Nuclear dualism without extensive DNA elimination in the ciliate Loxodes magnus

Abstract

Ciliates are unicellular eukaryotes with two distinct kinds of nuclei in each cell: transcriptionally active somatic macronuclei (MAC) and silent germline micronuclei (MIC). In the best-studied model species, both nuclei can divide asexually, but only germline MICs participate in meiosis, karyogamy, and development into new MACs. During MIC-to-MAC development, thousands of mobile element relics in the germline, called internally eliminated sequences (IESs), are excised. This genome editing enables IESs to persist by shielding them from somatic natural selection. Editing itself is a costly, time-consuming process, hypothetically maintained by evolutionary addiction. Loxodes magnus and its relatives (class Karyorelictea) are cytologically unusual because their MACs do not divide asexually, but must develop anew from mitotically generated MIC copies every cell division. Here, we report that Loxodes genome development is also unconventional. We found no canonical germline-limited IESs in Loxodes despite careful purification and long-read sequencing of MICs and MACs. The k-mer content of these nuclei overlapped, and indels found by read mapping were consistent with allele variants rather than IESs. Two other hallmarks of genome editing--domesticated DDE-family transposases and editing-associated small RNAs--were also absent. Nonetheless, histone marks, nucleosome and DNA N6-methyladenosine distributions in vegetative Loxodes cells are consistent with actively transcribed MACs and inactive MICs, like other ciliates. Both genomes, not only the MIC, were large and replete with retrotransposon sequences. Given the costs associated with genome editing, we hypothesize that karyorelicteans like Loxodes have lost or streamlined editing during MIC-to-MAC development, and have found a way out of the addictive cycle.