Supernumerary B chromosomes (Bs) are dispensable genetic elements found in thousands of species of plants and animals, and some fungi. Since their discovery more than a century ago, they have been a source of puzzlement, as they only occur in some members of a population and are absent from others. When they do occur, they are often harmful, and in the absence of “selfishness”, based on mechanisms of mitotic and meiotic drive, there appears to be no obvious reason for their existence. Cytogeneticists have long wrestled with questions about the biological existence of these enigmatic elements, including their lack of any adaptive properties, apparent absence of functional genes, their origin, sequence organization, and co-evolution as nuclear parasites. Emerging new technologies are now enabling researchers to step up a gear, to look enthusiastically beyond the previous limits of the horizon, and to uncover the secrets of these “silent” chromosomes. This book provides a comprehensive guide to theoretical advancements in the field of B chromosome research in both animal and plant systems.

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Understanding of the origin of species and their adaptability to new environments is one of the main questions in biology. This is fueled by the ongoing debate on species concepts and facilitated by the availability of an unprecedented large number of genomic resources. Genomes are organized into chromosomes, where significant variations in number and morphology are observed among species due to large-scale structural variants such as inversions, translocations, fusions, and fissions. This genomic reshuffling provides, in the long term, new chromosomal forms on which natural selection can act upon, contributing to the origin of biodiversity. This book contains mainly articles, reviews, and an opinion piece that explore numerous aspects of genome plasticity among taxa that will help in understanding the dynamics of genome composition, the evolutionary relationships between species and, in the long run, speciation.

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This Special Issue contains 28 papers covering a wide range of topics related to plant genomics and genetics. The papers utilize a variety of genomic tools and techniques to explore topics such as identification of quantitative trait loci associated with flax seed yield and oil quality, mucilage and hull content in flax seeds, and genomic prediction for pasmo resistance in flax. Other papers focus on the identification and characterization of specific genes, such as the EXO70 gene family in wheat and related species, hydroxycinnamoyl transferase in tea plants, and the WRKY transcription factor GmWRKY12 in soybeans. Several papers examine gene expression and regulation in response to abiotic stress, such as drought and salt tolerance in moso bamboo, and the response to chilling in bell peppers. Other topics include genetic diversity analysis of crested wheatgrass, genome-wide analysis of chitin-binding protein gene family in peppers, and genome-wide bioinformatics analysis of MAPK gene family in kiwifruit. Overall, this Special Issue provides a comprehensive view of the current state of plant genomics research, showcasing the diversity of approaches and tools being used to better understand and improve crop plants.

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Repetitive DNA is ubiquitous in eukaryotic genomes, and, in many species, comprises the bulk of the genome. Repeats include transposable elements that can self-mobilize and disperse around the genome, and tandemly-repeated satellite DNAs that increase in copy number due to replication slippage and unequal crossing over. Despite their abundance, repetitive DNA is often ignored in genomic studies due to technical challenges in their identification, assembly, and quantification. New technologies and methods are now providing the unprecedented power to analyze repetitive DNAs across diverse taxa. Repetitive DNA is of particular interest because it can represent distinct modes of genome evolution. Some repetitive DNA forms essential genome structures, such as telomeres and centromeres, which are required for proper chromosome maintenance and segregation, whereas others form piRNA clusters that regulate transposable elements; thus, these elements are expected to evolve under purifying selection. In contrast, other repeats evolve selfishly and produce genetic conflicts with their host species that drive adaptive evolution of host defense systems. However, the majority of repeats likely accumulate in eukaryotes in the absence of selection due to mechanisms of transposition and unequal crossing over. Even these neutral repeats may indirectly influence genome evolution as they reach high abundance. In this Special Issue, the contributing authors explore these questions from a range of perspectives.

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