The social amoeba Dictyostelium discoideum rescues Paraburkholderia hayleyella, but not P. agricolaris, from interspecific competition

Bacterial intracellular endosymbionts (hereafter called endosymbionts) can provide benefits for their eukaryotic hosts, but it is often less clear if endosymbionts themselves benefit from these relationships. The social amoeba Dictyostelium discoideum is known to associate with three species of Paraburkholderia endosymbionts including P. agricolaris and P. hayleyella. These Paraburkholderia are costly to host because they reduce the number of hardy spores produced by D. discoideum. However, they can be beneficial because they also allow D. discoideum to carry prey bacteria through the dispersal stage to seed new environments where a good bacterial food source may not be available. In laboratory experiments when no other species are present, P. hayleyella benefits from associating with D. discoideum while P. agricolaris does not. However, the presence of other species may influence symbioses like these. We tested if P. agricolaris and P. hayleyella benefit from the presence of their host in the context of resource competition with Klebsiella pneumoniae, D. discoideum’s typical laboratory prey. In the absence of D. discoideum, K. pneumoniae depressed the growth of both Paraburkholderia symbionts, consistent with competition between the bacteria. In addition, we found that P. hayleyella was harmed more by the presence of K. pneumoniae than was P. agricolaris. We also found that P. hayleyella was rescued from competition with K. pneumoniae by the presence of D. discoideum while P. agricolaris was not. This may be because P. hayleyella is more specialized as an endosymbiont of D. discoideum; it has a highly reduced genome compared to P. agricolaris and may have lost genes relevant for resource competition outside of its host.

细胞内细菌共生体（Bacterial intracellular endosymbionts，以下简称共生体）可为真核宿主带来益处，但共生体自身能否从这类共生关系中获益，往往尚不明确。已知社会变形虫盘基网柄菌（Dictyostelium discoideum）可与三种伯克霍尔德氏菌属（Paraburkholderia）共生体建立关联，其中包括农业伯克霍尔德氏菌（P. agricolaris）和海利伯克霍尔德氏菌（P. hayleyella）。这些伯克霍尔德氏菌会给宿主带来生存成本：它们会降低盘基网柄菌产生的坚韧孢子数量。不过，它们也能为宿主提供益处——可帮助盘基网柄菌携带猎物细菌度过扩散阶段，以便在缺乏优质细菌食物源的新环境中完成定植。在无其他物种存在的实验室实验中，海利伯克霍尔德氏菌可从与盘基网柄菌的共生关系中获益，而农业伯克霍尔德氏菌则不能。但其他物种的存在可能会影响这类共生关系的动态。我们针对以下问题开展了测试：在与盘基网柄菌的典型实验室猎物——肺炎克雷伯菌（Klebsiella pneumoniae）的资源竞争背景下，农业伯克霍尔德氏菌和海利伯克霍尔德氏菌能否从宿主的存在中获益？在无盘基网柄菌的情况下，肺炎克雷伯菌会抑制两种伯克霍尔德氏菌共生体的生长，这与细菌间的资源竞争规律一致。此外，我们发现海利伯克霍尔德氏菌受肺炎克雷伯菌存在的负面影响程度显著高于农业伯克霍尔德氏菌。我们还发现，盘基网柄菌的存在可使海利伯克霍尔德氏菌摆脱与肺炎克雷伯菌的竞争压力，而农业伯克霍尔德氏菌则无此保护效应。这一现象可能源于海利伯克霍尔德氏菌作为盘基网柄菌共生体的特化程度更高：相较于农业伯克霍尔德氏菌，其基因组已大幅简化，可能已丢失了在宿主外环境中进行资源竞争所需的相关功能基因。