Sunday, August 13, 2006

Life in a seed

Some species of insects complete their entire development in seeds. Protected by the seed coat and nourished by the seed's food supply a developing insect has everything it needs. However these reserves only normally accumulate in fertilized ovules - requiring female insects that wish to lay eggs inside the seeds to either wait until ovules are fertilized (and be able to discriminate), or to lay their eggs earlier and risk many being laid in unfertilized seed. A paper in the Proceedings of the Royal Society last year describes how a host-specific insect, the chalcid Megastigmus spermotrophus that lays its eggs in ovules of Douglas fir has managed to evolve a solution to this problem. This insect lays its eggs before fertilization has taken place in the plant and ovipoisiton of eggs not only prevents the expected degeneration and death of unfertilized ovules, but it induces energy reserve accumulation. Ovules that would otherwise develop as empty seed are redirected in their development by the insect to provide food for the developing larvae. This is the first report of this type of insect-host relationship.



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