Brock Debra A, Callison W Éamon, Strassmann Joan E, Queller David C
Department of Biology, Washington University in Saint Louis, Campus Box 1137, One Brookings Drive, Saint Louis, MO, USA
Department of Biology, Washington University in Saint Louis, Campus Box 1137, One Brookings Drive, Saint Louis, MO, USA.
Proc Biol Sci. 2016 Apr 27;283(1829). doi: 10.1098/rspb.2015.2727.
The social amoeba Dictyostelium discoideum is unusual among eukaryotes in having both unicellular and multicellular stages. In the multicellular stage, some cells, called sentinels, ingest toxins, waste and bacteria. The sentinel cells ultimately fall away from the back of the migrating slug, thus removing these substances from the slug. However, some D. discoideum clones (called farmers) carry commensal bacteria through the multicellular stage, while others (called non-farmers) do not. Farmers profit from their beneficial bacteria. To prevent the loss of these bacteria, we hypothesize that sentinel cell numbers may be reduced in farmers, and thus farmers may have a diminished capacity to respond to pathogenic bacteria or toxins. In support, we found that farmers have fewer sentinel cells compared with non-farmers. However, farmers produced no fewer viable spores when challenged with a toxin. These results are consistent with the beneficial bacteria Burkholderia providing protection against toxins. The farmers did not vary in spore production with and without a toxin challenge the way the non-farmers did, which suggests the costs of Burkholderia may be fixed while sentinel cells may be inducible. Therefore, the costs for non-farmers are only paid in the presence of the toxin. When the farmers were cured of their symbiotic bacteria with antibiotics, they behaved just like non-farmers in response to a toxin challenge. Thus, the advantages farmers gain from carrying bacteria include not just food and protection against competitors, but also protection against toxins.
社会性变形虫盘基网柄菌在真核生物中较为独特，它具有单细胞和多细胞两个阶段。在多细胞阶段，一些被称为哨兵细胞的细胞会摄取毒素、废物和细菌。这些哨兵细胞最终会从移动的蛞蝓体后部脱落，从而将这些物质从蛞蝓体中清除。然而，一些盘基网柄菌克隆（称为“农民型”）在整个多细胞阶段都携带共生细菌，而其他克隆（称为“非农民型”）则不携带。“农民型”从其有益细菌中获益。为防止这些细菌流失，我们推测“农民型”中的哨兵细胞数量可能会减少，因此“农民型”对病原菌或毒素的反应能力可能会减弱。作为证据，我们发现与“非农民型”相比，“农民型”的哨兵细胞更少。然而，在用毒素攻击时，“农民型”产生的存活孢子数量并不比“非农民型”少。这些结果与有益细菌伯克霍尔德菌提供毒素保护作用是一致的。“农民型”在有无毒素攻击情况下的孢子产量变化与“非农民型”不同，这表明伯克霍尔德菌的代价可能是固定的，而哨兵细胞可能是可诱导的。因此，“非农民型”只有在存在毒素时才会付出代价。当用抗生素治愈“农民型”的共生细菌后，它们在应对毒素攻击时的表现就和“非农民型”一样。因此，“农民型”携带细菌所获得的优势不仅包括食物和抵御竞争者，还包括抵御毒素。
