The genome of < i > Litomosoides sigmodontis < /i > illuminates the origins of Y chromosomes in filarial nematodes

by Lewis Stevens, Manuela Kieninger, Brian Chan, Jonathan M. D. Wood, Pablo Gonzalez de la Rosa, Judith Allen, Mark Blaxter Heteromorphic sex chromosomes are usually thought to have originated from a pair of autosomes that acquired a sex-determining locus and subsequently stopped recombining, leading to degeneration of the sex-limited chromosome. The majority of nematodes species lack heteromorphic sex chromosomes and determine sex using an X-chromosome counting mechanism, with males being hemizygous for one or more X chromosomes (XX/X0). Some filarial nematode species, including important parasites of humans, have heteromorphic XX/XY karyotypes. It has been assumed that sex is determined by a Y-linked locus in these species. However, karyotypic analyses suggested that filarial Y chromosomes are derived from the unfused homologue of an autosome involved in an X-to-autosome fusion event. Here, we generated a chromosome-level reference genome forLitomosoides sigmodontis, a filarial nematode with the ancestral filarial karyotype and sex determination mechanism (XX/X0). By mapping the assembled chromosomes to the rhabditid nematode ancestral linkage (or Nigon) elements, we infer that the ancestral filarial X chromosome was the product of a fusion between NigonX (the ancestrally X-linked element) and NigonD (ancestrally autosomal). In the two filarial lineages with XY systems, there have been two independent autosome-to-X chromosome fusion events involving different autosomal Nig...
Source: PLoS Genetics - Category: Genetics & Stem Cells Authors: Source Type: research