Locoweed biomass and fecundity +/- fungal endophyte in field 2011-2020

<p>Locoweeds (mostly <em>Astragalus </em>and <em>Oxytropis</em>) are legumes that contain swainsonine, a toxic alkaloid produced by the vertically-transmitted, fungal endophyte <em>Alternaria</em> section <em>Undifilum</em>. These plants cause economic losses through livestock poisoning, yet there is little evidence that swainsonine plays a role in anti-herbivory. Similarly, while swainsonine synthesis relies on nitrogen from the plant, and many fungal endophytes are thought to be mutualists, the ecological role of this endophyte across the mutualism-commensalism-parasitism continuum is yet unknown. We examined the fitness of <em>A. mollissimus</em> and <em>O. sericea</em> plants with and without the endophyte by measuring growth and fecundity parameters, and potential transgenerational effects, in a multi-year, common garden experiment located in or beyond the northern range of locoweeds. We hypothesized that field-grown plants with the endophyte would possess a fitness advantage over plants without the endophyte under growing conditions different from their maternal source. <em>Oxytropis sericea </em>and<em> Astragalus mollissimus</em> var. <em>mollissimus</em> and var. <em>thompsoniae</em> plants germinated from seeds collected in New Mexico, symbiosed with the <em>Alternaria</em> section <em>Undifilum </em>fungal endophyte (E+) and with the fungal endophyte mechanically removed (E-) by removal of the seed coat, were established in an outdoor, common garden in southwest Montana. To evaluate the ability of this vertically-transmitted endophyte to mediate transgenerational effects in progeny, two successive generations of E+ and E- plants were also established in the garden. We measured overwinter survival, photosynthetic gas exchange, flower and seed production, and progeny seed germination on these pairs of plants over nine years, and compared final above and below-ground biomass. Both <em>A. mollissimus </em>varieties grew as annuals under common-garden field conditions, ca. 1500 km north of their native range, with <em>A. mollissimus</em> var. <em>mollissimus </em>plants producing more reproductive stems, flowers, and seed pods, and surviving longer than <em>A. mollissimus</em> var. <em>thompsoniae</em>, regardless of endophyte presence or swainsonine production. The more broadly distributed <em>Oxytropis sericea</em> plants grew as perennials with survival unaffected by endophyte status; however, E+ <em>O. sericea</em> plants produced slightly more reproductive stems, flowers/reproductive stems, and reproductive stem and log crown biomass. Maternal effects detected in the parental generation, Gen 0, were highly variable, but this variation disappeared in subsequent generations, with no endophyte transgenerational effects detected across most variables. <em>Astragalus </em>and <em>Oxytropis </em>gas exchange, fecundity, and seed germination also were unaffected by endophyte status when grown under environmental conditions differing from the collection sites. Contrary to our initial hypothesis of mutualism, the <em>Undifilum</em> endophyte did not appear to improve locoweed survival or fecundity, and the presence or absence of the endophyte had no detectable transgenerational effects. However, the endophyte did slightly increase the number of reproductive stems and flowers/stem and the weight of the crown and reproductive stem tissue in the perennial <em>O. sericea</em>, suggesting more work is needed to examine endophyte effects on carbohydrate biochemistry and pollination parameters. Our findings highlight the importance of evaluating putative commensal host-endophyte relationships under field conditions.</p>