The MYB transcription factor family is one of the largest families in plants, and its members have various biological functions. R2R3-MYB genes are involved in the synthesis of pigments that yield petal colors. Liriodendron plants are widely cultivated as ornamental trees owing to their peculiar leaves, tulip-like flowers, and colorful petals. However, the mechanism underlying petal coloring in this species is unknown, and minimal information about MYB genes in Liriodendron is available. Herein, this study aimed to discern gene(s) involved in petal coloration in Liriodendron via genome-wide identification, HPLC, and RT-qPCR assays. In total, 204 LcMYB superfamily genes were identified in the Liriodendron chinense genome, and 85 R2R3-MYB genes were mapped onto 19 chromosomes. Chromosome 4 contained the most (10) R2R3-MYB genes, and chromosomes 14 and 16 contained the fewest (only one). MEME analysis showed that R2R3-MYB proteins in L. chinense were highly conserved and that their exon-intron structures varied. The HPLC results showed that three major carotenoids were uniformly distributed in the petals of L. chinense, while lycopene and β-carotene were concentrated in the orange band region in the petals of Liriodendron tulipifera. Furthermore, the expression profiles via RT-qPCR assays revealed that four R2R3-MYB genes were expressed at the highest levels at the S3P/S4P stage in L. tulipifera. This result combined with the HPLC results showed that these four R2R3-MYB genes might participate in carotenoid synthesis in the petals of L. tulipifera. This work laid a cornerstone for further functional characterization of R2R3-MYB genes in Liriodendron plants.

Liriodendron chinense (Hemsl.), a Tertiary relic tree, is mainly distributed in subtropical China. The causes of the geographical distribution pattern of this species are poorly understood. In this study, we inferred historical dispersal routes and glacial refugia of this species by combining genetic data (chloroplast DNA (cpDNA), nuclear ribosomal DNA (nrDNA), and nuclear DNA (nDNA)) and geospatial data (climate and geology) with the methods of landscape genetics. Additionally, based on sequence variation at multiple loci, we employed GenGIS and Barrier software to analyze L. chinense population genetic structure. Dispersal corridors and historical gene flow between the eastern and western populations were detected, and they were located in mountainous regions. Based on species distribution model (SDMs), the distribution patterns in paleoclimatic periods were consistent with the current pattern, suggesting the presence of multiple refuges in multiple mountainous regions in China. The genetic structure analysis clustered most eastern populations into a clade separated from the western populations. Additionally, a genetic barrier was detected between the eastern and western populations. The dispersal corridors and historical gene flow detected here suggested that the mountains acted as a bridge, facilitating gene flow between the eastern and western populations. Due to Quaternary climatic fluctuations, the habitats and dispersal corridors were frequently inhabited by warm-temperate evergreen forests, which may have fragmented L. chinense habitats and exacerbated the differentiation of eastern and western populations. Ultimately, populations retreated to multiple isolated mountainous refugia, shaping the current geographical distribution pattern. These dispersal corridors and montane refugia suggested that the mountains in subtropical China play a crucial role in the conservation of genetic resources and migration of subspecies or related species in this region.

Background: Adaptive genetic differentiation is a hotspot in the research of speciation mechanisms in evolutionary biology. Genomic resources are important for detecting ecological adaptive evolution of non-model plants. Using RNA-seq for non-model plants is a good approach to obtain their genomic resources. The combination of population transcriptome resources and environmental data can provide insights into the genetic mechanism of adaptive genetic differentiation.Results: Based on the population transcriptome data, we investigated the spatial distribution of genetic variations in Liriodendron to detect relationships between ecological factors and genetic differentiation. Environmental data and genetic variations from 17 populations were integrated to detect the population structure, adaptive genes and key environmental factors that shape the population genetic structure by landscape genetic approach. Here, we identified 16592 high-quality single nucleotide polymorphisms (SNPs). The population structure analysis results showed that 17 populations were divided into three groups: L. tulipifera, eastern group and western group of L. chinense. Redundancy analysis and latent factor mixed model analysis suggested that precipitation seasonality, precipitation in the driest quarter, diurnal temperature, and solar radiation in May were closely associated with the adaptive genetic differentiation of Liriodendron. Ecological niche differentiation analysis implied significant ecological niche divergence between L. chinense and L. tulipifera habitats. In total, 858 environment-related loci were identified, which were associated with 464 genes. Pathway enrichment analysis revealed that these genes were significantly enriched in multiple biological pathways. Related studies confirmed that these biological pathways play vital roles in plant growth, development, stress, immune response and photosynthesis.Conclusions: Our research provided empirical evidence that environmental factors may play a key role in driving adaptive genetic differentiation of species. Furthermore, the combination of population transcriptome resources and environmental datasets provides new insights into the study of adaptive genetic differentiation of species.