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    Additional description and genome analyses of Caenorhabditis auriculariae representing the basal lineage of genus Caenorhabditis
    (Nature Research, 2021) Dayi, Mehmet; Kanzaki, Natsumi; Sun, Simo; Ide, Tatsuya; Tanaka, Ryusei; Masuya, Hayato; Kikuchi, Taisei
    Caenorhabditis auriculariae, which was morphologically described in 1999, was re-isolated from a Platydema mushroom-associated beetle. Based on the re-isolated materials, some morphological characteristics were re-examined and ascribed to the species. In addition, to clarify phylogenetic relationships with other Caenorhabditis species and biological features of the nematode, the whole genome was sequenced and assembled into 109.5 Mb with 16,279 predicted protein-coding genes. Molecular phylogenetic analyses based on ribosomal RNA and 269 single-copy genes revealed the species is closely related to C. sonorae and C. monodelphis placing them at the most basal clade of the genus. C. auriculariae has morphological characteristics clearly differed from those two species and harbours a number of species-specific gene families, indicating its usefulness as a new outgroup species for Caenorhabditis evolutionary studies. A comparison of carbohydrate-active enzyme (CAZy) repertoires in genomes, which we found useful to speculate about the lifestyle of Caenorhabditis nematodes, suggested that C. auriculariae likely has a life-cycle with tight-association with insects.
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    The compact genome of Caenorhabditis niphades n. sp., isolated from a wood-boring weevil, Niphades variegatus
    (Bmc, 2022) Sun, Simo; Kanzaki, Natsumi; Dayı, Mehmet; Maeda, Yasunobu; Yoshida, Akemi; Tanaka, Ryusei; Kikuchi, Taisei
    Background The first metazoan genome sequenced, that of Caenorhabditis elegans, has motivated animal genome evolution studies. To date > 50 species from the genus Caenorhabditis have been sequenced, allowing research on genome variation. Results In the present study, we describe a new gonochoristic species, Caenorhabditis niphades n. sp., previously referred as C. sp. 36, isolated from adult weevils (Niphades variegatus), with whom they appear to be tightly associated during its life cycle. Along with a species description, we sequenced the genome of C. niphades n. sp. and produced a chromosome-level assembly. A genome comparison highlighted that C. niphades n. sp. has the smallest genome (59 Mbp) so far sequenced in the Elegans supergroup, despite being closely related to a species with an exceptionally large genome, C. japonica. Conclusions The compact genome of C. niphades n. sp. can serve as a key resource for comparative evolutionary studies of genome and gene number expansions in Caenorhabditis species.
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    Nearly Complete Genome Assembly of the Pinewood Nematode Bursaphelenchus xylophilus Strain Ka4C1
    (Amer Soc Microbiology, 2020) Dayi, Mehmet; Sun, Simo; Maeda, Yasunobu; Tanaka, Ryusei; Yoshida, Akemi; Tsai, Isheng Jason; Kikuchi, Taisei
    Bursaphelenchus xylophilus has been destroying pine forests in East Asia and western Europe. Here, we report its nearly complete genomic sequence containing five similar to 12-Mb scaffolds and one similar to 15-Mb scaffold representing six chromosomes. Large repeat regions that were previously unidentified are now reasonably integrated, particularly in the similar to 15-Mb scaffold.
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    Possible stochastic sex determination in Bursaphelenchus nematodes
    (Nature Portfolio, 2022) Shinya, Ryoji; Sun, Simo; Dayı, Mehmet; Tsai, Isheng Jason; Miyama, Atsushi; Chen, Anthony Fu; Hasegawa, Koichi
    Sex determination mechanisms evolve surprisingly rapidly, yet little is known in the large nematode phylum other than for Caenorhabditis elegans, which relies on chromosomal XX-XO sex determination and a dosage compensation mechanism. Here we analyze by sex-specific genome sequencing and genetic analysis sex determination in two fungal feeding/plant-parasitic Bursaphelenchus nematodes and find that their sex differentiation is more likely triggered by random, epigenetic regulation than by more well-known mechanisms of chromosomal or environmental sex determination. There is no detectable difference in male and female chromosomes, nor any linkage to sexual phenotype. Moreover, the protein sets of these nematodes lack genes involved in X chromosome dosage counting or compensation. By contrast, our genetic screen for sex differentiation mutants identifies a Bursaphelenchus ortholog of tra-1, the major output of the C. elegans sex determination cascade. Nematode sex determination pathways might have evolved by bottom-up accretion from the most downstream regulator, tra-1. In most species, sex is determined by genetic or environmental factors. Here, the authors present evidence that sex determination in Bursaphelenchus nematodes is instead likely to be regulated by a random, epigenetic mechanism.
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    Syntenic relationship of chromosomes in Strongyloides species and Rhabditophanes diutinus based on the chromosome-level genome assemblies
    (Royal Soc, 2024) Kounosu, Asuka; Sun, Simo; Maeda, Yasunobu; Dayi, Mehmet; Yoshida, Akemi; Maruyama, Haruhiko; Hunt, Vicky
    The Strongyloides clade, to which the parasitic nematode genus Strongyloides belongs, contains taxa with diverse lifestyles, ranging from free-living to obligate vertebrate parasites. Reproductive strategies are also diverse in this group of nematodes, employing not only sexual reproduction but also parthenogenesis, making it an attractive group to study genome adaptation to specific conditions. An in-depth understanding of genome evolution, however, has been hampered by fragmented genome assemblies. In this study, we generated chromosome-level genome assemblies for two Strongyloides species and the outgroup species Rhabditophanes diutinus using long-read sequencing and high-throughput chromosome conformation capture (Hi-C). Our synteny analyses revealed a clearer picture of chromosome evolution in this group, suggesting that a functional sex chromosome has been maintained throughout the group. We further investigated sex chromosome dynamics in the lifecycle of Strongyloides ratti and found that bivalent formation in oocytes appears to be important for male production in the mitotic parthenogenesis.This article is part of the Theo Murphy meeting issue 'Strongyloides: omics to worm-free populations'.
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    Telomere-to-Telomere Genome Assembly of Bursaphelenchus okinawaensis Strain SH1
    (Amer Soc Microbiology, 2020) Sun, Simo; Shinya, Ryoji; Dayi, Mehmet; Yoshida, Akemi; Sternberg, Paul W.; Kikuchi, Taisei
    Bursaphelenchus okinawaensis is a self-fertilizing, hermaphroditic, fungus-feeding nematode used as a laboratory model for the genus Bursaphelenchus, which includes the important pathogen Bursaphelenchus xylophilus. Here, we report the nearly complete genome sequence of B. okinawaensis. The 70-Mbp assembly contained six scaffolds (>11 Mbp each) with telomere repeats on their ends, indicating complete chromosomes.