Vernalization — the acquisition of flowering competence after prolonged winter cold — has evolved repeatedly in temperate angiosperms by genetically distinct routes: an FLC/FRIGIDA-based epigenetic switch in the Brassicaceae, a VRN1/VRN2-based regulatory loop in temperate grasses, and a cold-repressible FLC homolog in sugar beet. The problem asks for the evolutionary account of this repeated adaptation at its natural angiosperm-wide generality: the phylogenetic pattern of gains and losses of the vernalization requirement together with the selective conditions under which they occur, and a component-level determination of how much of the underlying network logic (a cold-activated gene, a flowering repressor, a florigen integrator, and chromatin-based memory of cold) reflects inherited constraint versus independent reinvention at each origin. Comparative studies from 2007 to 2026 have resolved pieces within single families, but no angiosperm-scale accounting exists, and the most recent primary literature still frames both halves of the question as unresolved.

Background

Many plants of seasonal temperate climates delay reproduction until spring by means of vernalization: an extended period of winter cold that does not itself induce flowering but renders the plant competent to flower once growth-promoting conditions return. A vernalization requirement is one of the clearest known cases of a complex adaptive trait that has appeared repeatedly and by visibly different genetic means in unrelated plant families, which is why its evolution is a persistent question in plant evolutionary genetics.

In the Brassicaceae (the mustard family, including Arabidopsis thaliana), the requirement is created by two genes. FRIGIDA (FRI), a coiled-coil scaffold protein, promotes high expression of FLOWERING LOCUS C (FLC), a MADS-box transcription factor (MADS-box genes form a large family of DNA-binding regulators, many of which control flowering) that represses flowering by inhibiting FT, the mobile leaf-to-shoot flowering signal (florigen). Prolonged cold epigenetically silences FLC: the Polycomb repressive complex 2 (PRC2), a conserved chromatin-modifying machine, deposits the repressive histone modification H3K27me3 across the FLC locus, and the silenced state is stably maintained through many subsequent cell divisions, forming a cellular memory of winter (Bouché et al. 2017; Dixon et al. 2019).

In the temperate cereals wheat and barley (grass subfamily Pooideae), the requirement is built from different, sequence-unrelated components. VRN1, an APETALA1/FRUITFULL-like MADS-box gene that promotes floral meristem identity, is switched on gradually during cold; VRN2, a repressor with a CCT domain (a zinc-finger DNA-binding module of the CONSTANS family), blocks the FT-like florigen VRN3 before winter; cold-induced VRN1 then represses VRN2, releasing VRN3 under spring long days (Yan et al. 2003; Woods et al. 2016). In sugar beet (family Amaranthaceae), a dicot lineage outside the mustards, cold-repressible FLC homologs again carry the requirement (Reeves et al. 2007).

The overall picture is thus one of repeated, largely independent assembly. Reviews of the two exemplar frameworks conclude that distinct genes underlie vernalization in different plant groups, indicating independent origins in lineages that diverged under warm paleoclimates where a cold response was not advantageous (Ream et al. 2012; Bouché et al. 2017), while the eLS review by Dixon et al. (2019) identifies a shared three-player network logic — a cold-activated gene, a flowering repressor, and an integrator (florigen) — present in both monocots and dicots, with the relative importance of allelic variation at these genes differing between the two groups. Comparative work has since mapped parts of this mosaic. Within the grasses, vernalization responsiveness is widespread across Pooideae and apparently arose early during their tropical-to-temperate transition, yet the signature VRN1–VRN2 regulatory loop evolved only late, within the core temperate grasses: cold repression of VRN2 is unique to that core, and VRN1-mediated suppression of VRN2 is absent in the model grass Brachypodium distachyon (McKeown et al. 2016; Woods et al. 2016). Vernalization-induced flowering evolved at least five times independently within the grass family alone, with different combinations of FRUITFULL-like genes recruited on several of these occasions (Paliocha et al. 2023), and short-day vernalization appears widespread and early-origin in Pooideae, likely through co-option of a GF14h–VRN2–FTL4 flowering regulon (Schubert et al. 2026). Within Brassicaceae, the FLC ortholog PEP1 of the perennial Arabis alpina shares only about 14% of its binding sites with Arabidopsis FLC: flowering-time targets are conserved while stress-response regulation was independently recruited (Mateos et al. 2017), and FLC-family functions have further diversified between annual and perennial species (Soppe et al. 2021). On the loss side, summer-annual Arabidopsis accessions have repeatedly shed the requirement through loss-of-function FRI alleles and independently weakened FLC alleles (Michaels et al. 2003), and Cardamine flexuosa modified its response by recruiting an age-sensing microRNA module into the vernalization pathway (Zhou et al. 2013). What no study has produced is the integrated, angiosperm-scale accounting of how often the requirement was gained and lost, under what selective conditions, and which parts of the network each origin reused versus rebuilt.

Problem Statement

Explain the repeated evolution of the vernalization requirement across the angiosperms at its natural generality, in two linked parts.

(1) Pattern and selective context. Determine, on a phylogenetically explicit basis across flowering plants, how many times a vernalization requirement (a cold-induced flowering-competency mechanism whose absence blocks or strongly delays flowering) has been gained and lost, in which lineages, and under what ecological and selective conditions the gains and losses occur — including the conditions associated with the transition from tropical to temperate seasonal regimes, the role of annual versus perennial life history, and why descendant lineages sometimes lose the requirement.

(2) Constrained versus reinvented logic. For each independent origin, establish which components of the underlying network are inherited constraints (homologous components redeployed, as with the FLC-based route shared by Brassicaceae and Amaranthaceae) and which are independently recruited from unrelated components that converge on the same three-player logic of a cold-activated gene, a flowering repressor, and a florigen integrator — including how often the chromatin-based epigenetic memory of cold is itself reinvented rather than reused.

A complete answer must provide: a phylogenetically resolved accounting of gains and losses across a representative angiosperm sampling, together with the criteria and evidence used to score the requirement in each species; a component-level constraint-versus-reinvention map for each inferred origin, supported by orthology assignments and functional or expression evidence in the exemplar lineages; and an explicit, evidence-linked statement of the selective conditions associated with the inferred transitions, distinguishing tested inference from conjecture. The question is not restricted to any single family, mechanism class, or answer form; partial family-level accounts (for example within Poaceae or Brassicaceae alone) are progress toward, not completion of, the stated objective.

References

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