Unraveling sex determination in Bursaphelenchus nematodes

Sex and sexual characteristics are the most important aspects of an organism’s life and are determined by a biological process known as sex determination. These ever-evolving mechanisms are broadly classified according to the type of “switch” that activates them. Genetic sex determination depends on sex chromosomes, such as the X and Y chromosomes in humans, while environmental sex determination depends on factors such as temperature and the local ratio between males and females. While most sex determination mechanisms are genetic or environmental, there is also a third type of sex determination, which depends on completely random factors. However, this has not been fully investigated.

The sex determination mechanism of Caenorhabditis elegans, a species of nematode, or our common roundworm, is one of the best understood aspects of its biology. In this case, embryos with two X chromosomes, or the XX embryos, develop into hermaphrodites, while the XO embryos, which have one sex chromosome — the X chromosome — develop into males. Several species of nematodes have a sex determination mechanism similar to that of C. elegans. Interestingly, however, some nematode species also rely on the XX/XY system for sex determination, with both X- and Y-type sex chromosomes, as well as environmental factors. Unfortunately, the mechanisms that cause this variation in sex determination between nematode species have remained a mystery until now.

Recently, a group of researchers led by associate professor Ryoji Shinya of Meiji University, Japan, professor Paul Sternberg of the California Institute of Technology, USA, and associate professor Taisei Kikuchi of the University of Miyazaki, Japan, conducted a study to test the sex determination. to understand. in two species of nematodes: Bursaphelenchus xylophilus and Bursaphelenchus okinawaensis. The team of dr. Shinya has long been involved in nematode research† In this new study, they performed a genus-specific genome-wide comparative analysis to determine the initial trigger of sex determination in the two Bursaphelenchus species, and genetic screening to determine the genetic cascade that followed the trigger.

In their study published in nature communication, the researchers report that there is no difference in the number of chromosomes, or genome, between males and females in B. xylophilus and between males and hermaphrodites in B. okinawaensis. This suggests that these sexes have identical genomes and no sex chromosomes in both nematode species. Sex determination in these species must therefore occur through non-genetic mechanisms.

To investigate this further, the team conducted an analysis to find out whether environmental factors such as temperature, nutrient availability and population density influenced sex determination in these organisms. They found that these factors had minimal effect on sex determination in the larvae of these species, and none of the larvae turned into males.

Since the progeny produced by self-fertilization in B. okinawaensis are essentially isogenic clones, it is clear that genetic differences are not required for sex determination in B. okinawaensis. Moreover, even under fixed environmental conditions, genetically identical individuals of B. okinawaensis differentiate into hermaphrodites and males. The team suggests that the sex of B. okinawaensis nematodes is primarily determined by stochastic expression of an unknown trigger gene and/or developmental noise. In other words, sex differentiation occurs as a result of random events during development.

The team also compared the orthologs, ie genes related by common descent, of similar sequences in C. elegans, B. xylophilus and B. okinawaensis. They found that only downstream genes were conserved in these three nematodes, indicating that the genus Bursaphelenchus has a different sex determination trigger than C. elegans. In addition, they performed genetic analyzes and identified one major sex-determining locus in B. okinawaensis, known as Bok-tra-1a. Using bioinformatics and RNA sequencing, they observed a conservation of putative targets in this regulatory gene, further supporting the findings that pointed to the conservation of downstream functions. This implies that nematode sex differentiation may have evolved from this downstream regulator.

“Our discovery of a striking new way of sexing in the nematode stock may not only aid laboratory studies of parasitic nematodes, but also contribute to population engineering,” noted an excited Dr. Shinya on.

dr. Shinya points to the importance of these findings in pest control: “The damage caused by plant-parasitic nematodes is estimated at USD 80 billion per year. Conventional nematicides are harmful to the environment. Understanding the sex-determination mechanisms of plant-parasitic nematodes can help develop sterile strains that are not parasitic, but can help reduce nematode populations in a safe and sustainable way.”

Who knew that sex determination research in parasitic nematodes would point the way to disease-free crop production and forest protection? We keep our fingers crossed for you!

Reference

authors Ryoji Shinya1Simo Sun2Mehmet Dayic2.3Isheng Jason Tsai4Atsushi Miyama1Anthony Fu Chen5Koichi Hasegawa6Igor Antoshechkin5Taisei Kikuchi2 & Paul W. Sternberg5

Title of original paper

Possible stochastic sex determination in Bursaphelenchus nematodes

log nature communication
DOI 10.1038/s41467-022-30173-2

preferences

School of Agriculture, Meiji University, Kawasaki, Japan1† Department of Parasitology, Faculty of Medicine, University of Miyazaki, Miyazaki, Japan2† Forestry Vocational School, Duzce University, Duzce, Turkey3† Biodiversity Research Center, Academia Sinica, Taipei, Taiwan4† Department of Biology and Biological Engineering, California Institute of Technology, Pasadena, USA5† Department of Environmental Biology, College of Bioscience & Biotechnology, Chubu University, Kasugai, Japan6

References

Publication date last article 11 May 2022

Research method Experimental research

Subject of research Animals

Statement of Conflict of Interest The authors declare no conflict of interest

About Associate Professor Ryoji Shinya

dr. Ryoji Shinya is an associate professor in the School of Agriculture at Meiji University, Japan. He graduated from Obihiro University of Agriculture and Veterinary Medicine in 2007 and received his Ph.D. from the Graduate School of Agriculture at Kyoto University in 2012. His doctoral work investigated the molecular mechanisms of pine wilt caused by the pine forest nematode Bursaphelenchus xylophilus using genetics, proteomics and plant pathological approaches. As a postdoctoral researcher at the Dr. Koichi Hasegawa Laboratory at Chubu University, Japan, and the Dr. Paul W. Sternberg laboratory in Caltech, USA, he studied C. elegans biology. He joined Meiji University as a faculty in 2017. He is working on setting up a new laboratory model system using B. okinawaensis.

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