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Evidence for adaptive changes in egg laying in crickets exposed to bacteria and parasites SHELLEY A.
initial acceptance 15 April 1998
Animals should increase their present reproductive output if their chances for future reproduction are low.
an animal’s ability to make this adjustment may be constrained by the physiological mechanisms mediating the response.
To examine this hypothesis,
I infected 2- and 5-week-old female crickets,
with either a pathogen (the bacterium Serratia marcescens) that induces antimicrobial immune responses,
or a parasite (larvae of the parasitoid fly Ormia ochracea) that induces an encapsulation immune response.
Females of both age groups infected with bacteria laid more eggs the day after injection than did saline-injected crickets.
A similar increase was elicited by injecting components of the bacterial cell wall (lipopolysaccharides).
The bacteria-induced increase in egg laying (1) was not the result of physical stress,
and (3) was probably not mediated by octopamine.
Females did not increase egg laying when infested with O.
even though this parasitoid invariably kills its host.
Injections of Sephadex beads,
which induced an immune response similar to that created by the parasitoids,
also had no effect on egg laying.
These results are consistent with the hypotheses that crickets can increase egg laying in response to infection and that increased egg output correlates with the activation of some,
that the host increases reproduction in response to infection (Minchella & Loverde 1981).
Oppliger et al.
lay more eggs when infected with malarial parasites,
although another study on the same species found that parasitized birds did not have larger clutches (Allander & Bennett 1995).
is positively correlated with the number of hematozoans in the blood (Sorci et al.
this study did not manipulate parasite levels,
and the authors were unable to determine whether higher parasite levels were caused by increased reproductive effort (shown to increase parasitic levels by Richner et al.
or whether increased reproductive effort was an adaptive response of the host to the presence of the parasites.
and are often infected by chronic,
nonlethal parasites and pathogens.
it is unclear what the animal’s best reproductive strategy will be in these cases (e.g.
but see Perrin & Christe 1996).
animals increase their reproductive output in response to
Parasites and pathogens can cause catastrophic declines in an animal’s reproductive fitness (e.g.
natural selection should strongly favour behaviours that prevent infection and/or that minimize its effects on fitness (Minchella 1985
snails infected with parasites increase their reproductive output,
thus reducing the fitness costs of serious illness (Minchella 1985
placing all of an individual’s resources into the present bout of reproduction is usually selected against because it diminishes future reproduction,
and thus lifetime fecundity (Williams 1966
If the probability of future reproduction declines,
it would be adaptive for an animal to increase its reproductive output at the expense of survival (Minchella 1985
Sorci et al.
few examples support this hypothesis.
infected with the trematode Schistosoma mansoni lay more eggs prior to parasitic castration than do uninfected snails,
NS B3H 4J1,
Canada (e-mail: [email protected]).
I examined these questions in an animal in which the benefits to the host of increased egg laying are unambiguous.
Murtaugh & Denlinger 1985,
its life span in the field is unknown.
Egg production in A.
domesticus occurs continuously in the adult female (Woodring et al.
eggs are stored in the lateral oviducts (10–100,
adult females of the related cricket,
retention of mature eggs is not an artefact of laboratory conditions.
domesticus females lay eggs daily for several weeks (Woodring et al.
Murtaugh & Denlinger 1985).
eggs are fertilized and laid in batches (Sugawara & Loher 1986).
an increased output of fertilized eggs translates directly into more offspring.
Because of the reproductive biology of A.
increases in egg output could occur by increased oviposition with or without increased provisioning of eggs.
domesticus could increase its reproductive output with less increase in total reproductive effort than would be required in many other animals.
this species might be expected to show an increase in egg laying in response to a decline in the possibility of future reproduction.
Whether an animal that increases its reproductive output in response to one pathogen,
has the ability to increase reproductive output in response to any parasite or pathogen,
depends on the underlying physiological mechanism.
There may be constraints on the range of organisms to which the animal can increase reproduction.
if the immune system mediates behavioural responses to infection,
organisms that activate different components of the immune system may induce different changes in host behaviour.
Whether a specific immune pathway is selected to influence reproductive behaviour will depend on competing selection pressures on that pathway.
In this study,
domesticus adaptively alter egg output,
and (2) whether two organisms that elicit different immune responses in the host (i.e.
a bacterium and a parasitoid) have different effects on host reproduction.
domesticus females either a bacterial infection (Serratia marcescens) or a parasitic infestation (Ormia ochracea),
and monitored the effect on egg output.
Serratia marcescens is a gram-negative,
red-pigmented bacterium found world-wide in both soil and water,
and has been recovered from six different orders of insects in the field,
including crickets (Steinhaus 1959).
for many years (Walker & Masaki 1989),
but the bacterium is not lethal unless it enters the hemocoel (Steinhaus 1959).
marcescens succumbed to the infection.
