The evolutionary origin of host association and polycistronic transcription in trypanosomatids

Trypanosomatids (Kinetoplastids) encompass multiple lineages of parasitic protists with monoxenous or dixenous life cycles, infecting insects, vertebrates, and plants; in vertebrate hosts, some are intracellular, while others are extracellular. To understand the origin and diversification of their host associations, we integrated comparative genomics across 47 genomes. Results highlight that monoxenous, extracellular trypanosomatids originated from predatory ancestors through reductive evolution, which diminished their metabolic and hunting capabilities. Intracellularity and dixenous lifestyle convergently originated three times independently. Progressive consolidation of genes into polycistronic transcription units (PTUs) was a central innovation that began in early Glycomonada and expanded through chromosome fission-fusion and gene relocation/inversion. In present-day PTUs, protein complex subunits and metabolic pathway enzymes are positioned for co-expression in temporal synchrony, and chromosomes minimize colinear PTUs to counter transcriptional readthrough. Together, these results provide a time-resolved origin of host-association and polycistronic transcription in trypanosomatids, possibly through an intermediate phase of facultative parasitism.