Programmed DNA elimination in the parasitic nematode Ascaris

• Published in: PLoS pathogens Vol. 19; no. 2; p. e1011087
• Main Authors: Estrem, Brandon, Wang, Jianbin
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 02.02.2023; Public Library of Science (PLoS)
• Subjects: Animals; Ascaris; Ascaris - genetics; Base Sequence; Biological evolution; Biology and Life Sciences; Cell cycle; Cellular control mechanisms; Chromosomes; Comparative analysis; Deoxyribonucleic acid; DNA; Drug resistance; Earth Sciences; Embryogenesis; Embryonic growth stage; Embryos; Gene sequencing; Gene silencing; Genetic aspects; Genomes; Host-parasite relationships; Humans; Infections; Karyotypes; Life span; Medical research; Medicine, Experimental; Nematoda; Nematoda - genetics; Nematodes; Neurosciences; Nucleotide sequence; Parasites; Review; Somatic cells; Swine; United States
• ISSN: 1553-7374; 1553-7366; 1553-7374
• DOI: 10.1371/journal.ppat.1011087

In most organisms, the whole genome is maintained throughout the life span. However, exceptions occur in some species where the genome is reduced during development through a process known as programmed DNA elimination (PDE). In the human and pig parasite Ascaris, PDE occurs during the 4 to 16 cell stages of embryogenesis, when germline chromosomes are fragmented and specific DNA sequences are reproducibly lost in all somatic cells. PDE was identified in Ascaris over 120 years ago, but little was known about its molecular details until recently. Genome sequencing revealed that approximately 1,000 germline-expressed genes are eliminated in Ascaris, suggesting PDE is a gene silencing mechanism. All germline chromosome ends are removed and remodeled during PDE. In addition, PDE increases the number of chromosomes in the somatic genome by splitting many germline chromosomes. Comparative genomics indicates that these germline chromosomes arose from fusion events. PDE separates these chromosomes at the fusion sites. These observations indicate that PDE plays a role in chromosome karyotype and evolution. Furthermore, comparative analysis of PDE in other parasitic and free-living nematodes illustrates conserved features of PDE, suggesting it has important biological significance. We summarize what is known about PDE in Ascaris and its relatives. We also discuss other potential functions, mechanisms, and the evolution of PDE in these parasites of humans and animals of veterinary importance.