A sex‐specific size–number tradeoff in clonal broods

Abstract

Polyembryonic parasitoids producing single‐sex broods of clonal offspring provide an unusually clear window into the classic tradeoff between the number and size of offspring. We conducted a laboratory study of the encyrtid parasitoid Copidosoma bakeri parasitizing the noctuid Agrotis ipsilon to examine the way that size and number of offspring tradeoff in broods of each sex and to determine how the fit between host and parasitoid brood is achieved. We found that brood mass (wasp body mass ×brood size) was proportional to host mass, independent of brood sex, indicating a tight fit between brood and host and ensuring a size–number tradeoff. By correcting brood size and body mass of each brood for host mass, we demonstrated the expected inverse relationship between wasp variables. We postulated that the wasp brood might achieve the fit to the host by (1) adjusting brood size based on information available early in host development before and during division of the embryo, (2) manipulating host size late in host development after completion of embryo division, or (3) simply adjusting individual wasp mass to fill the host. We evaluated host responses to parasitism – and correlations between brood size and host growth early and late in development – for broods of each sex. The data are consistent with adjustment of brood size to the amount of host growth early in host development and with manipulation of host mass late in host development. The tight link between host mass and brood mass also suggests a final adjustment by parasitoid growth to achieve complete filling. Within the tight fit, female broods were smaller but contained larger individuals than male broods. The sex‐specific balance point of the tradeoff and sex differences in balancing mechanisms and responses to host size suggest different selection pressures on each sex requiring future investigation.

Balancing the Size-Number Tradeoff in Clonal Broods

“…The independence coefficient b could not be directly estimated and was therefore set to a magnitude that produced an optimal brood size near values observed in recent empirical work (approximately 1800 female wasps per brood in female C. bakeri- Saeki et al 2009). Despite this free parameter, the model's behavior was strongly constrained by its structure and the relevant ranges of the other parameters.…”

“…Of particular interest are the results of Saeki et al (2009) on the relation to host mass H of both brood size n and body (5B and 8B) for females (but not males) is striking. If, as we expect, a > 1 and x < 0, then the pattern of data in Fig.…”

“…In a recent article, Saeki et al (2009) presented results of empirical work examining the size-number trade-off in Copidosoma bakeri. They were able to document the tradeoff and the mechanisms used by the developing broods to balance the trade-off differently for male and female broods.…”

Endophyte-mediated tritrophic interactions between a grass-feeding caterpillar and two parasitoid species with different life histories

“…The fly, Linnaemya comta (Fallen) (Diptera: Tachinidae) (formerly: Bonnetia comta [Fallen]; O'Hara and Wood 2004), is a rapidly developing solitary parasitoid wherein only the larval stage develops within the caterpillar host. The polyembryonic wasp, Copidosoma bakeri (Howard) (Hymenoptera: Encyrtidae), develops more slowly from egg to adult within its host, producing large single-sex broods of clonally identical offspring whose mass and number is adjusted to efficiently exploit the host (Saeki et al 2009). We predicted that both parasitoid species would be adversely affected when developing within a caterpillar feeding on endophytic grass, but that the consequences would be greater for C. bakeri because of its relatively long and intimate relationship with the host.…”

“…This study compared the intensity of endophyte-mediated tritrophic effects on two parasitoid species having different life history strategies that exploit a common host, A. ipsilon, in turfgrass settings (Saeki et al 2009;BixbyBrosi and Potter 2010). The fly, Linnaemya comta (Fallen) (Diptera: Tachinidae) (formerly: Bonnetia comta [Fallen]; O'Hara and Wood 2004), is a rapidly developing solitary parasitoid wherein only the larval stage develops within the caterpillar host.…”