Plant-herbivore interactions contribute to the high variety of many insect families, as changes in plant-host use can result in herbivorous insect speciation. The sequential link between genetic alterations and plant-host use is an important consideration. Genetic changes before a plant-host shift can be regarded as pre-adaptation for the transition, whereas genetic changes after a plant-host shift can be considered as a response to changes in plant-host utilization. The current consensus is that the host insect changes first, then the genes change, and one potential mechanism is that gut microbes play a role in the relationship between genetic changes on the insects and plant-host use. According to recent theory, animals should no longer be regarded as autonomous entities, but rather as a biomolecular network composed of the host insects plus its associated microbes, a "holobiont" in which the collective genome is referred to as the "hologenome" and the gut microbiome participates in host insect metabolism as "the third malleable genome." Microbial inheritance, or the direct transmission of bacteria from parent to offspring, is a critical component in determining the extent to which the holobiont can be viewed as a unit of selection, and it can erode the link between microbial fitness and host individual fitness. In the case of heritable microbiome, a holobiont is a complete unit of selection, and selecting heritable symbionts maximizes host insect longevity and reproduction. Thus, one can hypothesize that changes in gut microbiomes is a possible mechanism allowing shifts in host use.

Our study system consists of six closely related leaf beetle species (Galerucella spp.). Females of four of the analyzed species lay a fecal string on top of the egg, which may aid in the transfer of gut microbiome, whereas females of the other two species do not. In chapters I, II, and III of my PhD thesis, I explored the crucial functions of the fecal string, particularly for transgenerational transfer of the gut microbiome. First, the roles of ecology and host phylogeny in the composition of gut microbial communities have been distinguished. I discovered phylogenetically controlled co-occurrence patterns between each of six closely related Galerucella leaf beetle species and their species-specific gut bacteria, indicating the possibility of host-bacterium co-evolution (Chapter I). Although it is critical to understand the co-evolutionary processes that occur between hosts and their gut bacterial communities, gut microbes are not just inherited from ancestors, but can also be obtained from food or the environment or acquired through social interactions. Even in the absence of co-evolutionary processes, diverse transmission mechanisms can influence host ecological traits. In Chapter II, I discovered that fecal strings in Galerucella species aid in the vertical transfer of gut bacteria between host insect generations, influencing gut bacteria diversity and dynamics. To learn more about how gut microbiomes influence variations in host use, I utilized antibiotics and microbial transfers between beetle species, comparing the impact of microbial transfers in Galerucella beetles that fed on their own host plants vs non-host plants in Chapter III. In Chapter IV, I presented evidence that fecal strings defend against natural enemies.