A sex-specific size  number tradeoff in clonal broods Yoriko Saeki, Philip H. Crowley, Charles W. Fox and Daniel A. Potter Y. Saeki (yoriyoripp@uky.edu) and P. H. Crowley, Dept of Biology, Univ. of Kentucky, 101 T H Morgan Building, Lexington, KY 40506- 0225, USA.  C. W. Fox and D. A. Potter, Dept of Entomology, Univ. of Kentucky, USA. 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. The number and body size of progeny within broods are among the most studied traits in life history tradeoffs (Smith and Fretwell 1974, Stearns 1989) and should generally be inversely related when resources are limited (Sibly and Calow 1986). This numbersize tradeoff has been documented in plants (Stuefer et al. 2002), inverte- brates (reviewed by Fox and Czesak 2000), and vertebrates (Mappes and Koskela 2004). However, attempts to detect a sizenumber tradeoff may prove unsuccessful because of (1) the absence of a tradeoff (e.g. when brood size is genetically fixed), or because of masking of the tradeoff by (2) resource variation (van Noordwijk and de Jong 1986, Roff and Fairbairn 2007), (3) an interaction between resource variation and parental reproductive behavior (Mayhew 1998, Mayhew and Glaizot 2001), (4) effects of another trait mediating the tradeoff (e.g. development time: Klingenberg and Spence 1997), (5) other fitness compo- nents influencing the tradeoff (e.g. future reproduction: Lack 1947), (6) continuous acquisition of resources during reproduction (Fox and Czesak 2000), or (7) competition among offspring (Godfray and Parker 1991). In this study, we were able to by-pass many of these issues to address possible sex-differences in the sizenumber tradeoff. Sex-specific tradeoffs have been documented in previous work (Fischer and Fiedler 2000) but not to our knowledge for the sizenumber tradeoff, though two studies have demonstrated declines in parasitoid body size with greater brood size. The usual genetically diverse and mixed- gender broods, involving parentoffspring and sibling conflicts (Godfray 1986) and sometimes differential provi- sioning of offspring by gender (West and Sheldon 2002, Young and Badyaev 2004), can obscure or render moot any such gender differences in the sizenumber tradeoff. Here, we focus on the polyembryonic parasitoid Copi- dosoma bakeri, which produces large broods of clonally identical (and thus all male or all female) offspring. For endoparasitic wasps in general, food intake and biomass of the host constitute the resources used to produce a wasp brood. Interactions between endoparasitoids and their hosts affect parasitoid fitness through survival to the adult stage, development time, and adult body mass (Mackauer and Sequeira 1993, Godfray 1994). To increase their fitness, koinobiont endoparasitoids, which develop inside a host that continues to grow, may actively manipulate host growth (‘host regulator’; Vinson and Iwantsch 1980) or may passively adjust their own development to the host’s growth (‘host conformer’; Lawrence 1986). Parasitoids may interfere with the host’s endocrine system (Beckage and Riddiford 1982), increase the host’s dietary assimilation efficiency (Slansky 1978), alter its nutritional status (Vinson and Iwantsch 1980, Slansky 1986), and ameliorate or neutralize host defenses (Beckage 1997). Also, parasitoids Oikos 118: 15521560, 2009 doi: 10.1111/j.1600-0706.2009.17571.x, # 2009 The Authors. Journal compilation # 2009 Oikos Subject Editor: Matt Ayres. Accepted 3 April 2009 1552