Population-level analyses indirectly reveal cryptic sex and life history traits of Pseudoperkinsus tapetis (Ichthyosporea, Opisthokonta): a unicellular relative of the animals.

We use population genetics to detect the molecular footprint of a sexual cycle, of a haploid vegetative state, and of lack of host specificity in Pseudoperkinsus tapetis, a marine unicellular relative of the animals. Prior to this study, complete life cycles were not known for any of the unicellular lineages sharing common ancestry with multicellular animals and fungi. We established the first collection of conspecific cultures of any member from the unicellular opisthokont lineage ichthyosporea, isolating 126 cultures of P. tapetis from guts of marine invertebrates ranging from clams to sea cucumbers. We sequenced fragments of the elongation factor alpha-like (EFL) and heat-shock protein 70 (HSP70) genes for a subset of our isolates. Absence of heterozygotes from the EFL locus in 52 isolates provided evidence for haploidy. Phylogenetic incongruence and a lack of support for linkage between two loci from 34 sequenced isolates signified a history of recombination consistent with a sexual cycle. Shared haplotypes in different invertebrate species showed that P. tapetis was not host specific. Based on estimates of the frequency of sex and on observations of cultures, we propose that P. tapetis is transmitted between hosts via asexual endospores. New protists are continually being discovered, and, as this study illustrates, analysis of culturable collections from natural habitats can transform a species from a near unknown to a model system for better understanding the evolution of life histories.