Host-parasite-vector interactions across scales

How do within-host processes translate to parasite fitness?

Ironwood Forest National Monument, Arizona
Characterizing desert mistletoe infections in Ironwood Forest National Monument, Arizona

Aggregated distributions of parasite individuals across host individuals are nearly ubiquitous among parasitic taxa. The size and sex ratio of the population of one parasite species infecting a single host (“infrapopulation”) can influence
parasite fitness through intraspecific competition, mate availability and the ability to attract vectors for transmission of parasite propagules. Competition for resources and for pollen and seed vector services may limit reproductive success (pollen receipt, fruit production and seed dispersal) in large infrapopulations of parasitic plants, while mate limitation or reduced ability to attract vectors may limit this success in small infrapopulations.

Using a dioecious parasitic plant, desert mistletoe (Phoradendron californicum), we experimentally removed reproductive tissue from male parasites in whole infrapopulations to test for independent effects of infrapopulation size and within-host mate availability on female fitness. As desert mistletoe requires both pollen and seed vectors for successful reproduction, the species provides the opportunity to test how infrapopulation characteristics affect multiple components of parasite

The results of this work in collaboration with Judie Bronstein (The University of Arizona) is published in Journal of Ecology.

What are the ecological causes and consequences of host-associated genetic differentiation?

Desert mistletoe infection on velvet mesquite, Santa Rita Experimental Range, Arizona
Desert mistletoe infection on velvet mesquite, Santa Rita Experimental Range, Arizona

Host-associated differentiation, or host-race formation, occurs when parasite populations infecting different host species become genetically differentiated through a reduction in interbreeding. This process has been implicated in the speciation and diversification of many parasite lineages.  Parasites on different hosts experience different selective pressures, but adaptation to a given host species’ particular challenges can be opposed by gene flow from nearby parasites on different hosts.

While genetic divergence between parasite populations associated with different host species are often recorded, this pattern could also be due only to historical differences in host species geographic ranges and/or secondary contact between host races that have not yet been eroded by gene flow. One way to understand whether those genetic patterns are actually due to adaptation to host species is to search for ecological mechanisms preventing between host-race gene flow.  For example, I have found that host species identity correlates with flowering phenology, which influences the pollinator community that interacts with different desert mistletoe host races. Understanding how desert mistletoe responds to the available host community is important for furthering our knowledge both about the evolution of parasites in general and about the ecology of a species critical to Sonoran animal communities.

The results of this work have been published in collaboration with Judie Bronstein (The University of Arizona) in Oecologia.


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