On Compensation Loops in Genomic Duplications

In this paper, we investigate the compensation loops, a DNA rearrangement in chromosomes due to unequal crossing over. We study the effect of compensation loops over the gene duplication, and we formalize it as a restricted case of gene duplication in general. We study this biological process under the point of view of formal languages, and we provide some results about the languages defined in this way.

Special Issue: Repetitive DNA Sequences

Repetitive DNAs are ubiquitous in eukaryotic genomes and, in many species, comprise 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 DNAs are often ignored in genomic studies due to technical challenges in identifying, assembling, and quantifying them. New technologies and methods are now allowing unprecedented power to analyze repetitive DNAs across diverse taxa. Repetitive DNAs are of particular interest because they can represent distinct modes of genome evolution. Some repetitive DNAs form essential genome structures, such as telomeres and centromeres, that are required for proper chromosome maintenance and segregation, while 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 cause 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. However, 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.

A Game of Thrones at Human Centromeres I. Multifarious structure necessitates a new molecular/evolutionary model

Human centromeres form over arrays of tandemly repeated DNA that are exceptionally complex (repeats of repeats) and long (spanning up to 8 Mbp). They also have an exceptionally rapid rate of evolution. The generally accepted model for the expansion/contraction, homogenization and evolution of human centromeric repeat arrays is a generic model for the evolution of satellite DNA that is based on unequal crossing over between sister chromatids. This selectively neutral model predicts that the sequences of centromeric repeat units will be effectively random and lack functional constraint. Here I used shotgun PacBio SMRT reads from a homozygous human fetal genome (female) to determine and compare the consensus sequences (and levels of intra-array variation) for the active centromeric repeats of all the chromosomes. To include the Y chromosome using the same technology, I used the same type of reads from a diploid male. I found many different forms and levels of conserved structure that are not predicted by –and sometimes contradictory to– the unequal crossing over model. Much of this structure is based on spatial organization of three types of ~170 bp monomeric repeat units that are predicted to influence centromere strength (i.e., the level of outer kinetochore proteins): one with a protein-binding sequence at its 5’ end (a 17 bp b-box that binds CENP-B), a second that is identical to the first except that the b-box is mutated so that it no longer binds CENP-B, and a third lacking a b-box but containing a 19 bp conserved “n-box” sequence near its 5’ end. The frequency and organization of these monomer types change markedly as the number of monomers per repeat unit increases, and also differs between inactive and active arrays. Active arrays are also much longer than flanking, inactive arrays, and far longer than required for cellular functioning. The diverse forms of structure motivate a new hypothesis for the lifecycle of human centromeric sequences. These multifarious levels of structures, and other lines of evidence, collectively indicate that a new model is needed to explain the form, function, expansion/contraction, homogenization and rapid evolution of centromeric sequences.

Genomes and the origin of genetic variation

Error and chance events, random mutations, are necessary prerequisites for evolution to happen. In a perfect world with no mutations there would be no evolution because no genetic variation would be generated that natural selection or genetic drift could work upon. This chapter first reviews how DNA is organized into genomes and genes in bacteria, archaea, and, in greater detail, eukaryotes. A surprising finding is that only a small fraction of the eukaryote genome consists of coding sequence. Evolutionary processes that can explain the presence of large amounts of noncoding DNA and the repetitive structure of the genome are reviewed, with emphasis on the roles that selfish genetic elements and unequal crossing over play. The chapter further explores the mechanisms that cause mutation and how new genes and protein functions originate.

Alterations in Heterochromatic Knobs in Maize Callus Culture by Breakage-Fusion-Bridge Cycle and Unequal Crossing Over

The meiotic and mitotic behavior of regenerated plants derived from a long-term callus culture, designated 12-F, was analyzed. This culture was heterozygous for an amplification of the heterochromatic knob on the long arm of chromosome 7 (K7L). We aimed to investigate if the amplification resulted from a breakage-fusion-bridge (BFB) cycle or from unequal sister chromatid recombination. Therefore, C-banded mitotic metaphases and pachytene, diakinesis, and anaphase I of regenerated plants were analyzed. Additionally, the occurrence of alterations in K7L was investigated in C-banded metaphases from short-term callus cultures derived from lines related to the donor genotype of the 12-F culture. As a result, plants homozygous and heterozygous for the amplification were detected. Meiosis was normal with few abnormalities, such as a low frequency of univalents at diakinesis. In the callus cultures a chromosome 7 with knobs of different sizes in the sister chromatids was detected and interpreted as a result of unequal crossing over. Other chromosomal alterations were consistent with the occurrence of BFB cycles. The finding of unequal crossing over in the cultures supports the conclusion that the amplification in the culture 12-F would be derived from this mechanism. If the amplification was derived from a BFB cycle, the terminal euchromatic segment between knob and the telomere would be deleted, and possibly, homozygous plants would not be viable.

Structural variation and dynamics of the nuclear ribosomal intergenic spacer region in key members of the Gibberella fujikuroi species complex

The intergenic spacer (IGS) region of the ribosomal DNA was cloned and sequenced in eight species within the Gibberella fujikuroi species complex with anamorphs in the genus Fusarium, a group that includes the most relevant toxigenic species. DNA sequence analyses revealed two categories of repeated elements: long repeats and short repeats of 125 and 8 bp, respectively. Long repeats were present in two copies and were conserved in all the species analyzed, whereas different numbers of short repeat elements were observed, leading to species-specific IGS sequences with different length. In Fusarium subglutinans and Fusarium nygamai, these differences seemed to be the result of duplication and deletion events. Here, we propose a model based on unequal crossing over that can explain these processes. The partial IGS sequence of 22 Fusarium proliferatum isolates was also obtained to study variation at the intraspecific level. The results revealed no differences in terms of number or pattern of repeated elements and detected frequent gene conversion events. These results suggest that the homogenization observed at the intraspecific level might not be achieved primarily by unequal crossing-over events but rather by processes associated with recombination such as gene conversion events.