Sexual dimorphism in the tardigrade Paramacrobiotus metropolitanus transcriptome
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Background
In gonochoristic animals, the sex determination pathway induces different morphological and behavioral features that can be observed between sexes, a condition known as sexual dimorphism. While many components of this sex differentiation cascade shows high levels of diversity, factors such as the Doublesex-Mab-3-related transcription factor (DMRT) are highly conserved throughout animals. Species of the phylum Tardigrada exhibits remarkable diversity in morphology and behavior between sexes, suggesting a pathway regulating such dimorphism. Despite the wealth of genomic and zoological knowledge accumulated in recent studies, the sexual differences in tardigrades genomes have not been identified. In this study, we focused on the gonochoristic species Paramacrobiotus metropolitanus and employed omics analyses to unravel the molecular basis of sexual dimorphism.
Results
Transcriptome analysis between sex identified numerous differentially expressed genes, of which approximately 2,000 male-biased genes were focused on 29 non-male-specific genomic loci. From these regions, we identified two Macrobiotidae family specific DMRT paralogs, which were significantly upregulated in males and lacked sex specific splicing variants. Furthermore, phylogenetic analysis indicated all tardigrade genomes lacks the doublesex ortholog, suggesting doublesex emerged after the divergence of Tardigrada. In contrast to sex-specific expression, no evidence of genomic difference between the sexes were found. We also identified several anhydrobiosis genes exhibiting sex-biased expression, possibly suggesting a mechanism for protection of sex specific tissues against extreme stress.
Conclusions
This study provides a comprehensive analysis for analyzing the genetic differences between sexes in tardigrades. The existence of male-biased, but not male-specific, genomic loci and identification of the family specific male-biased DMRT subfamily would provide the foundation for understanding the sex determination cascade. In addition, sex-biased expression of several tardigrade-specific genes which are involved their stress tolerance suggests a potential role in protecting sex-specific tissue and gametes.
