The Spirogyra genome: signatures of shared and divergent division and differentiation

Zygnematophytes came as unexpected closest algal relatives to land plants because of their simple body plans, raising questions about the morphogenetic toolkit that was present in the last common ancestor of land plants and algae. Genomic analyses have unveiled that zygnematophytes are cell biological giants, sharing the homologous chassis for several phytohormones, secondary metabolites s well as key morphogenetic as well as transcriptional regulations. Zygnematophytes fall into five orders and each of these has chartered its own evolutionary path. Here, we have sequenced a contiguous genome of Spirogyra pratensis, epynomous for Spirogyrales and a classical system for evolutionary cell biology in the green lineage. Building on the genome, we transcriptionally profile the tractable life cycle of Spirogyra and its response to a bifactorial gradient of light and temperature. Our data highlight the deployment of quiescence and homeostatic programs. Yet what stands out most in Spirogyra is its spiral chloroplast, undulating intracellularly and abscising during mixed phragmoplast formation and furrowing. Leveraging the genome in tandem with co-expression network analyses we describe the molecular underpinnings of the unique cytokinetic process that govern both cell and plastid division. We find that Spirogyra deploys a molecular program characteristic of Phragmoplastophyta yet lacks the deeply conserved plastid division machinery of archaeplastid plastids.