Colonization of new habitats is expected to require genetic adaptations to overcome environmental challenges. Here, we use full genome re-sequencing and extensive common garden experiments to investigate demographic and selective processes associated with colonization of Japan by Lotus japonicus over the past~20,000 years. Based on patterns of genomic variation, we infer the details of the colonization process where L. japonicus gradually spread from subtropical conditions to much colder climates in northern Japan. We identify genomic regions with extreme genetic differentiation between northern and southern subpopulations and perform population structure-corrected association mapping of phenotypic traits measured in a common garden. Comparing the results of these analyses, we find that signatures of extreme subpopulation differentiation overlap strongly with phenotype association signals for overwintering and flowering time traits. Our results provide evidence that these traits were direct targets of selection during colonization and point to associated candidate genes.

AbstractColonization of new habitats is expected to require genetic adaptations to overcome environmental challenges. Here we use full genome re-sequencing and extensive common garden experiments to investigate demographic and selective processes associated with the recent colonization of Japan by Lotus japonicus. We carefully track the colonization process where L. japonicus gradually spread from subtropical conditions to much colder climates in northern Japan. We characterize the loss of diversity during this process and identify genomic regions with extreme genetic differentiation. Next, we perform population structure-corrected association mapping of phenotypic traits measured in a common garden and discover a number of genome-wide significant associations. Contrasting these analyses, we find that there is a strong concordance between phenotypic variation and extreme differentiation for overwintering and flowering time traits. Our results provide evidence that these traits were direct targets of selection by local adaptation during the colonization process and point to associated candidate genes.

In plants, timing of flowering is an essential factor that controls the survival rates of descendants. The circadian clock genes E1 and GIGANTEA (GI) play a central role in transmitting signals to flowering locus T (FT) in leguminous plants. Lotus japonicus is a wild Japanese species that ranges from northern Hokkaido to the southern Ryukyus and exhibits a wide range in terms of the time between seeding and first flowering. In this study, we first identified LjGI and analyzed polymorphisms of LjE1 and LjGI among wild populations covering the entire distribution range of this species in Japan. LjGI had a coding sequence (CDS) length of 3495 bp and included 14 exons. The homologies of DNA and amino acid sequences between LjGI and GmGI were 89 and 88% (positive rate was 92%), respectively. LjE1 harbored five nucleic acid changes in a 552 bp CDS, all of which were nonsynonymous; four of the changes were located in the core function area. LjE1 alleles exhibited partial north-south differentiation and non-neutrality. In contrast, the LjGI harbored one synonymous and one nonsynonymous change. Thus, our study suggests that LjE1 may be involved in the control of flowering times, whereas LjGI may be under strong purifying selection.

Flowering is an important step in the life cycle of plants and indicates adaptability to external climatic cues such as temperature and photoperiod. We investigated the expression patterns of core genes related to flowering time regulation in Japanese wild radish (Raphanus sativus var. raphanistroides Makino) with different vernalization requirements (obligate and facultative) and further identified climatic cues that may act as natural selective forces. Specifically, we analyzed flowering time variation under different cold and photoperiod treatments in Japanese wild radish collected from the Hokkaido (northern lineage) and Okinawa (southern lineage) islands, which experience contrasting climatic cues. The cultivation experiment verified the obligate and facultative vernalization requirements of the northern and southern wild radish accessions, respectively. The expression of major genes involved in flowering time indicated that RsFLC and RsCOL-5 (A&B) may interact to regulate flowering time. Notably, floral initiation in the northern lineage was strongly correlated with RsFLC expression, whereas flowering in the southern linage was correlated with induction of RsCOL5-A expression, despite high RsFLC transcript levels. These results suggested that the northern accessions are more sensitive to prolonged cold exposure, whereas the southern are more sensitive to photoperiod. These different mechanisms ultimately confer an optimal flowering time in natural populations in response to locally contrasting climatic cues. This study provides new insights into the variant mechanisms underlying floral pathways in Japanese wild radish from different geographic locations.

No abstract