Parasitism of seed of Douglas fir (Pseudotsuga menziesii) by the seed chalcid, Megastigmus spermotrophus, and its influence on seed hormone physiology

Abstract

Parasitism of Douglas fir (Pseudotsuga menziesii) megagametophytes by the seed chalcid, Megastigmus spermotrophus Wachtl, occurs naturally after pollination but before fertilization. In the absence of fertilization, the presence of insect larvae within the megagametophyte prevents abortion and the storage tissue continues to develop as if the seed had been fertilized. We investigated the effect of parasitism on the metabolism of abscisic acid (ABA), auxins, cytokinins, and gibberellins during early development of Douglas fir seeds. Hormones and hormone metabolites of infested and uninfested megagametophytes with or without pollination were analyzed by HPLC–ESI/MS/MS. At 1 week after Megastigmus introduction, the insect’s presence stimulated ABA accumulation in unpollinated megagametophytes compared to unpollinated, unparasitized megagametophytes. In pollinated material, parasitism did not stimulate ABA accumulation compared to levels present in unparasitized megagametophytes. In all four treatments, the metabolism of ABA occurred primarily through conjugation to the ABA glucose ester (ABAGE), while the 7′-, 8′- and, 9′-hydroxylation pathways were only minor. ABAGE levels declined with time in all treatments and this occurred to a greater extent in pollinated, parasitized megagametophytes, suggesting that the insect’s presence induced the dramatic decrease in ABAGE. Although there were temporal variations in the auxin, cytokinin, and gibberellin profiles of parasitized megagametophytes, the profiles were generally similar to those of unparasitized megagametophytes. Our results suggest that failure of parasitized megagametophytes to abort may be due to the insect inducing similar hormone profiles to those present during normal development of Douglas fir seed.