Single dominant diatom can host diverse parasitic fungi with different degree of host specificity

Abstract

Recent molecular surveys revealed an unexpected diversity of mycoplankton in lakes and oceans. The early diverging fungal lineages are known to be prominent parasites of phytoplankton. However, due to missing fungal reference data, their identity and ecology remain mostly unknown. To overcome this problem, we combined single‐spore‐based DNA barcoding during one sampling season of the eutrophic Lake Inba (Japan), which is dominated by two diatoms. By linking microscopically picked single fungal spore on two diatoms to subsequent DNA barcoding of ribosomal maker genes, we identified 12 distinct lineages, affiliated not only to the known parasitic phylum Chytridiomycota (chytrids), but also to the enigmatic phyla Rozellomycota and Aphelidiomycota. The detected Rozellomycota could be a hyperparasite of parasitic chytrid infecting diatoms. Host specificity appeared to be different among clades. However, the barcoding of single‐spore DNA could not clearly prove host specificity due to the limited number of samples and resolution in the targeted gene regions. The degree of host specificity was thus confirmed by the cross‐infection experiments. Five chytrid strains were generalists infecting all four diatoms strains, while the remaining three strains of chytrids were specialists exhibiting host preferences. Additional growth experiments indicated a trade‐off between host specificity and growth rate, so that generalists with more possible hosts grew slower than specialists with one host. Our results suggest that the combination of microscopy with single‐spore‐based barcoding is a promising approach to evaluate the complex host‐parasite interactions, while infection experiments can verify the interactions and shed light on the underlying ecological principles.

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