Outcrossing increases infection success in the holoparasitic mistletoe Tristerix aphyllus (Loranthaceae)

Abstract

Most studies on the fitness advantage of outbreeding in host-parasite systems have been assessed from the host rather than the parasite perspective. Here, we performed experimental pollination treatments to evaluate the consequences of outbreeding on fitness-related traits in the holoparasitic mistletoe Tristerix aphyllus in a 2-year field study. Results indicate that self-pollinated plants had a lower fruit production than outcrossed plants (20.4% and 29.5% reduction in 2002 and 2003, respectively), and resulting inbred fruits were smaller than outcrossed fruits in both years. No effect was detected for seed mass. The percentage of germination of inbred seeds was 15.1% and 6.0% lower than outcrossed seeds in 2002 and 2003, respectively. Inbred seedlings had shorter radicles, which translated to a 71.6% and 60.0% reduction in infection success compared with outcrossed plants in 2002 and 2003, respectively. Overall, our results revealed significant inbreeding depression on almost every trait that was examined. Although the mean value of traits varied from a year to another, the magnitude of inbreeding depression did not change significantly between years. Our findings constitute the first evidence that outcrossing increases infection success and probably virulence in parasitic plant populations.

Interactions of inbreeding and stress by poor host quality in a root hemiparasite

“…However, the effects of host species on the growth of R. alectorolophus were not influenced by pollination type, suggesting that inbreeding did not influence the ability of the parasites to overcome specific resistance mechanisms. In contrast, in the holoparasite Tristerix aphylla, which grows endophytically in South American cacti, inbreeding reduced many early fitness traits, including infection success (Gonz ales et al, 2007). It has been suggested that outbreeding is especially important for parasites, because they have to overcome defences evolving in different host genotypes (Agrawal and Lively, 2001;Gonz ales et al, 2007).…”

“…In contrast, in the holoparasite Tristerix aphylla, which grows endophytically in South American cacti, inbreeding reduced many early fitness traits, including infection success (Gonz ales et al, 2007). It has been suggested that outbreeding is especially important for parasites, because they have to overcome defences evolving in different host genotypes (Agrawal and Lively, 2001;Gonz ales et al, 2007). However, little is known about the genetics of host-parasite coevolution in Rhinanthus.…”

“…However, little is known about the effects of inbreeding in parasitic plants. In addition to direct effects on parasite fitness, inbreeding can reduce their infection success, as was shown for the holoparasite Tristerix aphylla (Gonz ales et al, 2007). It is not known if inbreeding affects the ability of hemiparasites to grow with hosts of different quality and how inbreeding influences the effect of hemiparasites on the growth of their hosts.…”

“…The evolution of new resistant host genotypes may require a rapid counteradaptation by the parasites, which is facilitated by outcrossing (Gemmill et al, 1997;Agrawal and Lively, 2001;Gonz ales et al, 2007). However, for parasitic plants selfing can be a means to ensure reproduction if crosspollination fails.…”

Ecology of neotropical mistletoes: an important canopy-dwelling component of Brazilian ecosystems

“…Some eff ort has been dedicated to the Ligaria and Tristerix in the temperate regions of Argentina (Galetto et al 1990;Rivera et al 1996;Tadey & Aizen 2001;Aizen & Harder 2007) and in the semiarid region of Chile (Gonzáles et al 2007). Gonzáles et al (2007) demonstrated that outcrossing could increase the percentage of germination in T. aphyllus, which had never been shown before in mistletoes. Studies of other neotropical mistletoe genera do not off er the same degree of complexity.…”