were developed in hexane-diethyl ether (95:5)and visualized under UV light after spraying with Rhodamine 6G (0.1% in methanol).
The hydrocarbons were extracted from the silica gel with diethyl ether.
A known amount of n-Octacosane was added as an internal standard at the
time of extraction. n-octacosane was not present in S. eriduniu hydrocarbon at
above 0.5% level of the total hydrocarbon at any time during the stages studied.
All gas chromatographic analyses were performed on a Hewlett Packard 5890
gas chromatograph which was controlled by a Hewlett-Packard 3393A integrator GC* terminal. Samples were analyzed on a 30 m x 0.32 mm I.D, 0.5 pm
stationary phase DB5 column, The oven was temperature-programmed from
200 to 310°C at 5"C/min, then held at 310°C for 15 min. The carrier gas was
helium, at a flow rate of 2.5 ml/min, and the carrier gas plus make-up gas at a
total flow rate of 30 ml/min.
Gas chromatography-massspectrometry (GC-MS) analyses were performed on
a Finnigan 4023 mass spectrometer interfaced with an INCOS data system. The
GC-MS system operated at 70 eV (GC condition: 30 m x 0.32 mm DB5 column,
30-200°C at 20"C/min, 200-28O0C/min at 5"C/min, held at 280°C for 15 min). The
carrier gas was helium at 8 PSI head pressure, linear velocity about 30 cm/s.
All GC peak retention times were expressed as equivalent chain lengths
(ECLs). ECL for each hydrocarbon was calculated by comparing the retention
time of a given peak with known n-alkanes. Mass spectra of methylalkanes
were interpreted according to the criteria of Nelson [16] andBlomquist et al. [3].
Determination of Radioactivity
Samples were assayed for radioactivity on a Beckman LS 1701liquid scintillation counter in a 10 ml fluor solution containing 0.4% 2,5-diphenyloxazole
in toluene at an efficiency of ca 96% for carbon-14.
RESULTS
Hydrocarbon Structure and Composition
The major components of the cuticular hydrocarbons of the Southern armyworm larvae are listed in Table 1. The same major components are present on
larvae, pupae, and adults, with differences noted in the percentage of each
component. The ECL values indicated that methyl-branched hydrocarbons
are the predominant components with the remainder being n-alkanes, principally n-C27, n-C29, and n-C31 (Table 1).No unsaturated hydrocarbons were
found. The percentages of the methyl-branched hydrocarbons increased
*Abbreviations used: ECL = equivalent chain length; FAS = fatty acid synthesis; GC = gas
chromatography, MS = mass spectrometry.
ECLb
19.00
20.00
21.00
22.00
23.00
24.00
25.00
26.00
27.00
28.00
29.00
29.27
29.37
30.00
31.00
31.26
31.37
Component
n-Nonadecane
n-Eicosane
n-Heneicosane
n-Docosane
n-Tricosane
n-Tetracosane
n-Pentacosane
n-Hexacosane
n-Heptacosane
n-Octacosane
n-Nonacosane
13-Methylnonacosane
7-Methylnonacosane
n-Triacontane
n-Hentriacontane
9-, 11-, 13-Methylhentriacontane
7-Methylhentriacontane
Peak”
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
1.5
1.4
1.4
0.9
1.0
0.7
0.8
0.8
2.7
1.8
16.2
2.1
0.6
1.9
6.5
1.3
2.2
3rd
Instar
2.8
0.4
0.7
0.2
0.9
0.4
0.6
0.7
0.7
2.1
1.7
15.3
1.6
0.6
1.8
5.1
1.2
1.8
1.5
1.4
1.3
1.0
0.7
0.6
0.7
0.7
3.2
1.6
11.0
2.3
0.7
1.4
2.9
1.3
3.0
0.4
0.5
0.3
0.8
0.3
0.2
2.6
0.7
5.5
0.7
3.9
1.5
0.4
0.5
1.7
3.5
Percentage composition‘
4th
5th
Instar
Instar
Pupae
0.8
1.4
1.0
0.5
0.5
0.3
0.3
1.1
0.5
10.8
0.9
12.9
0.2
0.2
0.5
1.5
0.7
Adult
(continued)
140,337,450
168,309,450
196,281,450
112,344,450
434
268
282
296
310
324
338
350
364
380
394
408
196,253,422
112,337,422
420
Diagnostic mass
spectral
ion fragment
TABLE 1. Composition of the Cutidar Hydrocarbons of the Southern Armyworm S. eridattia Third, Fourth, and Fifth h t a r Larvae;
Pupae and Adults*
35.67
11,23-Dimethylpentatriacontane
28
2.1
39.6
60.4
2.3
32.5
67.5
12.8
0.4
1.2
2.3
2.3
23.2
4.5
25.4
74.6
14.0
13.8
2.8
29.5
70.5
2.6
1.8
2.8
6.2
17.0
0.6
1.2
2.6
2.1
25.3
5.1
34.6
65.4
29.8
0.7
2.4
3.4
6.8
6.5
0.9
1.1
0.7
0.9
7.8
2.5
3.7
1.7
2.3
14.6
0.4
1.3
1.3
a.7
14.I
0.6
1.4
1.2
0.9
14.7
Adult
Percentage composition‘
4th
5th
Instar
Instar
Pupae
168,351,490
168,365,504;
196,337,504
168,196,
351,379,518
462
140,365,476;
168,337,476;
196,309,476
168,196,323,
351,490
476
450
Diagnostic mass
spectral
ion fragment
*Theinsects used were 24 hr-oId after the molt to the stage indicated.
