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

Locoweeds (mostly Astragalus and Oxytropis) are legumes that contain
swainsonine, a toxic alkaloid produced by the vertically-transmitted,
fungal endophyte Alternaria section Undifilum. 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 A. mollissimus and O. sericea 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. Oxytropis sericea and
Astragalus mollissimus var. mollissimus and var. thompsoniae plants
germinated from seeds collected in New Mexico, symbiosed with the
Alternaria section Undifilum 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 A. mollissimus
varieties grew as annuals under common-garden field conditions, ca. 1500
km north of their native range, with A. mollissimus var. mollissimus
plants producing more reproductive stems, flowers, and seed pods, and
surviving longer than A. mollissimus var. thompsoniae, regardless of
endophyte presence or swainsonine production. The more broadly distributed
Oxytropis sericea plants grew as perennials with survival unaffected by
endophyte status; however, E+ O. sericea 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. Astragalus and Oxytropis 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 Undifilum 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 O. sericea, 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.