Supernumerary B chromosomes (Bs) are extra karyotype units in addition to A chromosomes, and are found in some fungi and thousands of animals and plant species. Bs are uniquely characterized due to their non-Mendelian inheritance, and represent one of the best examples of genomic conflict. Over the last decades, their genetic composition, function and evolution have remained an unresolved query, although a few successful attempts have been made to address these phenomena. A classical concept based on cytogenetics and genetics is that Bs are selfish and abundant with DNA repeats and transposons, and in most cases, they do not carry any function. However, recently, the modern quantum development of high scale multi-omics techniques has shifted B research towards a new-born field that we call “B-omics”. We review the recent literature and add novel perspectives to the B research, discussing the role of new technologies to understand the mechanistic perspectives of the molecular evolution and function of Bs. The modern view states that B chromosomes are enriched with genes for many significant biological functions, including but not limited to the interesting set of genes related to cell cycle and chromosome structure. Furthermore, the presence of B chromosomes could favor genomic rearrangements and influence the nuclear environment affecting the function of other chromatin regions. We hypothesize that B chromosomes might play a key function in driving their transmission and maintenance inside the cell, as well as offer an extra genomic compartment for evolution.
Genomic Plasticity Mediated by Transposable Elements in the Plant Pathogenic Fungus Colletotrichum higginsianum.
Phytopathogen genomes are under constant pressure to change, as pathogens are locked in an evolutionary arms race with their hosts, where pathogens evolve effector genes to manipulate their hosts, whereas the hosts evolve immune components to recognize the products of these genes. Colletotrichum higginsianum (Ch), a fungal pathogen with no known sexual morph, infects Brassicaceae plants including Arabidopsis thaliana. Previous studies revealed that Ch differs in its virulence toward various Arabidopsis thaliana ecotypes, indicating the existence of coevolutionary selective pressures. However, between-strain genomic variations in Ch have not been studied. Here, we sequenced and assembled the genome of a Ch strain, resulting in a highly contiguous genome assembly, which was compared with the chromosome-level genome assembly of another strain to identify genomic variations between strains. We found that the two closely related strains vary in terms of large-scale rearrangements, the existence of strain-specific regions, and effector candidate gene sets and that these variations are frequently associated with transposable elements (TEs). Ch has a compartmentalized genome consisting of gene-sparse, TE-dense regions with more effector candidate genes and gene-dense, TE-sparse regions harboring conserved genes. Additionally, analysis of the conservation patterns and syntenic regions of effector candidate genes indicated that the two strains vary in their effector candidate gene sets because of de novo evolution, horizontal gene transfer, or gene loss after divergence. Our results reveal mechanisms for generating genomic diversity in this asexual pathogen, which are important for understanding its adaption to hosts. © The Author(s) 2019. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution.
A High-Quality Draft Genome Sequence of Colletotrichum gloeosporioides sensu stricto SMCG1#C, a Causal Agent of Anthracnose on Cunninghamia lanceolata in China.
Colletotrichum has a broad host range and causes major yield losses of crops. The fungus Colletotrichum gloeosporioides is associated with anthracnose on Chinese fir. In this study, we present a high-quality draft genome sequence of C. gloeosporioides sensu stricto SMCG1#C, providing a reference genomic data for further research on anthracnose of Chinese fir and other hosts.
Blast Fungal Genomes Show Frequent Chromosomal Changes, Gene Gains and Losses, and Effector Gene Turnover.
Pyricularia is a fungal genus comprising several pathogenic species causing the blast disease in monocots. Pyricularia oryzae, the best-known species, infects rice, wheat, finger millet, and other crops. As past comparative and population genomics studies mainly focused on isolates of P. oryzae, the genomes of the other Pyricularia species have not been well explored. In this study, we obtained a chromosomal-level genome assembly of the finger millet isolate P. oryzae MZ5-1-6 and also highly contiguous assemblies of Pyricularia sp. LS, P. grisea, and P. pennisetigena. The differences in the genomic content of repetitive DNA sequences could largely explain the variation in genome size among these new genomes. Moreover, we found extensive gene gains and losses and structural changes among Pyricularia genomes, including a large interchromosomal translocation. We searched for homologs of known blast effectors across fungal taxa and found that most avirulence effectors are specific to Pyricularia, whereas many other effectors share homologs with distant fungal taxa. In particular, we discovered a novel effector family with metalloprotease activity, distinct from the well-known AVR-Pita family. We predicted 751 gene families containing putative effectors in 7 Pyricularia genomes and found that 60 of them showed differential expression in the P. oryzae MZ5-1-6 transcriptomes obtained under experimental conditions mimicking the pathogen infection process. In summary, this study increased our understanding of the structural, functional, and evolutionary genomics of the blast pathogen and identified new potential effector genes, providing useful data for developing crops with durable resistance. © The Author(s) 2019. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. All rights reserved. For permissions, please e-mail: email@example.com.