Hybridization of an invasive shrub affects tolerance and resistance to defoliation by a biological control agent

Abstract

Evolution has contributed to the successful invasion of exotic plant species in their introduced ranges, but how evolution affects particular control strategies is still under evaluation. For instance, classical biological control, a common strategy involving the utilization of highly specific natural enemies to control exotic pests, may be negatively affected by host hybridization because of shifts in plant traits, such as root allocation or chemical constituents. We investigated introgression between two parent species of the invasive shrub tamarisk (Tamarix spp.) in the western United States, and how differences in plant traits affect interactions with a biological control agent. Introgression varied strongly with latitude of origin and was highly correlated with plant performance. Increased levels of T. ramosissima introgression resulted in both higher investment in roots and tolerance to defoliation and less resistance to insect attack. Because tamarisk hybridization occurs predictably on the western U.S. landscape, managers may be able to exploit this information to maximize control efforts. Genetic differentiation in plant traits in this system underpins the importance of plant hybridization and may explain why some biological control releases are more successful than others.

Can local adaptation explain varying patterns of herbivory tolerance in a recently introduced woody plant in North America?

“…Likewise, the date of leaf senescence was correlated with latitude for both Tamarix and P. deltoides genotypes in the common garden (Friedman et al ., 2011). Studies have also shown that northern populations of Tamarix have a larger root to shoot ratio, indicating that northern populations allocate more biomass to belowground tissues than southern populations (Sexton et al ., 2002; Williams et al ., 2014). For populations experiencing higher rates of freeze-induced dieback, it may be adaptive to have larger pools of carbon in the form of belowground biomass for replacement growth of dead tissues when the growing season resumes.…”

Hybrid ‘superswarm’ leads to rapid divergence and establishment of populations during a biological invasion

“…An increased genetic potential in hybrid populations may facilitate subsequent colonization and establishment, and allow genetically admixed individuals to tap into novel niches within the invaded range not typically occupied by any of its ancestors (Lockwood et al 2007;Yoder et al 2010;Williams et al 2014). For hybrids to persist, however, their distribution (in allopatry) and/or the balance between selection and gene flow (in sympatry or parapatry) should help establish reproductive isolation (Grant 1994;Buerkle et al 2000;Mallet 2007;Nolte & Tautz 2010;Abbott et al 2013;Seehausen et al 2014).…”

“…Thus, theoretical considerations notwithstanding, there is a need to identify systems appropriate for the study of the incipient stages of hybrid lineage formation and subsequent speciation (Buerkle et al 2000;Nolte & Tautz 2010;Seehausen et al 2014). The identification of newly formed hybrid lineages can not only provide key insights into the formation of new hybrid species, but also answer important questions related to the pace of hybrid lineage stabilization and the associated extent of genomic integration (Nolte & Tautz 2010;Abbott et al 2013;Seehausen et al 2014;Williams et al 2014). …”

“…Within such contact zones, distinct lineages can hybridize converting between-lineage genetic variation to within-population genetic variance (Mallet 2007;Prentis et al 2008;Abbott et al 2013;Seehausen et al 2014). This, in turn, increases standing genetic variation and reduces genetic constraints in newly formed hybrid populations, augmenting their genetic potential or adaptability (Mallet 2007;Prentis et al 2008;Nolte & Tautz 2010;Abbott et al 2013;Seehausen et al 2014;Williams et al 2014). Release from former genetic constraints may occur because allelic combinations fixed in parental lineages, expressed through their genetic variancecovariance matrices (VCVs), can be disrupted, the genetic covariance broken and the genetic variance broadened in ensuing hybrids (Buerkle et al 2000;Abbott et al 2013; …”

From Hybrid Swarms to Swarms of Hybrids

“…Friedman et al [47] and Gaskin and Kazmer [29] found that the F1, F2, and backcrosses to two parent species, T. ramosissima and T. chinensis, may represent 83-87% of the tamarisk invasion in the western United States. Recent studies have found that this complicates the biological control of the genus, because increased levels of T. ramosissima introgression resulted in higher investment in roots and tolerance to defoliation, and less resistance to biological control agents often used by managers to decrease the range and spread of this genus [48]. Plant hybridization, particularly in this harmful invasive species, may hinder containment efforts in natural areas.…”