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July 7, 2019

The first 50 plant genomes

Fifty-five plant genomes have been published to date representing 49 different species (Table 1 includes PubMed IDs for complete reference). What have we learned from the first wave of plant genomes? It has been said that plant genome papers (and genome papers in general) are dry and lack “biology” and that the days of high impact plant genome papers are drawing to a close unless they explore significant biology. However, with each new genome, earlier observations are refined and plant genome papers continue to reveal novel aspects of genome biology. For example, the tomato and banana genome papers refined current thinking on the whole genome duplications (WGD) that shaped dicot and monocot genome evolution (D’Hont et al., 2012; Tomato Genome Consortium, 2012). These observations were enabled not only by high quality genome assemblies but also by a greater number of genomes available for com- parisons. In addition, the initial round of plant genomes enabled the first generation of functional genomics that helped to define the roles of hundreds of genes, provided unprecedented access to sequence-based markers for breeding, and provided glimpses into plant evolutionary history. More genomes, representing the diverse array of species in Viridiplantae are still required to gain a full understanding of plant genome structure, evolution, and complexity.


July 7, 2019

Permanent draft genome sequences of the symbiotic nitrogen fixing Ensifer meliloti strains BO21CC and AK58.

Ensifer (syn. Sinorhizobium) meliloti is an important symbiotic bacterial species that fixes nitrogen. Strains BO21CC and AK58 were previously investigated for their substrate utilization and their plant-growth promoting abilities showing interesting features. Here, we describe the complete genome sequence and annotation of these strains. BO21CC and AK58 genomes are 6,985,065 and 6,974,333 bp long with 6,746 and 6,992 genes predicted, respectively.


July 7, 2019

Genome sequence of the clover-nodulating Rhizobium leguminosarum bv. trifolii strain SRDI565.

Rhizobium leguminosarum bv. trifolii SRDI565 (syn. N8-J) is an aerobic, motile, Gram-negative, non-spore-forming rod. SRDI565 was isolated from a nodule recovered from the roots of the annual clover Trifolium subterraneum subsp. subterraneum grown in the greenhouse and inoculated with soil collected from New South Wales, Australia. SRDI565 has a broad host range for nodulation within the clover genus, however N2-fixation is sub-optimal with some Trifolium species and ineffective with others. Here we describe the features of R. leguminosarum bv. trifolii strain SRDI565, together with genome sequence information and annotation. The 6,905,599 bp high-quality-draft genome is arranged into 7 scaffolds of 7 contigs, contains 6,750 protein-coding genes and 86 RNA-only encoding genes, and is one of 100 rhizobial genomes sequenced as part of the DOE Joint Genome Institute 2010 Genomic Encyclopedia for Bacteria and Archaea-Root Nodule Bacteria (GEBA-RNB) project.


July 7, 2019

Genome of an arbuscular mycorrhizal fungus provides insight into the oldest plant symbiosis.

The mutualistic symbiosis involving Glomeromycota, a distinctive phylum of early diverging Fungi, is widely hypothesized to have promoted the evolution of land plants during the middle Paleozoic. These arbuscular mycorrhizal fungi (AMF) perform vital functions in the phosphorus cycle that are fundamental to sustainable crop plant productivity. The unusual biological features of AMF have long fascinated evolutionary biologists. The coenocytic hyphae host a community of hundreds of nuclei and reproduce clonally through large multinucleated spores. It has been suggested that the AMF maintain a stable assemblage of several different genomes during the life cycle, but this genomic organization has been questioned. Here we introduce the 153-Mb haploid genome of Rhizophagus irregularis and its repertoire of 28,232 genes. The observed low level of genome polymorphism (0.43 SNP per kb) is not consistent with the occurrence of multiple, highly diverged genomes. The expansion of mating-related genes suggests the existence of cryptic sex-related processes. A comparison of gene categories confirms that R. irregularis is close to the Mucoromycotina. The AMF obligate biotrophy is not explained by genome erosion or any related loss of metabolic complexity in central metabolism, but is marked by a lack of genes encoding plant cell wall-degrading enzymes and of genes involved in toxin and thiamine synthesis. A battery of mycorrhiza-induced secreted proteins is expressed in symbiotic tissues. The present comprehensive repertoire of R. irregularis genes provides a basis for future research on symbiosis-related mechanisms in Glomeromycota.


July 7, 2019

Complete closed genome sequences of Mannheimia haemolytica serotypes A1 and A6, isolated from cattle.

Mannheimia haemolytica is a respiratory pathogen affecting cattle and related ruminants worldwide. M. haemolytica is commonly associated with bovine respiratory disease complex (BRDC), a polymicrobial multifactorial disease. We present the first two complete closed genome sequences of this species, determined using an automated assembly pipeline requiring no manual finishing.


July 7, 2019

Hammondia hammondi, an avirulent relative of Toxoplasma gondii, has functional orthologs of known T. gondii virulence genes.

