Diploid- and tetraploid wall barley exhibit alternative molecular responses but comparable reproductive performance in response to different ambient temperature regimes.

Diploid and allopolyploid relatives often differ in their responses to stressful or variable environments, but it remains unclear whether this is reflected in greater reproductive stability in such conditions, and whether any such advantage is associated with greater canalisation or plasticity of morphological and developmental traits or a more extensive transcriptional response. We tested this in the annual grass Hordeum murinum by comparing the reproductive performance, phenotypic responses, and transcriptomic responses of diploid and allotetraploid H. murinum subspecies under control and elevated temperature conditions. Gene-level nucleotide diversity was higher in the diploid ssp. glaucum than in either subgenome of the allotetraploid ssp. murinum, whereas divergence was greatest between the two subgenomes of the allotetraploid. Relative to ssp. glaucum, ssp. murinum altered expression in a larger proportion of its analysed transcriptome (10% versus 8%), spanned more enriched functional categories (36 versus 19), and showed pronounced subgenome-specific contributions to the response. In addition, 702 of 11 106 homeolog pairs showed temperature-dependent shifts in expression bias despite the absence of genome-wide dominance of either subgenome. By contrast, phenotypic responses were largely similar between subspecies. Although ssp. glaucum and ssp. murinum differed in phenotype, especially in phenology, temperature explained more phenotypic variation than subspecies and reduced reproductive biomass by 42% in ssp. glaucum and 47% in ssp. murinum. Overall, ssp. murinum showed neither greater plasticity nor better maintenance of reproductive performance. Thus, compared with diploid H. murinum, allopolyploid H. murinum showed broader transcriptional deployment under elevated temperature, but not greater reproductive stability or clear differences in the plasticity of morphological and developmental traits. These results point to a decoupling of trait and molecular differentiation from fitness under elevated temperature, suggesting that the allotetraploid's altered transcriptional response may represent an alternative route to maintaining fitness comparable to that of the diploid rather than a reproductive advantage.