Oophila is monophyletic within a three-taxon eukaryotic microbiome in egg masses of the salamander Ambystoma maculatum

Abstract

Tests of hypotheses of vertical transmission and competitive exclusion for particular resource-exchange mutualisms require assessments of the number and identity of partners. Unicellular green algae form a symbiosis with embryos of the northeastern yellow spotted salamander Ambystoma maculatum. The extent to which the composition of egg mass-associated taxa depends on temporal or spatial factors is unknown and the discovery of an endosymbiotic component to this symbiosis has increased the importance of knowing the specificity of the association. We assessed the diversity and relative abundance of algal and other unicellular eukaryotes in egg masses by subjecting DNA harvested from cells in egg capsules or water from breeding habitat to 18S amplicon sequencing. A nested sampling regime for two breeding seasons allowed comparisons among capsules within egg masses, among egg masses in ponds, between breeding locales, between pond water and capsule fluid, and between years. Based upon 4.33 X106 reads, we found no evidence inside fifty-eight egg capsules of algal lineages outside the Oophila clade and report that, whereas non-algal eukaryotic sequence diversity was low, cercozoan protists and chytridomycete fungi were sometimes abundant. There were weak or inconsistent egg mass, year or locale effects, but there was a very strong localization effect, indicating that capsules are rarefied spaces for eukaryotic aquatic taxa. This work provides a foundation for comparing microbial community structure within and among egg masses across the geographic range of the host.