LAUSR

“The days of ‘junk DNA’ are over,” according to Christoph Grunau and Christoph Grevelding, the senior authors of a new research article in Genome Biology and Evolution. Their study provides an in-depth look at an enigmatic superfamily of repetitive DNA sequences known as W elements in the genome of the human parasite Schistosoma mansoni (Stitz et al. 2021). Titled “Satellite-like W elements: repetitive, tran scribed, and putative mobile genetic factors with potential roles for biology and evolution of Schistosoma mansoni,” the analysis reveals structural, functional, and evolutionary aspects of these elements and shows that, far from being “junk,” they may exert an enduring influence on the biology of S. mansoni. “When we studied genetics at university in the 1980s, the common doctrine was that the non-protein coding parts of eukaryotic genomes consisted of interspersed, ‘useless’ sequences, often organized in repetitive elements like satellite DNA,” note Grunau and Grevelding. Since then, however, the common understanding of such sequences has fundamentally changed, revealing a plethora of regulatory sequences, noncoding RNAs, and sequences that play a role in chromosomal and nuclear structure. With their article, Grunau and Grevelding, along with their coauthors from Justus Liebig University Giessen, University of Montpellier, and Leipzig University, contribute further evidence to a growing consensus that such sequences play critical roles in evolution. Flatworms such as S. mansoni provide a fascinating subject for such a study. They have a complex life cycle, including an asexual reproductive phase that takes place inside an intermediate host—a freshwater snail—and a sexual reproductive phase that takes place inside the human host, where it causes schistosomiasis, a neglected tropical disease that nearly rivals malaria in terms of human morbidity and mortality. Unlike other trematodes that are exclusively hermaphroditic, schistosomes have two separate sexes. Even more unusual is that constant pairing between schistosome partners is required for the sexual maturation of the female (see fig. 1). Despite this irregular life history, sex in schistosomes is determined chromosomally, with males having two Z chromosomes and females having one Z and one W chromosome. The nonrecombining portion of the W chromosome is composed mostly of repetitive DNA sequences known as W elements. In the past, these sequences were assumed to be nonfunctional and female-specific, such that they were even used as a marker to identify the sex of schistosome larvae. Grevelding says that it was an accidental observation that first drew his attention to these W elements. “The first W elements were originally found only in the females of Puerto Rican S. mansoni isolates. When I started to investigate these W elements in a Liberian strain, it turned out that they also occurred in males. Further studies showed unexpected variation in W elements originating from mitotic recombination during the asexual phase in the snail intermediate host, indicating the presence of W elements on autosomes.” Unfortunately, at the time of this discovery, further investigations were made difficult due to the lack of genomic resources and appropriate techniques in S. mansoni. Recently, however, new genomic and transcriptomic data have allowed Grunau and Grevelding to take a deeper look at this mystery. In their comprehensive analysis of W elements across the genome, the authors identified 19 W element families, varying in copy number from 3 to 450 at one or several locations on the W chromosome. Notably, 15 of these families had related sequences—representing either full-length or partial W elements—on one or more autosomes, with 13 having representatives on all 7 autosomes. This corroborates Grevelding’s earlier studies and suggests that W elements have a mobile nature. Indeed, some of them exhibited similarities to known mobile genetic elements. A comparative analysis across three closely related schistosome species