GC-biased gene conversion in X-chromosome palindromes conserved in human, chimpanzee, and rhesus macaque

Codon usage bias and nucleotide bias are not influenced by the 5' flanking but by 3' and intronic region composition in SCID-associated genes

Compositional constraints, selectional and mutational forces, nucleotide skews, RNA folding free energy, gene expression, protein properties, and differential expression are a few attributes that define the molecular signatures of any gene. The absence of information regarding these attributes for genes associated with severe combined immunodeficiency disorder (SCID) prompted us to take up this study. The compositional bias influenced codon bias. Overall percent T composition is the lowest among all nucleotides. However, its distribution varies markedly across different codon positions and is not lowest at all codon positions. We, for the first time, determined the influence of intergenic elements and introns on nucleotide and codon bias on genes and found that in SCID-associated genes, the 5' flanking region neither influences the nucleotide nor codon bias, contrary to the intronic and 3' flanking region, which both influence nucleotide and codon bias in SCID associated genes. Codon usage in the SCID-associated gene set significantly differs from the codon usage present in overall human codon usage for 33 out of 59 codons (excluding start, stop, and trp encoding). Analysis of differentially expressed genes revealed that out of the 10 most differentially expressed genes, 07 genes are Zn finger proteins (ZNF728, ZNF726, ZNF676, ZNF667, ZNF439, ZNF257, and ZNF208). Applying the knowledge of codon bias, rare codons, minimum free energy, and codon adaptation index, codon deoptimization was carried out, and ZNF208 was the best suitable candidate. The study opened the area for the identification of peculiar molecular features and the development of more candidates for gene therapy purposes.

Genomic compliance with Chargaff's second parity rule may have originated non-adaptively, but stem-loops now function adaptively

Of Chargaff's four rules on DNA base quantity, his second parity rule (PR-2) is the most contentious. Various biometricians (e.g., Sueoka, Lobry) regarded PR-2 compliance as a non-adaptive feature of modern genomes that could be modeled through interrelations among mutation rates. However, PR-2 compliance with stem-loop potential was considered adaptively relevant by biochemists familiar with analyses of nucleic acid structure (e.g., of Crick) and of meiotic recombination (e.g., of Kleckner). Meanwhile, other biometricians had shown that PR-2 complementarity extended beyond individual bases (1-mers) to oligonucleotides (k-mers), possibly reflecting "advantageous DNA structure" (Nussinov). An "introns early" hypothesis (Reanney, Forsdyke) had suggested a primordial nucleic acid world with recombination-mediated error-correction requiring genome-wide stem-loop potential to have evolved prior to localized intrusions of protein-encoding potential (exons). Thus, a primordial genome was equivalent to one long intron. Indeed, when assessed as the base order-dependent component (correcting for local influences of GC%), modern genes, especially when evolving rapidly under positive Darwinian selection, display high intronic stem-loop potential. This suggests forced migration from neighboring exons by competing protein-encoding potential. PR-2 compliance may have first arisen non-adaptively. Primary prototypic structures were later strengthened by their adaptive contribution to recombination. Thus, contentious views may actually be in harmony.

Near-chromosomal-level genome of the red palm weevil (Rhynchophorus ferrugineus), a potential resource for genome-based pest control

The red palm weevil (RPW) is a highly destructive pest that mainly affects palms, particularly date palms (Phoenix dactylifera), in the Arabian Gulf region. In this study, we present a near-chromosomal-level genome assembly of the RPW using a combination of PacBio HiFi and Dovetail Omini-C reads. The final genome assembly is around 779 Mb in size, with an N50 of ~43 Mb, consistent with our previous flow cytometry estimates. The completeness of the genome was confirmed through BUSCO analysis, which indicates the presence of 99.5% of BUSCO single copy orthologous genes. The genome annotation identified a total of 29,666 protein-coding, 1,091 tRNA and 543 rRNA genes. Overall, the proposed genome assembly is significantly superior to existing assemblies in terms of contiguity, integrity, and genome completeness.

Disclosing complex mutational dynamics at a Y chromosome palindrome evolving through intra- and inter-chromosomal gene conversion

The human MSY ampliconic region is mainly composed of large duplicated sequences that are organized in eight palindromes (termed P1-P8), and may undergo arm-to-arm gene conversion. Although the importance of these elements is widely recognized, their evolutionary dynamics are still nuanced. Here, we focused on the P8 palindrome, which shows a complex evolutionary history, being involved in intra- and inter-chromosomal gene conversion. To disclose its evolutionary complexity, we performed a high-depth (50×) targeted next-generation sequencing of this element in 157 subjects belonging to the most divergent lineages of the Y chromosome tree. We found a total of 72 polymorphic paralogous sequence variants that have been exploited to identify 41 Y-Y gene conversion events that occurred during recent human history. Through our analysis, we were able to categorize P8 arms into three portions, whose molecular diversity was modelled by different evolutionary forces. Notably, the outer region of the palindrome is not involved in any gene conversion event and evolves exclusively through the action of mutational pressure. The inner region is affected by Y-Y gene conversion occurring at a rate of 1.52 × 10-5 conversions/base/year, with no bias towards the retention of the ancestral state of the sequence. In this portion, GC-biased gene conversion is counterbalanced by a mutational bias towards AT bases. Finally, the middle region of the arms, in addition to intra-chromosomal gene conversion, is involved in X-to-Y gene conversion (at a rate of 6.013 × 10-8 conversions/base/year) thus being a major force in the evolution of the VCY/VCX gene family.