Reproductive ecology of a parasitic plant differs by host species: vector interactions and the maintenance of host races

Abstract

Parasitic plants often attack multiple host species with unique defenses, physiology, and ecology. Reproductive phenology and vectors of parasitic plant genes (pollinators and dispersers) can contribute to or erode reproductive isolation of populations infecting different host species. We asked whether desert mistletoe, Phoradendron californicum (Santalaceae tribe Visceae syn. Viscaceae), differs ecologically across its dominant leguminous hosts in ways affecting reproductive isolation. Parasite flowering phenology on one host species (velvet mesquite, Prosopis velutina) differed significantly from that on four others, and phenology was not predicted by host species phenology or host individual. Comparing mistletoe populations on mesquite and another common host species (catclaw acacia, Senegalia greggii) for which genetically distinct host races are known, we tested for differences in interactions with vectors by quantifying pollinator visitation, reward production, pollen receipt, and fruit consumption. Mistletoes on mesquite produced more pollinator rewards per flower (1.86 times the nectar and 1.92 times the pollen) and received ~ 2 more pollen grains per flower than those on acacia. Mistletoes on the two host species interacted with distinct but overlapping pollinator communities, and pollinator taxa differed in visitation according to host species. Yet, mistletoes of neither host showed uniformly greater reproductive success. Fruit set (0.70) did not differ by host, and the rates of fruit ripening and removal differed in contrasting ways. Altogether, we estimate strong but asymmetric pre-zygotic isolating barriers between mistletoes on the two hosts. These host-associated differences in reproduction have implications for interactions with mutualist vectors and population genetic structure.

The cascade impacts of climate change could threaten key ecological interactions

“…, ). Nevertheless, mistletoes that rely on less specialized interactions seem not to be limited (e.g., Yule and Bronstein , b).…”

Infrapopulation size and mate availability influence reproductive success of a parasitic plant

“…and flies (especially Syrphidae and Calliphoridae). Flowers of both sexes produce nectar (Wiesenborn, ), but males have larger flowers that produce greater quantities (Yule & Bronstein, ). In addition to nectar, many of the floral visitors collect pollen (Wiesenborn, ).…”

“…While prediction 2, that competition for resources obtained from host trees limits reproduction, would best be tested by a pollen addition experiment across varying infrapopulation sizes, the reproductive structures of desert mistletoe preclude this type of manipulation: each adult female desert mistletoe can produce thousands of minute ( c . 1 mm diameter) flowers over a period of several months in winter and spring (January–April on acacia hosts; February–May on mesquite hosts) (Yule & Bronstein, ). In the absence of a pollen addition experiment, integrating the observational and experimental work, we present here allows us to test prediction 2 by connecting pollinator visitation, pollen receipt and offspring production for plants that differ in their potential number of competitors.…”

“…In 2016, 10 open flowers were collected from each focal female individual on 14 February 2016 on acacia and 15 March 2016 on mesquite. The flower collection dates on each host were chosen based on the approximate dates of the maximum proportion of plants flowering and pollen receipt in observational data (Yule & Bronstein, ). The stigmas of each flower were swabbed with fuchsin‐dyed glycerin gel to remove stained, deposited pollen grains, which were then counted using a light microscope (100×).…”

“…Female desert mistletoes from hosts lacking male parasites were analysed together with those from treated hosts, as they did not differ statistically for any response variable. Host species was included as a fixed effect because desert mistletoes infesting mesquite and acacia are known to form genetically distinct host races (Yule, Koop, Alexandre, Johnston, & Whiteman, ) with different reproductive ecologies (Yule & Bronstein, ) that may influence the relationship between infrapopulation characteristics and fitness. Stepwise backwards model selection, comparing nested models by likelihood ratio test, was performed for all response variables (Zuur, Ieno, Walker, Saveliev, & Smith, ).…”