Abstract Zygnematophytes emerged as the unexpected closest algal relatives of land plants despite their simple body plans, raising questions about the morphogenetic toolkit present in the last common ancestor of land plants and algae. Genomic analyses have revealed that zygnematophytes are cellular giants, sharing homologous frameworks for several phytohormones, secondary metabolites, and key morphogenetic and transcriptional regulatory processes. Zygnematophytes fall into 5 orders, each of which has charted its own evolutionary path. Here, we have sequenced a contiguous genome of Spirogyra pratensis, the eponymous representative of Spirogyrales and a classical model system for evolutionary cell biology in the green lineage. Building on this genome, we transcriptionally profiled the tractable life cycle of Spirogyra and its responses to a bifactorial gradient of light and temperature. Our data highlight the activation 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 processes 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 found in other archaeplastid plastids.
Elisa S. Goldbecker, Deepti Varshney, A. Holzhausen et al.· The Plant Cell· 1 citation
Chromosomal rearrangements are hypothesized to facilitate speciation by suppressing recombination in locally adapted genomic regions, yet how they shape evolutionary rates during rapid divergence remains poorly understood. Here, we investigate the genomic architecture of two sister Carex (Cyperaceae) species on Réunion Island, which rapidly diverged (∼0.5 Mya) to occupy contrasting tropical-montane and dry-subalpine habitats. Using chromosome-level assemblies and population genomics, we show that genomic divergence is not uniform across the genome but is concentrated within specific large-scale inversions. Crucially, genes within these structural variants exhibit significantly accelerated rates of protein evolution, as evidenced by elevated ω, compared to the collinear genome. This is consistent with recombination suppression and subsequent relaxation of purifying selection driving these patterns, which may complement or even outweigh the signal of positive selection. Functional analysis and environmental associations reveal that these "genomic accelerators" include key adaptive loci: Inversions on chromosomes 14 and 28 are enriched for mechanosensitive ion channels and auxin transport, which is consistent with facilitating the interspecific physiological shift to aridity. Partial redundancy analyses reveal that ongoing intraspecific ecological adaptation is highly polygenic across the collinear genome. Our results demonstrate that genomic architecture actively dictates evolutionary speed, suggesting that certain lineages boosted by structural variants may bypass the typical constraints of purifying selection to rapidly exploit vacant ecological opportunities.
Inés Gómez-Ramos, Rogelio Sánchez-Villegas, A. Mohan et al.· Proceedings of the National...· 0 citations
