Timing of reproduction in selfing and outcrossing desert annual plants: The role of pollinators and herbivores

Background/Question/Methods

Annual plants are thought to be critical in the diversity and functioning of desert ecosystems. Over half of species in deserts are annuals and the majority of selfing species are annuals. While selfing is widespread in this habitat, it is not uncommon to find congeneric obligate outcrossers and predominant selfers growing together. This suggests tradeoffs between the benefits and costs of these breeding systems that provide a viable niche for both. This research addresses the question, why do reproductive trade-offs promote outcrossing in some species and self-fertilization in others? Specifically, we explore how variable pollen and resource availability may permit different reproductive strategies to coexist in the same environment. Additionally, the timing of pollinator and herbivore activity should play a role in determining fitness given the potential for mismatch between species’ phenologies. An ideal system in which to examine the ecology of reproductive strategies in desert ecosystems is within the genus Camissonia (Onagraceae). Self-fertilization has evolved many times in this group and sister species with contrasting reproductive strategies frequently overlap in their distributions. This study utilizes field and greenhouse studies to explore relationships between mating system, resource availability, pollen limitation, and herbivory within pairs of selfing and outcrossing sister taxa.

Results/Conclusions

Field studies in the Mojave desert in 2008 and 2009 demonstrated that seed production is influenced by shifts in both pollen and resource availability throughout the flowering season. We determined pollen limitation by measuring seed set increase with supplemental pollination and the density of pollen tubes within stigmas. As the season progressed, outcrossing species became less pollen limited. Selfers demonstrated no pollen limitation and were less affected by resource availability than outcrossers. A decrease in resource availability was expressed by a reduction in flower size and number rather than a reduction in ovule or seed production. Fruit herbivory by the caterpillar Hyles lineata increased sharply towards the end of the season for all species. This demonstrates a strong cost to delay reproduction for both selfers and outcrosssers. Given a mismatch between plants and pollinators early in the season, there is an additional cost for the outcrossers to reproduce early. Greenhouse studies have further explored the role of pollen and resource availability in determining physiological and allocation patterns accompanying contrasting reproductive strategies. This study highlights the potential for decreased fitness in outcrossing species resulting from phenological mismatching given temporal and spatial variation in climate and population dynamics.