Fradin H, Kiontke K, Zegar C, Gutwein M, Lucas J, Kovtun M, Corcoran DL, Baugh LR, Fitch DHA, Piano F, Gunsalus KC.

Curr Biol. 2017 Oct 9;27(19):2928-2939.e6. doi:10.1016/j.cub.2017.08.038

Genome Architecture and Evolution of a Unichromosomal Asexual Nematode.

“By comparing divergence rates with those of Caenorhabditis, we estimate that the asexual clade originated ca. 18 mya”. “We obtained a genome assembly of 158 Mb with an N50 of 124 kb”. “the haploid size [...] is ~88 Mb”. “~4% average difference between [heterozygous] alleles.” “Genomic regions in D. pachys have generally maintained the same chro- mosomal identity since diverging from Caenorhabditis”. “ we used macro- synteny with C. elegans to infer an ancestral chromosome iden- tity (ACD) for each D. pachys scaffold.” “We identified specific patterns of rearrangements consistent with an order of ACD fusions.” “The data [of reciprocal reaggangements between ACDs] thus allow us to propose that four ancestral chromosomes became fused in the order I-X-III-II” “Telomeres are either absent from the D. pachys genome or are highly divergent from the ancestral telomeric sequence.” “The combined absence of telomere sequences and mainte- nance proteins suggests a connection between chromosome fusion and loss of telomeres”. “84% of homozygous regions localize to ACDs I and X, which are neighbors in the inferred chromosomal fusion”. ”The reductional division during meiosis I is skipped; the two chromosomes condense but do not pair or synapse.” “The phenotype of C. elegans rec-8 mutants is reminiscent of the modified meiosis in D. pachys. [...] rec-8 is one of the conserved meiosis genes that was not detected in the D. pachys and D. coronatus genomes”