Balancing selection on an MYB transcription factor maintains the twig trichome color variation in Melastoma normale.

BACKGROUND: The factors that maintain phenotypic and genetic variation within a population have received long-term attention in evolutionary biology. Here the genetic basis and evolution of the geographically widespread variation in twig trichome color (from red to white) in a shrub Melastoma normale was investigated using Pool-seq and evolutionary analyses. RESULTS: The results show that the twig trichome coloration is under selection in different light environments and that a 6-kb region containing an R2R3 MYB transcription factor gene is the major region of divergence between the extreme red and white morphs. This gene has two highly divergent groups of alleles, one of which likely originated from introgression from another species in this genus and has risen to high frequency (> 0.6) within each of the three populations under investigation. In contrast, polymorphisms in other regions of the genome show no sign of differentiation between the two morphs, suggesting that genomic patterns of diversity have been shaped by homogenizing gene flow. Population genetics analysis reveals signals of balancing selection acting on this gene, and it is suggested that spatially varying selection is the most likely mechanism of balancing selection in this case. CONCLUSIONS: This study demonstrate that polymorphisms on a single transcription factor gene largely confer the twig trichome color variation in M. normale, while also explaining how adaptive divergence can occur and be maintained in the face of gene flow.

Characterization of the plastome of Camellia pingguoensis (Theaceae), an endangered and endemic yellow camellia species in China.

Camellia pingguoensis D. Fang is a shrub which is found on limestone of karst forests in Guangxi, China. In this study, we characterized the whole plastid genome of C. pingguoensis using Illumina paired-end sequencing reads. The plastome is 156,621 bp in length, containing two copies of inverted repeat (IR) regions (26,046 bp), a large-single copy (LSC) region (86,289 bp), and a small-single copy (SSC) region (18,240 bp). A total of 114 unique genes in the genome has 80 protein-coding genes, 30 tRNA genes, and 4 rRNA genes. The phylogenetic result indicates C. pingguoensis is closely related to C. nitidissima C. W. Chi.

Molecular identification of natural hybridization between Melastoma malabathricum and Melastoama beccarianum in Sarawak, Malaysia.

Hybridization is very common in flowering plants and it plays a significant role in plant evolution and adaptation. Melastoma L. (Melastomataceae) comprises about 80-90 species in tropical Asia and Oceania, among which 41 species occur in Borneo. Natural hybridization is frequently reported in Melastoma in China, but so far there have been no confirmed cases of hybridization in Southeast Asia (including Borneo), where most species occur. Here, we identified a case of natural hybridization between Melastoma malabathricum L. and Melastoma beccarianum Cogn. in Sarawak, Malaysia, by using sequence data of three nuclear genes and one chloroplast intergenic spacer. Melastoma malabathricum is the most widespread species of this genus, occurring in almost the whole range of this genus, while M. beccarianum is a local species endemic to northern Borneo. Our results showed that natural hybridization and introgression occur between M. malabathricum and M. beccarianum, and the introgression was asymmetrical, mainly from M. malabathricum to M. beccarianum. As adaptive traits can be transferred by introgression, our study suggests that natural hybridization should be a significant mechanism for the evolution and adaptation of Melastoma in Southeast Asia. However, introgression from the common species M. malabathricum to the relatively rare species M. beccarianum may cause the decline of M. beccarianum, incurring conservation concern. With a large number of species of Melastoma and almost year-around flowering in Southeast Asia, more cases of natural hybridization are expected to be found and identified in near future.

RNA-Seq analysis reveals the distinctive adaxial-abaxial polarity in the asymmetric one-theca stamen of Canna indica.

Proper establishment of adaxial-abaxial polarity is essential for the development of lateral organs, while former researches were mostly focused on the polarity regulation in leaves, and little is known in stamens, especially in the asymmetric ones. Canna indica (Zingiberales: Cannaceae) is a widely cultivated ornamental plant and the representative species to study the evolutionary development of Zingiberales. The androecium of Canna indica comprises 3-4 petaloid staminodes and a fertile stamen (FS), which consists of a one-theca anther and a petaloid appendage. The partially petaloid stamen is considered as an intermediate state organ from a two-thecae stamen to a completely petaloid staminode. Using RNA-Seq, we quantified the expressions of the transcripts in anther and petaloid appendage, and detected 64,430 and 57,041 unigenes in these two organs, respectively. 4574 unigenes were down-regulated, and 3525 were up-regulated in petaloid appendage compared with those in anther. GO enrichment analysis indicated that the function of cytokinin is more related to cell differentiation in anther, while auxin is more to cell division in petaloid appendage. B- and C-class floral homeotic genes were expressed in these two androecium parts. Most of the class III HD-ZIP family members, which specify adaxial identity, were expressed lower in petaloid appendage than in anther; while KANADIs and YABBYs, which promote abaxial identity, exhibited opposite expression patterns. In situ hybridization showed that the adaxial marker gene was mainly expressed in the region between the two protrusions of the anther, while the abaxial marker was mainly expressed in petaloid appendage. We hypothesize that the adaxial-abaxial polarity participates in the distinctive anther-petaloid appendage patterning within the asymmetric FS of Canna indica.

Similar Morphologies but Different Origins: Hybrid Status of Two More Semi-creeping Taxa of Melastoma.

Inferring the origins of hybrid taxa based on morphology alone is difficult because morphologically similar hybrids can arise from hybridization between different populations of the same parental species or be produced by hybridization of different parental species. In this study, we investigated the origins of two semi-creeping taxa in Melastoma, which are morphologically similar to a natural hybrid, M. intermedium, by sequencing a chloroplast intergenic spacer, nuclear ribosomal internal transcribed spacer and two low-copy nuclear genes (tpi and cam) in these taxa and their putative parental species. Our sequence analysis provides compelling evidence for the hybrid status of the two semi-creeping taxa: one originating from hybridization between M. dodecandrum and M. malabathricum, and the other between M. dodecandrum and M. normale. The origins of these hybrids are therefore clearly different from M. intermedium, and morphological similarity for the three hybrids is most likely due to their origins from hybridization between the same creeping species M. dodecandrum and a different erect species in each of the three cases. We also observed low rate of introgression from M. normale to M. dodecandrum, and genetic exchange between them may transfer adaptive traits to M. dodecandrum. Rare occurrence of these two hybrids may be due to small range overlaps between parental species in one case, and different flowering periods between parental species in the other.