The locoweed–endophyte complex as commensalism: a multigenerational study

Abstract

Plants in the genera Astragalus and Oxytropis, collectively referred to as “locoweeds,” contain swainsonine, a toxic alkaloid synthesized by their fungal endophyte Alternaria sect. Undifilum. The ecological role of this endophyte across the mutualism–commensalism–parasitism continuum is unknown. We examined the fitness traits of Astragalus and Oxytropis species growing with and without the endophyte, in a 9-yr, common-garden experiment. Silky crazyweed (Oxytropis sericea Nutt.) and woolly loco (Astragalus mollissimus Torr.) plants germinated from seeds that naturally host the endophyte (E+) and with it mechanically removed (E−) were established in a common garden in southwest Montana. We measured mortality, gas exchange, flower and seed production, seed germination, and final biomass. Astragalus mollissimus plants grew as annuals under common-garden conditions regardless of endophyte status. Oxytropis sericea plants grew as perennials with survival unaffected by endophyte; however, E+ O. sericea plants produced slightly more reproductive stems, flowers per stem, and crown and stem biomass. Maternal effects detected in the parental generation disappeared in subsequent generations. Gas exchange, fecundity, and seed germination were unaffected by endophyte. Contrary to our initial hypothesis of mutualism, the endophyte did not improve host survival or fecundity, nor did we detect transgenerational effects. However, the endophyte did slightly increase the number of reproductive stems and flowers per stem and crown and stem mass in O. sericea, suggesting endophytic effects on carbohydrate biochemistry and pollination parameters should be examined. Lack of selection for or against endophyte-containing plants allows both nontoxic and toxic swainsonine-producing plants to persist in Astragalus and Oxytropis populations, posing a continued threat to grazing livestock.