Hemlock woolly adelgid (Adelges tsugae) and bacterial symbionts
For over 30 years, the exotic hemlock woolly adelgid (HWA) has been gradually decimating the native hemlock (Tsuga canadensis and T. caroliniana) in eastern North America. Introduced from Japan, HWA, like aphids, causes damage to the tree by sucking and depleting nutrients that are necessary for the host (Young et al. 1995, McClure et al. 2003).
Sap sucking insects often have associations with mutualistic, intracellular bacteria. These bacteria, referred to as symbionts, can be involved in converting precursors available in the nutritionally poor sap-based diet to nutrients that are essential for animal survival and growth (e.g. amino acids and vitamins, Moran 2006). Some of the most foundational work on the roles and evolution of bacterial symbionts of insects has been completed using aphids as a model system. Given that aphids and adelgids are closely related, we can use our knowledge of aphid symbionts to form hypotheses about adelgid symbionts.
Nutritional symbionts can be sensitive to high temperatures. For example, the nutritional symbiont of pea aphids, Buchnera, can decrease in density to < 10% of its normal levels during a short burst of heat (heat shock), which has drastic effects upon the fitness of the aphids themselves (Dunbar et al. 2007). Other intracellular symbionts provide defense against environmental stress, including factors such as parasitoids, and heat stress (Oliver et al. 2009). In pea aphids, Serratia symbiotica protects against heat stress, possibly by lysing (resulting in a decrease in density) and releasing metabolites that protect the Buchnera (nutrient provisioning) symbionts (Montllor et al. 2002, Burke et al. 2009).
To date, the diversity and roles of microbial symbionts in adelgids is mostly unknown. A recently published survey of symbionts present in HWA revealed that there are two symbionts present in all HWA populations, named Annandia adelgestsuga and Pseudomonas adelgestsugas (von Dohlen et al. 2013). One additional symbiont type, Serratia symbiotica was identified in HWA populations in ENA and the source lineage Japanese adelgids. The roles these symbionts play in HWA biology is currently unknown. However, we can form predictions based upon the known functions of aphid Buchnera, which is distantly related to Annandia adelgestsuga, and aphid Serratia symbiotica, which is very closely related to Serratia symbiotica infecting HWA. Our research aims to better understand the biology of HWA through the characterization of HWA symbionts and their role in nutrition, heat susceptibility, and overall fitness.
We would like to thank Garden Delights for generously donating hemlock trees for our research.
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