Gamete number and size correlate with adult size in the egg parasitoid Trichogramma euproctidis

Abstract

International audienceIn parasitoids, the size of the adult is influenced by the size and quality of the host in which it develops. Body size is generally positively correlated with several adult fitness proxies (fecundity, longevity, and mating capacity). The initial resources available to an individual can influence gamete production (sperm and oocytes), and the number and quality of gametes produced directly influence the expected fitness of both males and females. Gamete production in relation to adult body size was quantified in Trichogramma euproctidis (Girault) (Hymenoptera: Trichogrammatidae), a short-lived egg parasitoid of lepidopteran species. To avoid host quality variation, male and female parasitoids of different body sizes were produced using superparasitism by allowing mated and virgin female parasitoids to oviposit on Trichoplusia ni Hu¨bner (Lepidoptera: Noctuidae) eggs. Seminal vesicles and ovaries of their offspring were dissected to count oocytes and to measure sperm length and oocytes volume. Tibia length was also measured to estimate body size. The number of oocytes, volume of oocytes, maternal investment index [= (number of oocytes * mean volume of oocytes) ⁄ 10 000] and sperm length were all significantly positively correlated to body size. These results show that initial resources acquired during larval stage induce phenotypic plasticity in gamete production in both male and female T. euproctidis. Whereas number of sperm and oocytes can influence the fitness of males and females through increased mating capacity and fecundity, variation in gamete size (sperm length and oocyte volume) could also affect the fitness of an individual through sperm and larval competition

Size-induced reproductive constraints in an egg parasitoid

“…The reduction in sperm number thus seems to be relatively more important than the reduction in size; at least when size is estimated using tibia length. Sperm length was not influenced by the size of the adult male in A. listronoti contrarily to what is reported in T. euproctidis (Durocher-Granger et al, 2011;Martel et al, 2011). In T. euproctidis, sperm length was shown to influence the probability of exiting the spermatheca, small sperm being favored in the race for fertilizing the oocyte (L. Durocher-Granger unpublished data).…”

“…A reduction in fecundity with a reduction of size of the female has been observed in most parasitoid species studied (Honek, 1993;Mills and Kuhlmann, 2000;Durocher-Granger et al, 2011) including several Mymaridae species (Cronin and Strong, 1996;Jepsen et al, 2007;Irvin and Hoddle, 2009) (but see Riddick, 2005a for a counter example). Our study suggests a decrease in fitness in small females with a reduced maximum capacity for egg production (Parker and Begon, 1986).…”

Using optimality models to improve the efficacy of parasitoids in biological control programmes

“…The quality of the males available and the number of previous mates they had will then influence the capacity of the females to react as predicted by the local mate competition model and the host-quality model. Sperm production can incur sizable costs for the males and they should be under selection pressure to optimise their use of it (Martel et al, 2008;Durocher-Granger et al, 2011). After several matings, males may exhaust their sperm supply, permanently so in species in which males emerge with their full sperm complement but produce none during their adult life (prospermatogenic), but temporarily in species in which males produce some sperm throughout their life (synspermatogenic) (Boivin et al, 2005;Boivin & Martel, 2012).…”

Suitability assessment of three Trichogramma species in the control of Mythimna separata (Lepidoptera: Noctuidae)

“…Hind tibia length reflects body size of Trichogramma. The larger the individual is, the more the egg load is, and the stronger the parasitic capacity is (Durocher-Granger, Martel, & Boivin, 2011;Thorne, Pexton, Dytham, & Mayhew, 2006). Pavlik (1993b) found that based on wasp species or different strains, the body size of Trichogramma wasps could be used to assess Trichogramma quality.…”