Sporophyte and gametophyte generations differ in their thermotolerance response in the moss Microbryum

• Background and Aims: Actively growing post-embryonic sporophytes of desert mosses are restricted to the cooler, wetter months. However, most desert mosses have perennial gametophytes. It is hypothesized that these life history patterns are due in part to a reduced thermotolerance for sporophytes relative to gametophytes. • Methods: Gametophytes with attached embryonic sporophytes of Microbryum starckeanum were exposed whilst desiccated to thermal episodes of 35°C (1 hr), 55°C (1 hr), 75°C (1 hr) and 75°C (3 hr), then moistened and allowed to recover for 35 d in a growth chamber. • Key Results: All of the gametophytes survived the thermal exposures and produced protonemata, with the majority also producing shoot buds. Symptoms of gametophytic stress (leaf burning and discoloration of entire shoots) were present in lower frequencies in the 55°C exposure. Sporophyte resumption of growth and maturation to meiosis were significantly negatively affected by thermal treatment. Not a single sporophyte exposed to the two higher thermal treatments (75°C for 1 h and 75°C for 3 h) survived to meiosis, and those sporophytes exposed to 75°C that survived to the post-embryonic phenophase took significantly longer to reach this phase. Furthermore, among the thermal treatments where some capsules reached maturity (35°C and 55°C), maternal shoots that produced a meiotic capsule took longer to regenerate through protonemata than maternal shoots aborting their sporophyte, suggestive of a resource trade-off between generations. • Conclusions: Either (1) the inherent sporophyte thermotolerance is quite low even in this desert moss, and/or (2) a gametophytic thermal stress response controls sporophyte viability.