The parasite (parasitoid) used in this study was the larvae of the tachinid fly,
female fly uses acoustic cues to find male crickets (Cade 1975),
both male and female crickets are infested in the field (Walker & Wineriter 1991
during which time they consume muscle and fat body,
but tend to spare reproductive tissue,
especially ovaries (Adamo et al.
the fly larvae use the host’s immune response to build a structure for themselves called the respiratory funnel (Vinson 1990).
although both the bacterium and the parasitoid can decrease a cricket’s reproductive fitness by killing it,
each organism induces a different type of immune response (Gillespie et al.
I used lipopolysaccharides,
derived from the cell wall of S.
to stimulate some of the host’s antimicrobial immune responses (Armstrong 1991).
I studied the effects of different kinds of stress on egg laying to test whether the change in egg laying might be a nonspecific response to the stress of infection.
Animals The A.
domesticus crickets used in this study came from a long-established laboratory population.
The animals were reared in mixed-sex groups at 22C on a 12:12 h light:dark cycle and were supplied with dried cat food and water ad libitum.
My co-workers and I removed 2-week-old,
inseminated females from the colony,
and placed them in separate containers (9 cm diameter,
We counted the number of eggs each female laid daily for 5 days.
We randomly assigned the remaining animals to different treatments,
Effect of S.
marcescens on Egg Laying To determine whether bacterial infection increases the number of eggs laid by females,
we randomly divided the isolated females into four groups: (1) unhandled controls,
(2) controls injected with Grace’s medium,
(3) crickets injected with 100 ìg/cricket of lipopolysaccharides (LPS) derived from the bacterium S.
marcescens (Sigma) and dissolved in Grace’s sterile medium,
this dose of LPS is 1/30 of the amount required to elicit behavioural fever in the beetle Onymacris plana (McClain
(4) crickets injected with 5104 cells of S.
marcescens in Grace’s medium,
this dose is less than the LD50 (personal observations).
I determined cell dilution using a Petroff Hausser cell counter.
made using a 10-ìl Hamilton syringe.
Crickets that died from the bacterial injection did so 12–36 h later.
we recorded the ratio of nymphs produced to the number of eggs laid the day following treatment.
To determine whether the effect of bacterial infection on egg laying depends on female age,
we injected 2- and 5-week-old female crickets with either saline (N=9/group) or S.
We recorded the number of eggs laid by each female as described above.
ochracea on Egg Laying To determine whether parasites induce an increase in egg laying,
we divided 2-week-old female crickets into the following four groups.
I removed the larviparium of gravid flies and transferred three fly larvae to each cricket.
I placed the larvae on the membrane below the pronotum.
but did not place fly larvae on these animals.
We injected the third group (N=25) with approximately 25 A-25 Sephadex beads (Sigma) in 5 ìl Grace’s medium.
ochracea larvae emerged from their hosts (8 days after infestation).
I examined the Sephadex beads using phase contrast microscopy to determine whether they had been encapsulated (Lavine & Beckage 1995).
ochracea-infested crickets had respiratory funnels,
which are formed as a result of the cricket’s immune response (i.e.
Effect of Three Different Stressors on Egg Laying To test whether the response to bacteria and/or O.
ochracea might be part of a generalized stress response,
we exposed crickets to two acute stresses (physical tumbling and 5 min of enforced exercise),
and one chronic stress (food deprivation).
We tumbled the second group (N=81) at approximately 10 rpm for 15 min while they were inside their individual cages.
We exercised the third group (N=18) by placing them into a clear drum that was rotated at approximately 4 rpm,
which causes the crickets to run constantly to avoid being flipped.
we withheld food from the crickets for 2 days,
although water remained available.
This treatment did
not increase cricket mortality compared to unhandled controls (0 crickets died in either group).
We recorded the number of eggs laid the day following treatment.
insect stress responses (see Woodring et al.
Adamo et al.
and the control of egg laying (Lange & Orchard 1986),
as well as participating in the immune system of cockroaches and lepidopterans (Baines et al.
To examine whether octopamine is involved in mediating the bacteria-induced increases in egg laying,
we divided isolated female crickets into six groups.
an antagonist of octopamine in insects (Evans 1985).
We injected the fourth group (N=10) with both OA (2.5 ìl of 210 2 M) and PA (2.5 ìl of 210 2 M) in one injection.
We injected the fifth group (N=10) with both LPS (100 ìg/cricket) and PA (2.5 ìl of 210 2 M) in one injection.
We injected the sixth group (N=11) with 10% ethanol and 90% insect saline,
Statistical Analyses The data were not normally distributed.
I used SYSTAT for some tests and ranking of scores.
I tested all randomly assigned groups (using a Kruskal–Wallis test) and found that they did not differ in their pretreatment rate of egg laying.
I corrected alpha values to account for multiple tests when appropriate (Meddis 1984).
RESULTS Female crickets removed from the colony at 2 weeks of age laid approximately 18 eggs/day for at least the next 5 days (Fig.
There was no evidence for an increase or decrease in the number of eggs laid over a 5-day period (test for trends,
Effect of S.
marcescens on Egg Laying Female crickets given either S.
marcescens or lipopolysaccharides (LPS) derived from S.
marcescens laid more eggs than saline-injected animals (test for trends: z=3.41,