Selective oviposition in fertilized seed of Ilex integra by the wasp Macrodasyceras hirsutum (Hymenoptera: Torymidae)

Etsuro Takagi,Katsumi Togashi,Masanori Suzuki,Kazunobu Iguchi

doi:10.14411/eje.2010.026

Abstract

It is speculated that the wasps that attack the seeds of gymnosperm trees (conifers) before they are fertilized can induce unfertilized seeds to accumulate storage material whereas those that attack after the seeds are fertilized selectively oviposit in fertilized seeds. Moreover, in the case of the wasps that oviposit after fertilization of seed, the presence of unfertilized seeds and seedless fruit may increase plant fitness via reduced parasitism of the viable seed. To determine the relationship between the two strategies, host manipulation or selective oviposition, and the time of fertilization of the seeds of angiosperm host plants, fertilized seed of Ilex integra Thunb. was dissected out of berries either immediately after the flight of the seed wasp Macrodasyceras hirsutum Kamijo in the field or the death of adults in the laboratory. The wasps oviposited mostly in fertilized seeds and rarely in unfertilized seeds. Unfertilized seeds, produced by flowers enclosed in pollen exclusion bags, and then exposed to wasps did not contain immature wasps or storage material, which indicates that the wasp did not oviposit in unfertilized seeds. These results support the above mentioned hypothesis and indicate that the substantial proportion of seedless berries do not function as an egg sink.

Highlights

Some insects insert their eggs into seeds and their larvae consume its storage matter, often inhibiting the survival and growth of seeds and seedlings and so reduce the fitness of host plants (Janzen, 1970)
The present study showed that Macrodasyceras hirsutum lays eggs in fertilized seeds but rarely in unfertilized seeds in the field, indicating selective oviposition
Macrodasyceras hirsutum larvae develop and pupate in the seed; it is suggested that the selection of nutritious seeds by female wasps may increase their fitness via the increased survival and reproduction ability of their progeny

Summary

Introduction

Some insects insert their eggs into seeds and their larvae consume its storage matter, often inhibiting the survival and growth of seeds and seedlings and so reduce the fitness of host plants (Janzen, 1970). When the larvae cannot move from seed to seed, selective oviposition into the most nutritious seeds, such as fertilized rather than unfertilized seeds, may increase the fitness of the parents, because there is more storage matter in fertilized seeds (Desouhant, 1998; Stamps & Linit, 2002). Once fertilization has occurred, unpollinated ovules usually quickly degenerate but if they contain wasp larva they do not degenerate and even accumulate energy reserves, as is the case when Megastigmus spermotrophus Wachtl (Hymenoptera: Torymidae) larvae are present in unfertilized ovules of Douglas fir (Pseudotsuga menziesii (Mirbel) Franco) (von Aderkas et al, 2005a, b). When the seed wasp lays its eggs after the seeds are fertilized they may differentiate between fertilized and unfertilized megagametophytes (Rouault et al, 2004). Some angiosperms, costly in terms of resources, have non-viable seeds and/or fruits containing non-viable seeds, which serve as a decoy for seed predators or as a sink for eggs deposited by them, resulting in the increased likelihood of viable seeds surviving to seed dispersal (Zangerl et al, 1991; Traveset, 1993; Ghazoul & Satake, 2009)

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