To endure predictable, periodic frost, plants must have evolved to integrate both phenology and complex physiological adjustments. Most importantly, plants need to maintain the integrity of cell membranes to avoid osmotic stress during prolonged freezing ( Thomashow, 1999). The capacity for surviving environments with subzero temperatures depends on the acquisition of frost tolerance. The fact that only a restricted number of plant lineages have transitioned into the temperate region suggests that it is challenging to evolve the coordinated set of physiological changes needed to withstand low temperatures ( Donoghue, 2008). The successful colonizers of the emerging temperate climates faced gradually more severe environmental stresses and evolved adaptations to frost, increased temperature seasonality, and short growing seasons ( Zachos et al., 2001 Eldrett et al., 2009 Mudelsee et al., 2014).

Throughout the Oligocene the temperatures continued to drop, and together with increased seasonality ( Zachos et al., 2001 Eldrett et al., 2009) this triggered the greater expansion of temperate biomes. Around the Eocene-Oligocene (E-O) transition 34 million years ago, global temperatures fell dramatically ( Pound and Salzmann, 2017). TodayвЂ™s cold climate is the result of gradual cooling over the last 50 million years, and the temperate biomes originated relatively recently in EarthвЂ™s history when the global climate started to cool in the late Eocene ( Potts and Behrensmeyer, 1992 Morley, 2000 Zachos et al., 2001 Fine and Ree, 2006 Eldrett et al., 2009 Liu et al., 2009 StrГ¶mberg, 2011). These lineages represent several independent transitions from tropical to temperate environments and represent a variety of evolutionary adaptations to both unpredictable, short incidents of episodic frost and predictable periods of periodic frosts (i.e. Only a few ancestrally tropical angiosperm lineages have colonized temperate biomes ( Judd et al., 1994 Wiens and Donoghue, 2004 Kerkhoff et al., 2014). How Pooideae evolved to transition from tropical to temperate biomes sheds light on how complex traits evolve in the light of climate changes.ДЇrost is one of the most dramatic abiotic stresses a plant can experience, and exposure to frost is a limiting factor for many species to diversify in temperate and arctic regions. Evolution of adaptations to seasonally cold climates is regarded as particularly difficult. Nevertheless, conservation of cold-induced gene expression of certain genes indicates that the Pooideae ancestor may have possessed some molecular machinery to mitigate cold stress. Our results reflect that selection pressure resulting from global cooling must have acted on already diverged lineages.

We saw evidence of protein-coding and regulatory sequence evolution as well as the origin of novel genes and functions contributing toward evolution of a cold response in Pooideae. Furthermore, we studied the correlation between gene family size and number of cold-responsive genes as well as between selection pressure on coding sequences of genes and their cold responsiveness. However, comparing differential gene expression after cold treatment in transcriptomes of five phylogenetically diverse species revealed widespread species-specific responses of genes with conserved sequences. Here, we investigated the evolution of cold acclimation in Pooideae and found that a phylogenetically diverse set of Pooideae species displayed cold acclimation capacity. However, understanding how complex traits like cold acclimation evolve remains a major challenge in evolutionary biology. One such adaptation is cold acclimation, wherein plants increase their frost tolerance in response to gradually falling temperatures and shorter days in the autumn. The grass subfamily Pooideae dominates the grass floras in cold temperate regions and has evolved complex physiological adaptations to cope with extreme environmental conditions like frost, winter, and seasonality.

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