Phytochytrium and Sparrowiella, two new polycentric genera in Cladochytriales

Cladochytriales comprise multiple monocentric and polycentric lineages of chytrids, which are commonly found as saprobes in algae and decaying plant material in aquatic and terrestrial ecosystems. Although monocentric species are more common among the Chytridiomycota, additional polycentric chytrids have been described in recent years. During our ongoing effort to add to the knowledge of the diversity of chytrids, we found two polycentric chytrids on onion skin bait that produce rhizomycelia like none of described cladochytrialean genera or like those of genera in other orders. The rhizomycelia of these two newly found isolates consist of isodiametric filaments that lack swellings such as those found in either Cladochytrium or Nowakowskiella. Both zoosporangia and resting spores develop on short, right-angled offshoots from these filaments. Our analyses of the small and large subunits of rDNA indicate that these two isolates belong to separate branches of a predominantly monocentric clade of the Cladochytriales. Phytochytrium stagnum gen. et sp. nov. groups with two monocentric strains and Sparrowiella insolita gen. et sp. nov. are in an independent lineage distantly related to a monocentric species. We have expanded the concept of Septochytriaceae to include this clade. The ancestral state reconstruction allowed us to hypothesize for the first time a possible growth pattern of the common ancestral that gave origin to the order, besides to corroborate the unreliable nature of thalli's development as a taxonomic character.

Automated summary

Cladochytriales is a group of chytrids that includes both monocentric and polycentric lineages, commonly found in algae and decaying plant material in aquatic and terrestrial ecosystems. Recently, two polycentric chytrids with unique rhizomycelia structures were discovered on onion skin, distinct from known cladochytrialean genera or those in other orders. Phylogenetic analysis revealed that these two isolates belong to separate branches of a predominantly monocentric clade. The discovery of these new chytrids expands our understanding of their diversity and supports the unreliable nature of thalli's development as a taxonomic character.