Polyploidy (genome duplication) is a pivotal force in evolution. However, the interactions between parental genomes in a polyploid nucleus, frequently involving subgenome dominance, are poorly understood. Here we showcase analyses of a bamboo system (Poaceae: Bambusoideae) comprising a series of lineages from diploid (herbaceous) to tetraploid and hexaploid (woody), with 11 chromosome-level de novo genome assemblies and 476 transcriptome samples. We find that woody bamboo subgenomes exhibit stunning karyotype stability, with parallel subgenome dominance in the two tetraploid clades and a gradual shift of dominance in the hexaploid clade. Allopolyploidization and subgenome dominance have shaped the evolution of tree-like lignified culms, rapid growth and synchronous flowering characteristic of woody bamboos as large grasses. Our work provides insights into genome dominance in a remarkable polyploid system, including its dependence on genomic context and its ability to switch which subgenomes are dominant over evolutionary time.

Bulbophyllum is the largest genus in Orchidaceae with a pantropical distribution. Due to highly significant diversifications, it is considered to be one of the most taxonomically and phylogenetically complex taxa. The diversification pattern and evolutionary adaptation of chloroplast genomes are poorly understood in this species-rich genus, and suitable molecular markers are necessary for species determination and phylogenetic analysis. A natural Asian section Macrocaulia was selected to estimate the interspecific divergence of chloroplast genomes in this study. Here, we sequenced the complete chloroplast genome of four Bulbophyllum species, including three species from section Macrocaulia. The four chloroplast genomes had a typical quadripartite structure with a genome size ranged from 156,182 to 158,524 bp. The chloroplast genomes included 113 unique genes encoding 79 proteins, 30 tRNAs and 4 rRNAs. Comparison of the four chloroplast genomes showed that the three species from section Macrocaulia had similar structure and gene contents, and shared a number of indels, which mainly contribute to its monophyly. In addition, interspecific divergence level was also great. Several exclusive indels and polymorphism SSR loci might be used for taxonomical identification and determining interspecific polymorphisms. A total of 20 intergenic regions and three coding genes of the most variable hotspot regions were proposed as candidate effective molecular markers for future phylogenetic relationships at different taxonomical levels and species divergence in Bulbophyllum. All of chloroplast genes in four Bulbophyllum species were under purifying selection, while 13 sites within six genes exhibited site-specific selection. A whole chloroplast genome phylogenetic analysis based on Maximum Likelihood, Bayesian and Parsimony methods all supported the monophyly of section Macrocaulia and the genus of Bulbophyllum. Our findings provide valuable molecular markers to use in accurately identifying species, clarifying taxonomy, and resolving the phylogeny and evolution of the genus Bulbophyllum. The molecular markers developed in this study will also contribute to further research of conservation of Bulbophyllum species.

Tropical and subtropical Asia are major orchid diversity and endemism centers. However, the evolutionary dynamics of orchids in these areas remain poorly studied. Gastrochilus D. Don, a species-rich orchid genus from tropical and subtropical Asian forests, was employed to investigate the issue. We firstly used eight DNA regions to reconstruct the phylogeny and estimate the divergence times within Gastrochilus. We inferred the ancestral ranges and conducted a diversification analysis based on empirical and simulated data. Subsequently, we assessed the ancestral niche state and tested for phylogenetic signals in the evolution of niche conditions. Our results suggested that the most recent common ancestor of Gastrochilus occurred in the subtropical area of the East Asiatic region in the late Miocene (8.13 Ma). At least eight dispersal events and four vicariant events were inferred to explain the current distribution of Gastrochilus, associated with the global cooling from the Plio-Pleistocene. The genus experienced a slowly decreasing diversification rate since its origin, and no significant correlation between current niches and phylogenetic relatedness was observed. The diversification of Gastrochilus was attributed to accumulation through time, integrated with the intensification of the Asian Monsoon system during the Plio-Pleistocene, pollination, and epiphytism.

Pseudostellaria heterophylla produces both closed (cleistogamous, CL) and open (chasmogamous, CH) flowers on the same individual but in different seasons. The production of CH and CL flowers might be in response to environmental changes. To better understand the molecular mechanisms of CH and CL flowering, we compared the transcriptome of the two types of flowers to examine differential gene expression patterns, and to identify gene regulatory networks that control CH and CL flowering. Using RNA sequencing, we identified homologues of 428 Arabidopsis genes involved in regulating flowering processes and estimated the differential gene expression patterns between CH and CL flowers. Some of these genes involved in gene regulatory networks of flowering processes showed significantly differential expression patterns between CH and CL flowers. In addition, we identified another 396 differentially expressed transcripts between CH and CL flowers. Some are involved in environmental stress responses and flavonoid biosynthesis. We propose how the differential expression of key members of three gene regulatory modules may explain CH and CL flowering. Future research is needed to investigate how the environment impinges on these flowering pathways to regulate CH and CL flowering in P. heterophylla.

The endangered epiphytic orchid, Dendrobium chrysotoxum, is known for its ornamental and medicinal uses. However, knowledge of this orchid’s symbiotic seed germination, protocorm anatomy, and developmental morphology is completely unknown. In this study, we investigated the process of protocorm development of D. chrysotoxum during symbiotic germination using anatomical technologies and scanning electron microscopy. There are six development stages that were morphologically and anatomically defined during symbiotic seed germination. The embryo transformed into a protocorm at stage two, and a protrusion developed from the top of the protocorm at stage three and elongated to form the cotyledon at stage four. The stem apical meristem (SAM) was initiated at stage three and well developed at stage four. The first leaf and the root appeared at stages five and six, respectively. The hyphae entered through the micropylar end of the seed at stage one and then invaded the protocorm through rhizoids when rhizoids formed. Invading fungal hyphae colonized the inner cortex at the base of protocorms, formed pelotons, and were digested by host cells later. We conclude that protocorm development is programmed by the embryo, which determines the structure and function of the protocorm. The two developmental zones in a polarized D. chrysotoxum embryo include the smaller cells zone, which forms the cotyledon and a shoot apical meristem at the apical end, and the larger cells zone, which forms the mycorrhiza to house the symbiont at the basal end. These results will provide important insights for further research on the mechanisms underlying orchid-fungi symbiosis and enhance the understanding of orchid evolution.