“Peak number refer to peaks identified in Figure 1.
bECL = equivalent chain length.
‘Number represent the average of three determinations, 10 insects per group. The range of values were less than 20% of the mean.
35.75
10.8
34.00
34.15
34.32
35.31
n-Tetratnacontane
Unknown
11-Methyltetratriacontane
11-, 13-Methylpentatriacontane
24
25
26
27
29
Unknown
Total straight chain hydrocarbons
Total methyl-branched hydrocarbons
0.2
1.1
1.9
2.5
33.61
11,21-Dimethyltritriacontane
23
18.8
32.00
32.35
32.65
33.00
33.32
n-Dotriacontane
Unknown
Unknown
n-Tritriacontane
9-, 11-, 13-Methyltritriacontne
18
19
20
21
22
0.6
1.7
1.0
1.3
14.4
ECLb
Component
3rd
Instar
Peak’
TABLE 1. (Continued)
24
Cuo and Blomquist
steadily from third (60.4%) instar larva to fifth instar larva (70.5%),and with
the pupa containing the highest percentage of methyl-branched hydrocarbon
(74.6%)(Table 1).
GC-MS was used to characterize each of the major components. The mass
spectra of GC peaks (Fig. 1)22 and 23 are presented in Figure 2A and B and
they are representative of the spectra obtained. Under electron impact,
methylalkanes fragment preferentially on either side of a methyl branch. The
mass spectrum of peak 22 (Fig. l),which has the ECL 33.32, is identified as a
mixture of 9-, 11-and 13-methyltritriacontane (Fig. 2A). The major isomer appears to be the 11-methyl component which gives rise to ions at miz 1681169
and 336/337. The strong m/z 168/169 and 336/337 ions are interpreted as arising from a-cleavage on either side of the methyl-branch point. The ions at m/z
168 and 336 result from the loss of a hydrogen atom from the fragment at 169
and 337, respectively. The preferred a-cleavage is the one which loses the bigger radical, the 22-carbon tail. Isomers with methyl groups at positions 9 and
13 are also present.
Peak 23 from Figure 1is interpreted as 11,21-dimethyltritriacontane(Fig. 2B).
The two clusters with a significant even-mass ion at m/z 1681169 and 1961197
are interpreted as arising from the a-cleavage internal to the methyl-branch
points which yield a 12-carbon and a 14-carbon fragment ion, respectively.
The two ion clusters with only a significant odd-mass ion at m/z 351 and 379
are interpreted as arising from the a-cleavage external to the methyl-branch
points yielding a 25-carbon and a 27-carbon fragment ion, both of which have
a methyl branch which suppresses the formation of the even-mass ions [3].
The n-alkanes were identified by GC retention times and their characteristic
mass spectra. The n-alkanes have carbon numbers from C19 to C34 with odd
numbered components predominating, mainly C27, C29, and C31 (Table 1).
The mass spectra of the cuticular hydrocarbon from fourth instar larvae and
pupae were also obtained and the same hydrocarbon components were present as in fifth instar larvae.
Accumulation of Hydrocarbon
Essentially all of the cuticular hydrocarbons on fourth instar larvae just prior
to ecdysis were recovered on the cast skins (Table 2). Of the 11.0 2 1.2 pg of
internal hydrocarbon in the fourth instar larvae, 3.7 ? 0.7 pg were found in the
cuticular hydrocarbons of newly emerged fifth instar larvae and 7.1 ? 0.8 were
recovered in internal extracts (Table 2). These data strongly suggest that
Fig. 1. CC trace of the cuticular hydrocarbonsof the Southern armyworm, S. eridania 1-day-old
fifth instar larvae. The nomenclature used for GC peaks is described in Table 1.
