The plant parasites that are normally referred to as the heteroecious aphids and the heteroecious rust fungi provide an example of parallel evolution that is little short of incredible. The term 'heteroecious' is derived from the Greek, and it means 'different houses'. Heteroecious plant pathosystems are those in which the parasite is obliged to change its species of host in order to complete its life cycle. Each pathosystem thus involves two entirely different host species, which are both parasitised by different stages of the one parasite species. The parasite has various 'forms' whose functions depend on their position in the life cycle. The parallel evolution is revealed by the virtually identical life cycles of the fungal and insect parasites (Fig. 8.1).

0 0

Post a comment