Gut microorganisms are crucial for many biological functions playing a pivotal role in the host’s well-being. We studied gut bacterial community structure of marine iguana populations across the Galápagos archipelago. Marine iguanas depend heavily on their specialized gut microbiome for the digestion of dietary algae, a resource whose growth was strongly reduced by severe “El Niño”-related climatic fluctuations in 2015/2016. As a consequence, marine iguana populations showed signs of starvation as expressed by a poor body condition. Body condition indices (BCI) varied between island populations indicating that food resources (i.e., algae) are affected differently across the archipelago during ‘El Niño’ events. Though this event impacted food availability for marine iguanas, we found that reductions in body condition due to “El Niño”-related starvation did not result in differences in bacterial gut community structure. Species richness of gut microorganisms was instead correlated with levels of neutral genetic diversity in the distinct host populations. Our data suggest that marine iguana populations with a higher level of gene diversity and allelic richness may harbor a more diverse gut microbiome than those populations with lower genetic diversity. Since low values of these diversity parameters usually correlate with small census and effective population sizes, we use our results to propose a novel hypothesis according to which small and genetically less diverse host populations might be characterized by less diverse microbiomes. Whether such genetically depauperate populations may experience additional threats from reduced dietary flexibility due to a limited intestinal microbiome is currently unclear and calls for further investigation.

肠道微生物对于在宿主健康中起关键作用的许多生物功能至关重要。我们研究了加拉帕戈斯群岛上海鬣蜥种群的肠道细菌群落结构。海鬣蜥在很大程度上依赖于其专门的肠道微生物组来消化膳食藻类，而该藻类的生长在2015/2016年因与“厄尔尼诺”相关的严重气候波动而大大降低。结果，海鬣蜥种群显示出饥饿的迹象，如身体状况不佳所表示。岛屿人口之间的身体状况指数（BCI）有所不同，表明在“厄尔尼诺”事件期间，整个群岛的粮食资源（即藻类）受到不同的影响。尽管此事件影响了海鬣蜥的食物供应，我们发现，由于“厄尔尼诺”相关的饥饿而导致的身体状况的减轻并没有导致细菌肠道群落结构的差异。相反，肠道微生物的物种丰富度与不同宿主种群中中性遗传多样性的水平相关。我们的数据表明，与遗传多样性较低的种群相比，基因多样性和等位基因丰富度较高的海鬣蜥种群可能具有更多的肠道微生物组。由于这些多样性参数的低值通常与人口普查和有效人口规模有关，因此我们使用我们的结果提出了一个新的假设，根据该假设，较小且遗传上较少多样性的宿主种群可能具有较少多样性的微生物群落。