25
Hydrocarbon Production in the Southern Armyworm
A
I
1111
I
168
,111 L
100
100
150
200
250
-
-
50
--
-
337
309
11.
365
It
II
111
I.
-
-
"I 96
1111
I II....,, I
II
.,,I
.
I
,I
. .I1
1
,,
I
Ill11
L
100-
-
-
I
I
351
196
-
50
--
I
323
-.
,I.
.#I.
I
I
'
'
11,
351
'
,
'
I,
l
'
'
'
'
l
'
r
'
~
Fig. 2, Mass spectra of peak 22, Figure 1, identified as 9-, 11-, and 13-methyltritriacontane (A)
and peak 23, Figure 1 , identified as 11,21-dirnethyltritriacontane(B).
S. eridania loses all of its cuticular hydrocarbons during each molt and the internal hydrocarbon pool is used to provide hydrocarbons for the newly molted
insect. The amount of cuticular hydrocarbon stayed relatively constant during
each stadium while the amount of internal hydrocarbon increased dramatically during the first half of each stadium (Fig. 3 ) . During the fourth larval
stadium, both the amount of cuticular and internal hydrocarbons increased
l
Guo and Blomquist
26
TABLE 2. Amounts of the Hydrocarbon Present in S. eridunia During the Fourth to Fifth
Larval Ecdysis
Hydrocarbon mass (Fglinsect 2 SD)
Cuticular
Internal
Total
Stage
48 h after 3rd to 4th stadium ecdysis
Shed cast skin
Immediately following fourth to fifth stadium ecdysis
-
-
70
60
5.4 4 0.6
5.5 ? 0.6
-3.7 % 0.7
-
11.0
+ 1.2
16.42 1.8
7.1 -t 0.8 10.8 k 1.5
-
Cuticular hydrocarbons
Internal hydrocarbons
Fig. 3. Amounts of cuticular and internal hydrocarbons during fourth and fifth instar larval
and pupal development. E, ecdysis.
during the first 16 h, and then the amount of cuticular hydrocarbons remained
almost constant while the amount of internal hydrocarbon increased until the
next ecdysis. During the fifth stadium, the amount of cuticular hydrocarbons
remained essentially constant whereas the accumulation of internal hydrocarbon increased dramatically following the fourth to fifth stadium molt, and
then declined about 40 h later. During the pupal stage, the amount of cuticular hydrocarbon remained relatively constant, but the amount of internal hydrocarbon increased up to 24 h, leveled off until 96 h, and then declined
significantly (Fig. 3).
Biosynthesis of Hydrocarbon
The incorporation of [l-I4C]acetateinto hydrocarbons was used to estimate
the rate of h drocarbon formation. Large fluctuations in the percent incorporation of [I-'Clacetate into cuticular and internal hydrocarbons during fifth
stadium larval and pupal development were observed (Fig. 4). Consistent with
the data obtained from studies measuring hydrocarbon accumulation, a high
rate of incorporation of [1-l4C]acetateinto cuticular and internal hydrocarbons
Hydrocarbon Production in the Southern Armyworm
1.2
-
1.0
-
=
Cuticular hydrocarbons
Internal hydrocarbons
27
T
T
I
hours
0.0
E
48
E
4th
I
48
96
-
48
96
5th
LARVA
PUPA
144
192
-
Fig. 4. Incorporation of [1-’4C]acetate into cuticular and internal hydrocarbons during larval
and pupal development. E, ecdysis.
following the fourth to fifth instar molt was observed. Unlike the decline in
the amount of internal hydrocarbon of fifth instar larvae at about 40 h after
fourth to fifth stadium molt, the rate of incorporation of [l-14C]acetate into
internal hydrocarbon declined much earlier. The fifth instar larvae showed the
highest rate of hydrocarbon production at about 16 h after the fourth to fifth
stadium molt (Fig. 4).
Whereas almost no incorporation of [l-14C]acetateinto cuticular hydrocarbons was observed during pupal development, a dramatic increase of
[l-I4C]acetate incorporation into internal hydrocarbon was observed irnmediately after pupal ecdysis (Fig. 4). This decreased from 48-96 h after pu a
tion. After 96 h, there was again an increase in the incorporation of [l-lC]
acetate into internal hydrocarbons which remained high until the end of the
pupal stage. Other than at time points immediately following ecdysis and at
16 h after pupation, no labeled hydrocarbon was recovered in cuticular extracts
from pupae.
f-
DISCUSSION
The cuticular lipids of most insect species contain an isomeric mixture of
methyl-branched alkanes [2-41 which commonly include mono- and dimethylalkanes and less commonly tri- and tetramethylalkanes [16-181. The cuticular
n-alkanes of most insect species occur as continuous homologous series, and
n-C23, n-C25, n-C27, nC-29, and n-C31 usually predominate [4].The cuticular hydrocarbons of the Southern armyworm, S. eridania, contain n-alkanes
and both mono- and dimethyl-branched alkanes and have a high proportion
28
Cuo and Blomquist
of methyl-branched alkanes. The methyl-branched alkanes of S. eridaniu were
similar in structure to those found in many other insects [l-41, including different families of insects. However, some common hydrocarbon components
in insects, such as 2-methyl and 3-methylalkanes and alkenes were not found
in S. eridaniu.
