Adaptation and species divergence in extreme environments are key questions in plant evolutionary biology and molecular ecology.The approximately 80 mangrove species, woody plants inhabiting tropical and subtropical intertidal zones, have long been the focus of research. These plants have adapted to the stressful environmental conditions of high salinity, tidal inundation, hypoxia, and strong UV (Duke, 2014(Duke, , 2017. Morphological traits typical of mangrove plants, such as viviparity, salt-excreting leaves, aerial roots, and high stem
Zygophyllum xanthoxylon and Z. fabago are two important desert plants from Zygophyllaceae, which are both widely distributed in north-western China. Here, we report the complete chloroplast genome sequences of Z. xanthoxylon and Z. fabago, which are 109,577 bp and 108,695 bp in length, respectively. The inverted repeat regions, the large single-copy region and the small single-copy region of Z.xanthoxylon are 5084 bp, 83,735 bp, and 15,674 bp in length, respectively, while those of Z. fabago are 4669 bp, 82,293 bp and 17,064 bp in length, respectively. A total of 98 genes were annotated in the genome of Z. xanthoxylon including 29 tRNA, 4 rRNA and 65 protein-coding genes, and 100 genes were annotated in the genome of Z. fabago including 31 tRNA, 4 rRNA and 65 protein-coding genes. Phylogenetic analysis showed Z. xanthoxylon clustered and Z. fabago formed a monophyletic group sister to Tetraena mongolica.
SUMMARY
Adaptation to new environments is a key evolutionary process which presumably involves complex genomic changes. Mangroves, a collection of approximately 80 woody plants that have independently invaded intertidal zones >20 times, are ideal for studying this process. We assembled near‐chromosome‐scale genomes of three Xylocarpus species as well as an outgroup species using single‐molecule real‐time sequencing. Phylogenomic analysis reveals two separate lineages, one with the mangrove Xylocarpus granatum and the other comprising a mangrove Xylocarpus moluccensis and a terrestrial Xylocarpus rumphii. In conjunction with previous studies, we identified several genomic features associated with mangroves: (i) signals of positive selection in genes related to salt tolerance and root development; (ii) genome‐wide elevated ratios of non‐synonymous to synonymous substitution relative to terrestrial relatives; and (iii) active elimination of long terminal repeats. These features are found in the terrestrial X. rumphii in addition to the two mangroves. These genomic features, not being strictly mangrove‐specific, are hence considered pre‐adaptive. We infer that the coastal but non‐intertidal habitat of X. rumphii may have predisposed the common ancestor to invasion of true mangrove habitats. Other features including the preferential retention of duplicated genes and intolerance to pseudogenization are not found in X. rumphii and are likely true adaptive features in mangroves. In conclusion, by studying adaptive shift and partial shifts among closely related species, we set up a framework to study genomic features that are acquired at different stages of the pre‐adaptation and adaptation to new environments.
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