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

Abstract

In 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.