Gene silencing and sex determination by programmed DNA elimination in parasitic nematodes

Abstract

Maintenance of genome integrity is essential. However, programmed DNA elimination removes specific DNA sequences from the genome during development. DNA elimination occurs in unicellular ciliates and diverse metazoa ranging from nematodes to vertebrates. Two distinct groups of nematodes use DNA elimination to silence germline-expressed genes in the soma (ascarids) or for sex determination (Strongyloides spp.). Data suggest that DNA elimination likely evolved independently in these nematodes. Recent studies indicate that differential CENP-A deposition within chromosomes defines which sequences are retained and lost during Ascaris DNA elimination. Additional studies are needed to determine the distribution, functions, and mechanisms of DNA elimination in nematodes.

Figure 1. Programmed DNA elimination during early embryo mitoses in Parascaris and Ascaris

A,B. P. univalens embryos. A. Two-cell embryo showing one cell with the single pair of germline chromosomes. The big arrows indicate the heterochromatic arms of the chromosomes (H) and the small arrows point to the euchromatic regions of the genome (Eu). B. Somatic cell undergoing programmed DNA elimination from a two-cell embryo. The retained portions of the germline chromosomes (Eu) are fragmented into ~ 2n = 70 chromosomes. The heterochromatic arms that will be eliminated (H, big arrows) remain visible. Adapted from Muller & Tobler [5]. C,D. Ascaris embryos. C. Four-cell embryo with two cells undergoing DNA elimination. D. Six-cell embryo with one cell undergoing chromatin diminution. Note that DNA to be eliminated is present as fragments (artificially colored in red) between chromosomes segregating in early anaphase (C); DNA fragments (red) derived from a previous cell diminution can be seen in the cytoplasm of cells to the right (D). Adapted from Wang & Davis [1].

Figure 2. Ascaris early embryo development, cell lineage, and DNA elimination

Primordial germ cells (P) are in red, cells undergoing DNA elimination are represented by yellow filled circles surrounded by dots, and blue cells (S) are precursor somatic cells and lineages. The primordial germ cells numbers correspond to their division state. P0 is the zygote, whereas P1 through P4 represent the primordial germ cell derived from each subsequent cleavage of the germ cells as illustrated. S1–S4 cells are successive precursor somatic cells derived from each division of a germ cell Adapted from Boveri [50,51] and Muller & Tobler [4,5].

Figure 3. Model for DNA elimination and mechanism for the loss of chromosomal regions from holocentric chromosomes

A. Model of DNA elimination in Ascaris [18]. Following alignment, chromosomes in somatic cells undergo chromosome breaks producing fragments of chromosomes (1). Chromosome fragments that are retained (blue) have centromeric sites for microtubule attachment to facilitate chromosome segregation, whereas chromosomal fragments that will be eliminated (red) remain at the metaphase plate, are not segregated, and are lost (2). B. Monocentric chromosomes have a single centromere (blue box) where spindle microtubule attachment occurs. Fragmentation of a monocentric chromosome would lead to a loss of acentric chromosomal regions during chromosome segregation. C. Holocentric chromosomes are generally thought to have multiple centromeric regions distributed along the chromosome length that serve as sites for microtubule attachment. This distribution of microtubule attachment sites would predict no loss of chromosomal fragments following chromosome breakage during DNA elimination. D. CENP-A is reduced in genomic regions that remain at the metaphase plate and will be lost during Ascaris DNA elimination. Immunohistochemical staining of CENP-A on a 4-cell Ascaris embryo with two cells undergoing DNA elimination mitoses (anaphase) indicates the DNA to be eliminated (red arrows) has much less CENP-A than the DNA that will be segregated and retained. E. CENP-A and centromeres/kinetochores in germline Ascaris chromosomes are distributed along the length of the chromosomes. During development, CENP-A deposition is lost on regions of chromosomes that will be lost during DNA elimination. Thus, dynamic CENP-A deposition defines and regulates which portions of chromosomes with be lost and retained during DNA elimination.

Figure 4. DNA elimination in Strongyloides spp

Chromosomal configuration in females (left) and males (right) in S. ratti (top) and S. papillosus (bottom). Chromosomes and chromosomal regions present in two copies in both sexes are in blue; chromosomes and regions present in two copies in females but only one copy in males are in red. Note in S. ratti, one whole chromosome is lost in males, while in S. papillosus, only part of a chromosome is lost during programmed DNA elimination.