Studies On Inhibition Of Spermatogenic Cells Of Plutella Xylostella By Two Endoparasitoids

In the present sudy, we studied the effects of parasitization on the testicular growth and male germ cell development of the diamondback moth, Plutella xylostella (Linnaeus) (Lepidoptera: Plutellidae), the most important crucifer pest worldwide, parasitized by two major larval endparasitoids, Diadegma semiclausum Hellén (Hymenoptera: Ichneumonidae) or Cotesia vestalis (Haliday) (Hymenoptera: Braconidae), respectively, by using micromorphological method, cell biological technology and in vitro culture. We then compared the effects of Bracovirus PDV or Ichvovirus PDV carried by braconid or ichneumonid, respectively, on the devlopment of germ cell of male host P. xylostella. The main results of this study are summarized as follows.1. The development of germ cells in male host Plutella xylostella larvaeThe spermatogenesis and spermiogenesis of different stages and different larval instars of Plutella xylostella were investigated. The results showed that the testes of early 2nd instar larva were small and testicular sheath was colourless and almost transparent. The growth of testicular mainly occurred in the final (4th) instar larva. In the end, abundant yellow materials were acculated in the 4th larval testicular sheath. The spermatogonium predominated in 2nd instar larva or before. The spermatocytes were first seen from the 3rd instar larval stage. The bundles of sperm started just after the 4th instar larva. The occurrence of apyrene spermatogenesis was in larval stage. The onset of the eupyrene spermiogenesis occurred in pupal and adult stages.2. Comparative of parasitic effects on the spermatogenic cells development of host Plutella xylostella larvae between two endoparasitoidsThe current work intends to compare the spermatogenic cell development in Plutella xylostella parasitized by these two endoparasitoids, in order to make clear of the regulative capabilities of parasite factors on host reproductive system. The results showed that parasitization by two endoparasitoids both resulted in a reduction in the number of sperm bundles and that the spermatogenesis and spermiogenesis were retarded too. The abnormal testis structure or deformed sperm bundles were also observed for the first time in P. xylostella parasitized by D. semiclausum or C. vestalis, respectively. The degree of testis degeneration was more severe in parasitized P. xylostella by C. vestalis than that by D. semiclausum. After superparasitization, compared with a singly parasitism, a higher degree of testis degeneration was induced, including smaller testicular volume, severe abnormal testis morphology and little sperm bundles. These are most strongly related to more aliquots of wasp-associated materials injected into the host hemocoel. Pseudoparasitization also resulted in testis degeneration, and the degree of degeneration was almost as severe as in naturally parasitized hosts, indicating that polydnavirus and venom are mainly involved in effecting host castration. It is suggested that the mechanisms exploited by the bracovirus and ichnovirus in host castration are different.3. Effects on testicular development of different larval instars of Plutella xylostella by two endoparasitoidsTo clarify the mechanism of parasitic castration by endoparasitoid, the parasitic effects of two endoparasitoids, Cotesia vestalis and Diadegma semiclausum, carrying different types of polydnaviruses, on host germ cell and testicular growth were investigated when parasitism occurred on different host ages. The host-parasitoid combinations studied here allowed us to compare the degree of castration and the role played by PDVs in the process of spermatogenesis and spermiogenesis between hosts parasitized at different ages. The results showed that both C. vestalis bracovirus and D. semiclausum ichnovirus can inhibit host spermatogenesis and spermiogenesis, there were no sperm bundles or only a few in parasitized host testes. Host parasitized at early stadium, spermiogenesis was almost completely inhibited, however, when hosts were parasitized at the 4th (final) stadium, the degree of castration was less severe. The testicular growth in hosts parasitized by two endoparasitoids was also inhibited. Parasitization at the early stadium resulted in smaller testes in size than observed when parasitization occurred on the high stadium. The degree of the castration depends on the age of the host at the time of parasitization. C. vestalis always has a significantly stronger castration effect on host germ cell and testicular growth than D. semiclausum.4. Effects of polydnaviruses from two endoparasitoids, Cotesia vestalis or Diadegma semiclausum on the germ cells of their male host Plutella xylostella in vitroThe physiological effects of polydnaviruses from Cotesia vestalis or Diadegma semiclausum were assayed on spermatogenic cells of Plutella xylostella in vitro. PDVs from C. vestlis or D. semiclausum displayed obviously effects on the morphology of germ cells of P. xylostella after being incubated for 18 h. The morphology of the incubated spermatocytes changed compared with the control. The spermatocytes began to swell. Addtionally, a few vacuoles occurred in spermatocytes, and some apermatocytes began to disintegrate. When host germ cells were incubated in polydnaviruses from C. vestlis or D. semiclausum for 20 h, the majority of spermatocytes underwent apoptosis with fragmentated cell wall. Furthermore , this was determined after dying with trypan blue. The spermatocytes incubated with PDVs dyed blue, in contrast, the controls displayed normal morphology with integrate and clear nucleus. These results show that PDVs may play a key role in apoptosis induction of the host germ cells.In conclusion, there are differences in the degree of inhibition of host testicular development and male germ cells induced by Cotesia vestalis or Diadegma semiclausum, respectively. C. vestalis always has a significantly stronger effect on host testicular development and germ cells than D. semiclausum. These results suggested that the wasp-associated factors carried by these two endoparasitoids played a major role in induction of apoptosis of host spermatogenetic cells.