While much is known about how small endogenous viral elements (EVEs) integrate into host eukaryotic genomes, the prevalence of larger EVEs that originate from double-stranded DNA (dsDNA) viruses remains relatively unexplored. In a new study appearing in this week's Nature, a group of Virginia Tech scientists investigate whether nucleocytoplasmic large DNA viruses (NCLDVs) — a group of eukaryotic viruses that include the largest viruses characterized to date — represent a prominent component of eukaryotic EVEs. Using bioinformatics and other technologies, they uncover widespread endogenization of NCLDVs in diverse green algae, with EVEs reaching sizes greater than 1 million base pairs and contained as many as around 10 percent of the total open reading frames in some genomes. The EVEs were found to frequently share genes with host genomic loci and contain numerous spliceosomal introns and large duplications, suggesting tight assimilation into host genomes, the study's authors write. "The widespread endogenization of NCLDV into chlorophytes therefore represents an underappreciated aspect of eukaryotic genome evolution and suggests that many eukaryotic lineages have access to a much larger array of genomic material than previously thought," they add.
The wide range of genome sizes across animals, seemingly uncorrelated to morphological complexity and gene content, remains an unanswered puzzle in evolutionary biology. And while there are many hypotheses for genome expansions, the evolutionary drivers of genome reduction are more enigmatic. To gain potential insights, a team led by researchers from the University of Bergen sequenced and analyzed the compact 73.8-megabase genome of Dimorphilus gyrociliatus, a meiobenthic segmented worm that, despite its miniature size, retains ancestral annelid traits. As reported in Nature Ecology & Evolution, the investigators uncover canonical features of genome regulation, excluding the presence of operons and trans-splicing. "Instead, the gene-dense D. gyrociliatus genome presents a divergent Myc pathway, a key physiological regulator of growth, proliferation, and genome stability in animals," they write. "Altogether, our study characterizes an alternative, more conservative route to genome compaction, and furthermore provides an exciting new system and genomic resources to investigate the evolutionary plasticity and function of core cellular mechanisms in animals."