Parasitism is a partial or complete nutritional dependence of one organism, the parasite, on another living organism, a host that often experiences dramatic reductions in performance and can even die. However, the dynamic host-parasite complex is constantly changing and co-evolving, and a number of studies have suggested that selection may favour less aggressive parasites, since they can benefit from establishing a longer-lasting relationship with their hosts. Along these lines, some authors have proposed that for some perennial parasites, their relations with their hosts may be closer to commensalism, or even to mutualism, than to parasitism. In this contribution we focus on Cytinus hypocistis, a perennial holoparasitic angiosperm whose relationships with different shrubby Cistaceae hosts date back at least to the Miocene, to examine the evolutionary consequences of a close, well-established partnership between a parasite and its host. We studied three host races of the parasite C. hypocistis and their three hosts during four years, in six populations, to determine: (i) if pollination niche overlap occurs between parasite and host; (ii) the magnitude of the impact of the parasite on flower, fruit, and seed production by the host; and (iii) the parasite effect on host mortality.
Results/Conclusions
We observed a high asynchrony in the flowering time of the holoparasite and its host species. Cytinus hypocistis produced small flowers at ground level that were pollinated by ants, while the three Cistaceace species produced large generalist flowers that were pollinated mainly by bees and flies. The parasite reduced host flower production in only four of the six populations, and it had no impact on host fruit set and seed production. The mortality of parasitised and non-parasitised plants was not significantly different in five of the six populations. Our data confirm the expectations of a low impact of C. hypocistis on their hosts in terms of both influence on pollination success and fecundity. The relationship Cytinus-Cistaceae may have been maintained by a long history of co-adaptation established between the parasite and its hosts.
