Microbial transfer through fecal strings on eggs affects leaf beetle microbiome dynamics

Gut microbiomes of holometabolous insects can be strongly affected by metamorphosis. Previous studies suggest that microbiome colonization and community development often rely on specialized transmission routes between host life stages. However, there is a lack of comparative studies of microbial community dynamics from different transmission mechanisms. We compared the gut microbial community dynamics across life stages in five Galerucella species that differ in their potential microbial transfer mechanism by sequencing amplicons of the 16S rRNA gene. Females of three of the studied species place a fecal string on top of the egg, which may enhance the transfer of gut microbes, whereas females of the two other species do not. We found that the α-diversity was more stable between life stages in fecal string-placer species compared with the non-fecal string-placer species. Moreover, there were consistent microbiome differences between species, with multiple taxa in each species consistently appearing in all life stages. Fecal strings placed on eggs seem to play an important role in the diversity and dynamics of gut bacteria in Galerucella species, facilitating the vertical transfer of gut bacteria between host insect generations. Alternative, but less efficient, transmission routes appear to occur in non-fecal string-placer species.

We explore the consequences of having different mechanisms for transferring and establishing the gut microbiome between generations on gut microbial community dynamics. This process is often problematic in holometabolous insects, which have a complete metamorphosis between larval and adult stages. In our previous research, we found that females of some species within the genus Galerucella (Chrysomelidae) place a fecal string on the eggs, which is later consumed by the hatching larvae, whereas other species in the same genus do not have this behavior. In this paper, we therefore quantify the microbial community dynamics across all life stages in five Galerucella beetles (three with and two without fecal strings). Our results also indicate that the dynamics are much more stable in the species with fecal strings, particularly in the early life stages.