Toxoplasma gondii is a ubiquitous protozoan parasite capable of infecting all warm-blooded animals, including humans. Its closest extant relative, Hammondia hammondi, has never been found to infect humans and, in contrast to T. gondii, is highly attenuated in mice. To better understand the genetic bases for these phenotypic differences, we sequenced the genome of a H. hammondi isolate (HhCatGer041) and found the genomic synteny between H. hammondi and T. gondii to be >95%. We used this genome to determine the H. hammondi primary sequence of two major T. gondii mouse virulence genes, TgROP5 and TgROP18. When we expressed these genes in T. gondii, we found that H. hammondi orthologs of TgROP5 and TgROP18 were functional. Similar to T. gondii, the HhROP5 locus is expanded, and two distinct HhROP5 paralogs increased the virulence of a T. gondii TgROP5 knockout strain. We also identified a 107 base pair promoter region, absent only in type III TgROP18, which is necessary for TgROP18 expression. This result indicates that the ROP18 promoter was active in the most recent common ancestor of these two species and that it was subsequently inactivated in progenitors of the type III lineage. Overall, these data suggest that the virulence differences between these species are not solely due to the functionality of these key virulence factors. This study provides evidence that other mechanisms, such as differences in gene expression or the lack of currently uncharacterized virulence factors, may underlie the phenotypic differences between these species.


July 7, 2019

Neolithic mitochondrial haplogroup H genomes and the genetic origins of Europeans.

Haplogroup H dominates present-day Western European mitochondrial DNA variability (>40%), yet was less common (~19%) among Early Neolithic farmers (~5450 BC) and virtually absent in Mesolithic hunter-gatherers. Here we investigate this major component of the maternal population history of modern Europeans and sequence 39 complete haplogroup H mitochondrial genomes from ancient human remains. We then compare this ‘real-time’ genetic data with cultural changes taking place between the Early Neolithic (~5450 BC) and Bronze Age (~2200 BC) in Central Europe. Our results reveal that the current diversity and distribution of haplogroup H were largely established by the Mid Neolithic (~4000 BC), but with substantial genetic contributions from subsequent pan-European cultures such as the Bell Beakers expanding out of Iberia in the Late Neolithic (~2800 BC). Dated haplogroup H genomes allow us to reconstruct the recent evolutionary history of haplogroup H and reveal a mutation rate 45% higher than current estimates for human mitochondria.


July 7, 2019

Complete genome sequence of Liberibacter crescens BT-1.

Liberibacter crescens BT-1, a Gram-negative, rod-shaped bacterial isolate, was previously recovered from mountain papaya to gain insight on Huanglongbing (HLB) and Zebra Chip (ZC) diseases. The genome of BT-1 was sequenced at the Interdisciplinary Center for Biotechnology Research (ICBR) at the University of Florida. A finished assembly and annotation yielded one chromosome with a length of 1,504,659 bp and a G+C content of 35.4%. Comparison to other species in the Liberibacter genus, L. crescens has many more genes in thiamine and essential amino acid biosynthesis. This likely explains why L. crescens BT-1 is culturable while the known Liberibacter strains have not yet been cultured. Similar to Candidatus L. asiaticus psy62, the L. crescens BT-1 genome contains two prophage regions.


July 7, 2019

Absence of genome reduction in diverse, facultative endohyphal bacteria.

Fungi interact closely with bacteria, both on the surfaces of the hyphae and within their living tissues (i.e. endohyphal bacteria, EHB). These EHB can be obligate or facultative symbionts and can mediate diverse phenotypic traits in their hosts. Although EHB have been observed in many lineages of fungi, it remains unclear how widespread and general these associations are, and whether there are unifying ecological and genomic features can be found across EHB strains as a whole. We cultured 11 bacterial strains after they emerged from the hyphae of diverse Ascomycota that were isolated as foliar endophytes of cupressaceous trees, and generated nearly complete genome sequences for all. Unlike the genomes of largely obligate EHB, the genomes of these facultative EHB resembled those of closely related strains isolated from environmental sources. Although all analysed genomes encoded structures that could be used to interact with eukaryotic hosts, pathways previously implicated in maintenance and establishment of EHB symbiosis were not universally present across all strains. Independent isolation of two nearly identical pairs of strains from different classes of fungi, coupled with recent experimental evidence, suggests horizontal transfer of EHB across endophytic hosts. Given the potential for EHB to influence fungal phenotypes, these genomes could shed light on the mechanisms of plant growth promotion or stress mitigation by fungal endophytes during the symbiotic phase, as well as degradation of plant material during the saprotrophic phase. As such, these findings contribute to the illumination of a new dimension of functional biodiversity in fungi.


July 7, 2019

Current status of genome sequencing and its applications in aquaculture

Aquaculture is the fastest-growing food production sector in agriculture, with great potential to meet projected protein needs of human beings. Aquaculture is facing several challenges, including lack of a sufficient number of genetically improved species, lack of species-specific feeds, high mortality due to diseases and pollution of ecosystems. The rapid development of sequencing technologies has revolutionized biological sciences, and supplied necessary tools to tackle these challenges in aquaculture and thus ensure its sustainability and profitability. So far, draft genomes have been published in over 24 aquaculture species, and used to address important issues related to aquaculture. We briefly review the advances of next generation sequencing technologies, and summarize the status of whole genome sequencing and its general applications (i.e. establishing reference genomes and discovering DNA markers) and specific applications in tackling some important issues (e.g. breeding, diseases, sex determination & maturation) related to aquaculture. For sequencing a new genome, we recommend the use of 100–200 × short reads using Illumina and 50–60 × long reads with PacBio sequencing technologies. For identification of a large number of SNPs, resequencing pooled DNA samples from different populations is the most cost-effective way. We also discuss the challenges and future directions of whole genome sequencing in aquaculture.


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