Epi-parasitic mistletoes don't parasitise their host's host and this supports the immunity hypothesis for host choice

Mistletoes have much higher transpiration rates than their hosts. This is the basis for the ecological "compatibility hypothesis" for host choice. The inability of mistletoes to penetrate a host and connect to its vascular system is the competing "immunity hypothesis". Epi-parasites would have to transpire faster than their host mistletoe, which must transpire faster than its host. Therefore, epi-parasites should be able to parasitise their host's host. I surveyed epi-parasite host choice at a site and found this did not happen. Globally about a quarter of all epi-parasites are obligate. This supports the immunity hypothesis. Le taux de transpiration du gui est beaucoup plus eleve que celui de ses hotes. C'est la base de l' & DLANGBRAC; hypothese de compatibilite & drangbrac; ecologique pour le choix de l'hote. L'incapacite du gui a penetrer dans un hote et a se connecter a son systeme vasculaire est l'& DLANGBRAC; hypothese concurrente de l'immunite & drangbrac;. Les epi-parasites devraient transpirer plus vite que leur hote, le gui, qui doit transpirer plus vite que son hote. Par consequent, les epi-parasites devraient etre capables de parasiter l'hote de leur hote. J'ai etudie le choix de l'hote des epi-parasites sur un site et j'ai constate que ce n'etait pas le cas. Au niveau mondial, environ un quart de tous les epi-parasites sont stricts. Cela confirme l'hypothese de l'immunite.