A suite of selective pressures supports the maintenance of alleles of a Drosophila immune peptide

The innate immune system provides hosts with a crucial first line of
defense against pathogens. While immune genes are often among the fastest
evolving genes in the genome, in Drosophila, antimicrobial peptides (AMPs)
are notable exceptions. Instead, AMPs may be under balancing selection,
such that over evolutionary timescales multiple alleles are maintained in
populations. In this study, we focus on the Drosophila antimicrobial
peptide Diptericin A, which has a segregating amino acid polymorphism
associated with differential survival after infection with the
Gram-negative bacteria Providencia rettgeri. Diptericin A also helps
control opportunistic gut infections by common Drosophila gut microbes,
especially those of Lactobacillus plantarum. In addition to genotypic
effects on gut immunity, we also see strong sex-specific effects that are
most prominent in flies without functional diptericin A. To further
characterize differences in microbiomes between different diptericin
genotypes, we used 16S metagenomics to look at the microbiome composition.
We used both lab reared and wild caught flies for our sequencing and
looked at overall composition as well as the differential abundance of
individual bacterial families. Overall, we find flies that are homozygous
for one allele of diptericin A are better equipped to survive a systemic
infection from P. rettgeri, but in general have shorter lifespans
after being fed common gut commensals. Our results suggest a possible
mechanism for the maintenance of genetic variation of diptericin A through
the complex interactions of sex, systemic immunity, and the maintenance of
the gut microbiome.