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1999,

Article No.

available online at on Evidence for adaptive changes in egg laying in ...

Description

ANIMAL BEHAVIOUR,

1999,

available online at http://www.idealibrary.com on

Evidence for adaptive changes in egg laying in crickets exposed to bacteria and parasites SHELLEY A.

ADAMO

Department of Psychology,

Dalhousie University (Received 17 February 1998

initial acceptance 15 April 1998

Animals should increase their present reproductive output if their chances for future reproduction are low.

However,

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,

Acheta domesticus,

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.

Parus major,

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).

The reproductive output of the common lizard,

Lacerta vivipara,

is positively correlated with the number of hematozoans in the blood (Sorci et al.

1996).

However,

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.

1995),

or whether increased reproductive effort was an adaptive response of the host to the presence of the parasites.

Many of the animal systems in which this question is currently being addressed are complex.

Subjects have multiple breeding seasons,

and are often infected by chronic,

nonlethal parasites and pathogens.

Although longevity can be reduced by these chronic conditions (Richner et al.

1995),

it is unclear what the animal’s best reproductive strategy will be in these cases (e.g.

Forbes 1993

but see Perrin & Christe 1996).

To determine whether,

animals increase their reproductive output in response to

Parasites and pathogens can cause catastrophic declines in an animal’s reproductive fitness (e.g.

Brown & Reed 1997).

Therefore,

natural selection should strongly favour behaviours that prevent infection and/or that minimize its effects on fitness (Minchella 1985

Hart 1988).

For example,

snails infected with parasites increase their reproductive output,

thus reducing the fitness costs of serious illness (Minchella 1985

De Jong-Brink 1995).

In iteroparous animals,

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

Minchella & Loverde 1981).

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

Forbes 1993

Sorci et al.

1996).

Although this seems logical,

few examples support this hypothesis.

Snails,

Biomphalaria glabrata,

infected with the trematode Schistosoma mansoni lay more eggs prior to parasitic castration than do uninfected snails,

Adamo,

Department of Psychology,

Dalhousie University,

Halifax,

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.

The cricket Acheta domesticus lives about 60 days as an adult in the laboratory (Woodring et al.

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.

Renucci & Strambi 1983),

eggs are stored in the lateral oviducts (10–100,

In the field,

adult females of the related cricket,

Gryllus integer,

Murray,

retention of mature eggs is not an artefact of laboratory conditions.

After insemination,

domesticus females lay eggs daily for several weeks (Woodring et al.

Murtaugh & Denlinger 1985).

During oviposition,

eggs are fertilized and laid in batches (Sugawara & Loher 1986).

There is no parental care (Bate 1971)

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.

Therefore,

domesticus could increase its reproductive output with less increase in total reproductive effort than would be required in many other animals.

For these reasons,

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.

For example,

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,

I used three different approaches to examine: (1) whether infected A.

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.

First,

I gave A.

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).

Crickets have probably been in contact with Serratia spp.

for many years (Walker & Masaki 1989),

but the bacterium is not lethal unless it enters the hemocoel (Steinhaus 1959).

In our experiment,

marcescens succumbed to the infection.

The parasite (parasitoid) used in this study was the larvae of the tachinid fly,

Although the

female fly uses acoustic cues to find male crickets (Cade 1975),

both male and female crickets are infested in the field (Walker & Wineriter 1991

Adamo et al.

1995c).

The fly larvae develop within the cricket for approximately 9 days,

during which time they consume muscle and fat body,

but tend to spare reproductive tissue,

especially ovaries (Adamo et al.

1995b).

The fly larvae elicit an immune response in the host called encapsulation.

Although this immune response usually asphyxiates multicellular intruders,

the fly larvae use the host’s immune response to build a structure for themselves called the respiratory funnel (Vinson 1990).

The host dies after the fly larvae exit the cricket (Adamo et al.

1995b).

Thus,

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.

1997).

Second,

I examined the effect of stimulating different types of immune responses on egg laying.

I used lipopolysaccharides,

derived from the cell wall of S.

to stimulate some of the host’s antimicrobial immune responses (Armstrong 1991).

I used Sephadex beads to induce an immune response similar to that induced by the parasitoids (i.e.

Lavine & Beckage 1995).

Third,

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.

METHODS

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.

During the first 2 days we determined baseline rates of oviposition for each individual.

We returned to the colony any female that did not lay eggs during the first 2 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

ADAMO: ADAPTIVE CHANGES IN HOST EGG LAYING 119

1988)

(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.

The volume of all injections was 5 ìl,

made using a 10-ìl Hamilton syringe.

We injected crickets by inserting the needle into the membrane below the pronotum.

Crickets that died from the bacterial injection did so 12–36 h later.

To determine the hatching success for each group,

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.

Effect of the Parasitoid O.

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 infested the first group (N=12) with larvae of the tachinid fly,

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.

I handled the second group (N=12) in a manner similar to the infested group,

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.

We injected the fourth group with 5 ìl Grace’s medium (N=28).

Each day we recorded the number of eggs laid by each cricket until the O.

ochracea larvae emerged from their hosts (8 days after infestation).

The crickets die soon after parasitoid emergence (Adamo et al.

1995b).

At the end of the experiment,

I dissected the crickets.

I examined the Sephadex beads using phase contrast microscopy to determine whether they had been encapsulated (Lavine & Beckage 1995).

I noted whether O.

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 divided isolated 2-week-old females into four groups.

The first group (N=40) consisted of an unhandled control.

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.

In the fourth group (N=85),

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.

Effect of Octopamine on Egg Laying Octopamine is involved in orthopteran energy regulation,

insect stress responses (see Woodring et al.

Orchard et al.

Adamo et al.

1995a),

and the control of egg laying (Lange & Orchard 1986),

as well as participating in the immune system of cockroaches and lepidopterans (Baines et al.

Dunphy & Downer 1994).

To examine whether octopamine is involved in mediating the bacteria-induced increases in egg laying,

we divided isolated female crickets into six groups.

We injected the first group (N=38) with 100 ìg/cricket of LPS.

We injected the second group (N=18) with 10 2 M octopamine (OA) dissolved in insect saline.

We injected the third group (N=35) with 10 2 M phentolamine (PA),

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,

All injection volumes were 5 ìl.

We recorded the number of eggs laid the day before and the day after injection.

Statistical Analyses The data were not normally distributed.

Therefore,

I analysed the data using nonparametric statistical tests (Meddis 1984).

Values are given as medians and 1st and 3rd quartiles.

I used SYSTAT for some tests and ranking of scores.

Prior to treatment,

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,