Sequence acquisition
All fish gut bacterial 16S rRNA sequences were obtained from GenBank except for the
guppy (Poecilia reticulata) library, which were generated through cloning and sequencing
(See Supporting information for details). All nonguppy data sets were identified through
Entrez queries using targeted search terms, or through a review of the published literature on
fish and their gut microbes. Sequences from each of these studies had been similarly
generated via Sanger sequencing from PCR products that were amplified with ‘universal’
bacterial primers as stated by the authors of the studies. Within a particular study, all
sequences from one fish species were counted as a single library when acquired by the same
method (i.e. culture-dependent or culture-independent techniques). However, in cases where
the study specified that individuals came from different fish populations, sequences from
separate populations were analysed separately as distinct libraries (exceptions are noted in
Supporting information).