The amount of methyl-branched hydrocarbons vary considerably among different insect species. The methyl-branched hydrocarbons range from 98% of
the cuticular hydrocarbons in 7'. ni [19] to 2% in Eurychora sp. "1. This may be
related to their ability to prevent water loss. In a comparison of three species
of tiger beetles, Cicindela obsoletu, C. oregonu, and C. tranquebaricu, Hadley and
Schultz [20] found that the species with the most hydrocarbon of the highest
saturation and methyl-branching had the least water loss. S. eridurtiu larval
cuticular hydrocarbons were comprised exclusively of saturated hydrocarbons
with 60-70% of them being methyl-branched components. The percentage increase of the methyl-branched hydrocarbon from third instar larva to fifth instar larva may suggest that as the insect size increases, the prevention of water
loss becomes more crucial. The pupal stage contains the highest percentage
of methyl-branched hydrocarbon and may reflect the fact that the pupal stage
is most sensitive to desiccation.
All of the cuticular hydrocarbon on the surface of S. eriduniu is apparently
lost on the cast skin during the fourth to fifth instar molt. About one-third of
the internal hydrocarbons of S. eridania just prior to ecdysis were present on
the cuticle of the newly emerged insect and about two-thirds of them remained
internal. In a study with the cabbage looper, T. ni, it was shown that the cabbage looper doesn't reabsorb its cuticular hydrocarbons prior to molting. Instead, in the three different types of molts examined (larva-larva [ll]; larva-pupa
and pupa-adult [7]) they are discarded with the cast skins. The larva-larva
molt of S. eridunia also showed that no cuticular hydrocarbons were reabsorbed
and suggested that this may be a common phenomena in insects.
Hydrocarbons are synthesized by the cells associated with the epidermis,
probably the oenocytes [3-41 and are subsequently transferred to an undetermined internal storage site. In several insect species, including the American
cockroach and migratory locust [21,22], a major lipid class associated with
lipophorin is hydrocarbon, and the hydrocarbons associated with the lipophorin
have the identical components as do the cuticular hydrocarbons. Thus, lipophorin
is suggested as a storage site for hydrocarbons. At present, the exact role of
lipophorin in the storage and transportation of cuticular hydrocarbon is not
clear. In S. eridania, the large fluctuation in the accumulation and synthesis of
hydrocarbons observed during the last larval and pupal stage clearly indicated
the existence of a regulatory mechanism for the production and deposition of
hydrocarbon on the cuticle.
In both the last larval and pupal stage, the decline in the rate of incorporation of [l-14C]acetateinto internal hydrocarbon occurs much earlier than does
the decrease in internal hydrocarbon accumulation. Thus as the rate of hydrocarbon synthesis decreases, the insects accumulate more hydrocarbon internally.
From studies with T. ni, de Renobales et al. [7] suggested the existence of
two pools of internal hydrocarbon, one destined for the cuticle (synthesized
some time before)and another one that remains internal at all times. The study
reported here on S. eridania is consistent with this suggestion. The variation
Hydrocarbon Productionin the Southern Armyworm
29
in accumulation during fifth instar larval and pupal stadium may only reflect
the change in one of the internal hydrocarbon pools. In several other insect
species studied [7-9, 111, the developmental regulation of hydrocarbon biosynthesis and accumulation was also observed.
The relationship between the regulation of hydrocarbon synthesis and the
regulation of fatty acid synthesis in insects needs to be further studied. In
T. ni, during the fifth larval instar, FAS activity correlated with the synthesis
of hydrocarbons and other lipid classes. However, throughout the pupal stage,
negligible level of FAS activity was found in spite of the very high rate of hydrocarbon formation [23]. This suggested that the biosynthesis of long-chain
methyl-branched hydrocarbons is by a different system than that of fatty acids
in T. ni. Further work with S. eridaniu is necessary to understand the enzymology, regulation, and transport of hydrocarbons.
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hydrocarbonic, lepidopteranoctuidae, southern, identification, eridani, spodoptera, armyworm, cramer, accumulation, biosynthesis, cuticular
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