Project description:Determination of the complete genome of European mountain ash ringspot-associated emaravirus from Sorbus intermedia reveals two additional genome segments
Project description:Background and aimsInterspecific hybridization and polyploidy are key processes in plant evolution and are responsible for ongoing genetic diversification in the genus Sorbus (Rosaceae). The Avon Gorge, Bristol, UK, is a world 'hotspot' for Sorbus diversity and home to diploid sexual species and polyploid apomictic species. This research investigated how mating system variation, hybridization and polyploidy interact to generate this biological diversity.MethodsMating systems of diploid, triploid and tetraploid Sorbus taxa were analysed using pollen tube growth and seed set assays from controlled pollinations, and parent-offspring genotyping of progeny from open and manual pollinations.Key resultsDiploid Sorbus are outcrossing and self-incompatible (SI). Triploid taxa are pseudogamous apomicts and genetically invariable, but because they also display self-incompatibility, apomictic seed set requires pollen from other Sorbus taxa - a phenomenon which offers direct opportunities for hybridization. In contrast tetraploid taxa are pseudogamous but self-compatible, so do not have the same obligate requirement for intertaxon pollination.ConclusionsThe mating inter-relationships among Avon Gorge Sorbus taxa are complex and are the driving force for hybridization and ongoing genetic diversification. In particular, the presence of self-incompatibility in triploid pseudogamous apomicts imposes a requirement for interspecific cross-pollination, thereby facilitating continuing diversification and evolution through rare sexual hybridization events. This is the first report of naturally occurring pseudogamous apomictic SI plant populations, and we suggest that interspecific pollination, in combination with a relaxed endosperm balance requirement, is the most likely route to the persistence of these populations. We propose that Avon Gorge Sorbus represents a model system for studying the establishment and persistence of SI apomicts in natural populations.
Project description:Sorbus amabilis Cheng ex Yü, a small excellent ornamental tree species, is only distributed in Eastern China. In this study, we assembled and annotated the complete chloroplast (cp) genome of the species using the next-generation sequencing for the first time. The cp genome was 160,006 bp in size, consisting of two copies of invert repeat (IR) regions of 26,405 bp, one large single-copy (LSC) region of 87,870bp, and one small single-copy (SSC) region of 19,326 bp. The overall GC content of the genome was 36.55%. The genome was predicted to contain 128 genes, including 88 protein-coding genes, 37 tRNA genes, and eight rRNA genes. Phylogenetic analysis of 25 chloroplast genomes in Rosaceae indicated that S. amabilis is most closely related to S. commixta. These findings may provide useful information to the phylogeny of the genus Sorbus.
Project description:Sorbus harrowiana (≡Pyrus harrowiana), previously considered a synonym of Sorbus insignis (≡Pyrus insignis) in the Flora of China, is re-instated here and shown to be distinct from S. insignis, based on morphometric analysis, coupled with herbarium and field investigation. We also present for the first-time full descriptions, distributional records and notes for S. harrowiana and S. insignis.
Project description:Background and aimsLarge-scale ploidy surveys using flow cytometry have become an essential tool to study plant genome dynamics and to gain insight into the mechanisms and genetic barriers framing ploidy diversity. As an ideal complement to traditional techniques such as chromosome counting, the analysis of cytotype diversity in plant systems such as Sorbus provides primary investigation into the potential patterns and evolutionary implications of hybrid speciation.MethodsPloidy was assessed by means of relative nuclear DNA content using propidium iodide flow cytometry in 474 Sorbus samples collected from 65 populations in southern Wales and South-West England. Statistical tests were applied to evaluate the utility of this technique to confidently discriminate ploidy in the genus.Key resultsFlow cytometric profiles revealed the presence of four cytotypes (2x, 3x, 4x and 5x), confirming in many cases chromosome counts previously reported and demonstrating cytotype heterogeneity within specific Sorbus aggregates. Diploid cytotypes were restricted to the potential parental species and homoploid hybrids. Most of the samples processed were polyploid. The occurrence of the pentaploid cytotype had previously only been reported from a single specimen; it is now confirmed for two taxa occurring at different sites.ConclusionsFlow cytometry results obtained have proved useful in shedding light on the taxonomy of several controversial taxa and in confirming the presence of cytoypes which occur at very low frequencies. Notably, the coexistence of several cytotypes in Sorbus populations has probably been facilitated by the overlapping distribution of many of the species studied, which might also explain the high incidence of potential hybrid apomictic polyploids. These results will provide a solid baseline for molecular research aiming to better understand the genetic pathways controlling the formation and establishment of polyploid Sorbus.