| dc.description.abstract | The desert locust, Schistocerca gregoria is a major agricultural pest especially in the
gregarious phase when it can cover an infestation area of approximately 20% of the total land
mass. The solitary phase on the other hand, occur in low densities with discontinuous
distribution of no economic importance. The two locust phases differ in their anatomy,
behaviour, morphology, pigmentation, physiology and biochemistry. Swarming behaviour, a
characteristic feature of the gregarious locusts, is associated with long distance flights of upto
5000 km.
Locusts mainly use carbohydrates at the initiation of flight while, lipid is the main
fuel in prolonged flight. Thus, lipid metabolism is of prime importance in the maintenance
of long distance flights as in locust swarms. Due to the high flight activity in swarming
(gregarious) locusts, in addition to the need for adequate lipid reserve, locusts require an
efficient system for lipid mobilization, transport through the haemolymph and the ultimate
utilization at the flight muscles. Although lipid metabolism plays a crucial role in prolonged
flight, only limited studies have been carried out to determine how phase transition in locusts
affects lipid metabolism.
In order to understand how lipid metabolism is affected by phase transition in locusts,
lipophorin, the major insect haernolyruph lipoprotein involved in the transport of lipids from
the fat body to the flight muscles, was chosen for the study. Differences between the two
phases could in the long run be exploited in designing novel control strategies. Furthermore,
it could also be important in distinguishing the locust phases either from field isolates or
laboratory reared colonies.
High- and low density- lipophorin (HDLp and LDLp) were isolated from both the
solitary and gregarious S. gregaria by KBr density gradient ultracentrifugation (206,000 xg,
4° C, 4 h). HDLp isolated from the two phases had identical native molecular size of M, -
620,000 as determined by non-denaturing-PAGE. Analysis by SDS-PAGE showed the
presence of similar subunits, apolipophorin-I (apoLp-I, M, - 224,00) and apolipophorin-II
(apoLp-II, M, - 81,000). Both apoproteins were shown to be glycosylated with mannose
rich oligosaccharide chains. LDLp isolated from both phases showed the presence of an
additional apoprotein, apolipophorin-III (apoLp-III, M, - 20,000). The lipid percent content
of the lipoproteins was determined gravimetrically in both phases. Further analysis of lipid
moiety by gas chromatography indicated that the major lipid classes present in both HDLp
and LDLp were phospholipids and diacylglycerides. Diacylglycerides in HDLp constituted
43.5 and 35.5 % of the total lipids in gregarious and solitary locusts, respectively. On the
other hand, it constituted 60 and 48 % in LDLp of the gregarious and solitary locusts,
respectively. Immunological cross-reactivity was demonstrated between the lipoproteins
isolated from the two phases as well as with antibodies raised against HDLp from Locusta
migratoria.
Haemolymph titres of lipophorin during the development of the locusts were estimated "
by single radial immunodiffusion. Starting dom the third nymphal instar, there was a
general increase in the levels as the insect developed, with the gregarious locusts having
higher titres of the protein. r Exceptions were observed during the fifth nymphal instar of the
solitary locusts, when there was a large increase, which was higher than in the corresponding
gregarious stage.
ApoLp-III was purified from LDLp by exploiting its solubility properties, stability to
heat in addition to affinity chromatography on concanavalin-A-Sepharose. Analysis using
SDS-PAGE showed that, apoLp-lII isolated from the two phases had similar molecular size
(M, - 20,000). However, analysis on non-denaturing PAGE showed the presence of two
isoforms in each case. Immunological cross-reactivity was demonstrated between apoLp-III
isolated from both phases as well as with antibodies raised against L. migratoria apoLp-III.
Estimation of the lipid reserve available in the locust fat body revealed that the
gregarious locusts had a higher lipid reserve (79.02 ± 2.77%) as compared to the solitary
ones (64.75 ± 2.55 %). Analysis of the fat body lipids by gas chromatography revealed that
triacylglycerides was the major lipid type and constituted 83.9% and 73.85% of the total
lipids in solitary and gregarious locusts, respectively.
Lipid mobilization in response to administration of adipokinetic hormone (AKH), was
demonstrated in both phases. A maximum response was observed 90 min after the hormone
administration when the diacylglyceride levels reached a peak value. Furthermore, evidence
of lipoprotein shift was obtained using gel permeation chromatography on an AcA22 column.
The response in gregarious locusts, led to the formation of a larger LDLp molecule than that
formed in the solitary locusts. Increase in' particle size was shown to be due to increase in
diacylglyceride levels as well as association with more apoLp-III molecules. The response to
a range of doses of AKH showed that the gregarious locusts were more sensitive to low
doses below 2 pmol and the response was more tightly controlled than in solitary locusts.
Thus, whereas gregarious locusts had a maximum response from 5 pmol, the solitary locusts
peaked above 10 pmol. The EDso was estimated to be 6.60 and 1.53 pmol for solitary and
gregarious locusts, respectively.
Activities of the fat body triacylglyceride lipase was studied using 14C-Iabelled triolein
as the substrate tracer. The resting levels of the lipases was estimated to be 1.98 ± 0.24 and
1.95 ± 0.53 nmol/h/mg protein for solitary and gregarious locusts, respectively. Evidence
of the activation of the lipase in response to AKH administration was deduced. Thus,
administration of 2 pmol of AKH resulted in the elevation of the levels to 2.47 ± 0.39 and
2.30 ± 0.43 nmollh/mg for gregarious and solitary locusts, respectively. Estimation of
kinetic parameters showed that the solitary locusts Kill value was 18.75 compared to 46.67
fLM for the gregarious locusts. The Vlllax was estimated to be 2.52 and 10.29 nmollh/mg for
solitary and gregarious locusts, respectively. The lipase from the gregarious locusts showed
higher catalytic ability but lower affinity for the substrate. These properties of the enzyme
may reflect a physiological adaptation arising from their metabolic requirements . | en |
