Whole-genome duplication has been recognized as a major process in speciation of land plants, especially in ferns. Whereas genome downsizing contributes greatly to the post-genome shock responses of polyploid flowering plants, diploidization of polyploid ferns diverges by maintaining most of the duplicated DNA and is thus expected to be dominated by genic processes. As a consequence, fern genomes provide excellent opportunities to study ecological speciation enforced by expansion of protein families via polyploidy. To test the key predictions of this hypothesis, we reported the de novo genome sequence of Adiantum nelumboides, a tetraploid homosporous fern. The obtained draft genome had a size of 6.27 Gb assembled into 11,767 scaffolds with the contig N50 of 1.37 Mb. Repetitive DNA sequences contributed with about 81.7%, a remarkably high proportion of the genome. With 69,568, the number of predicted protein-coding genes exceeded those reported in most other land plant genomes. Intragenomic synteny analyses recovered 443 blocks with the average block size of 1.29 Mb and the average gene content of 16 genes. The results are consistent with the hypothesis of high ancestral chromosome number, lack of substantial genome downsizing, and dominance of genic diploidization. As expected in the calciphilous plants, a notable number of detected genes were involved in calcium uptake and transport. In summary, the genome sequence of a tetraploid homosporous fern not only provides access to a genomic resource of a derived fern, but also supports the hypothesis of maintenance of high chromosome numbers and duplicated DNA in young polyploid ferns.

The mitogenome of Bauhinia variegate was assembled and characterized in this study. The mitogenome size was 437,271 bp, and its GC content was 45.5%. 36 protein-coding genes, 17 tRNAs and 3 rRNAs were annotated in the mitogenome. A total of 12 MTPTs, ranging from 71 bp to 3562 bp, were identified in the mitogenome and covered 1.46% (6373 bp) of the mitogenome. Phylogenetic analysis of 15 species of Leguminosae based on 23 core protein-coding genes showed that B. variegata was sister to Tylosema esculentum, another member from the subfamily Cercidoideae. The mitogenome of B. variegata provides a valuable genetic resource for further phylogenetic studies of this family.

The mangrove fern Acrostichum aureum is widely distributed in the Indo West-Pacific and Atlantic East-Pacific regions. Here we assembled and annotated its chloroplast genome based on the Illumina sequencing reads. The complete chloroplast genome of A. aureum was 154,805 bp in length with the GC content of 38.38%. It contains a large single copy (LSC) region of 82,826 bp and a small single copy (SSC) region of 21,617 bp, separated by a pair of inverted repeat region (IRs) of 25,181 bp each. It contains 84 protein coding genes, 27 tRNA genes, and four rRNA genes. Phylogenetic analysis shows that A. aureum is closest to Ceratopteris cornuta in the subfamily Parkerioideae. The chloroplast genome of A. aureum reported here offers a useful resource for its phylogeography and conservation genetics studies.

Understanding the genetic basis of species differences in flowering time and inflorescence architecture can shed light on speciation and molecular breeding. Melastoma shows rapid speciation, with about 100 species formed in the past few million years, and, meanwhile, possesses high ornamental values. Two largely sympatric and closely related species of this genus, M. candidum and M. normale, differ markedly in flowering time and flower number per inflorescence. Here, we constructed an F2 population between M. candidum and M. normale, and used extreme bulks for flowering time and flower number per inflorescence in this population to identify genomic regions underlying the two traits. We found high differentiation on nearly the whole chromosome 7 plus a few regions on other chromosomes between the two extreme bulks for flowering time. Large chromosomal inversions on chromosome 7 between the two species, which contain flowering-related genes, can explain recombinational suppression on the chromosome. We identified 1872 genes with one or more highly differentiated SNPs between the two bulks for flowering time, including CSTF77, FY, SPA3, CDF3, AGL8, AGL15, FHY1, COL9, CIB1, FKF1 and FAR1, known to be related to flowering. We also identified 680 genes with one or more highly differentiated SNPs between the two bulks for flower number per inflorescence, including PNF, FIL and LAS, knows to play important roles in inflorescence development. These large inversions on chromosome 7 prevent us from narrowing down the genomic region(s) associated with flowering time differences between the two species. Flower number per inflorescence in Melastoma appears to be controlled by multiple genes, without any gene of major effect. Our study indicates that large chromosomal inversions can hamper the identification of the genetic basis of important traits, and the inflorescence architecture of Melastoma species may have a complex genetic basis.

Mitochondrial genome sequences are very useful for understanding the mitogenome evolution itself and reconstructing phylogeny of different plant lineages. Bauhinia purpurea, a species from the legume family Leguminosae, is widely distributed in South China and has high ornamental value. Here, we sequenced and assembled the mitogenome of B. purpurea to provide a useful genetic resource for further evolutionary studies. We assembled and characterized the complete mitogenome of B. purpurea based on Illumina sequence data. The mitogenome size was 525,727 bp, and its GC content was 45.38%. A total of 35 protein-coding genes, 16 tRNA genes, and 3 rRNA genes were identified in the mitogenome. We also identified 124 pairs of repeats and 6 mitogenome sequences of plastid origin (MTPTs). These MTPTs range from 108 bp to 751 bp, covering 0.65% of the mitogenome.