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Global gene expression during early differentiation of Xenopus (Silurana) tropicalis gonad tissues
Citation:
Haselman, J., A. Olmstead, AND S. Degitz. Global gene expression during early differentiation of Xenopus (Silurana) tropicalis gonad tissues. GENERAL AND COMPARATIVE ENDOCRINOLOGY. Academic Press Incorporated, Orlando, FL, 214:103-113, (2015).
Impact/Purpose:
This work extends the knowledge of the molecular machinery responsible for reproductive development in Xenopus and supports elucidation of adverse outcome pathways and cross-species extrapolation. Xenopus is the preferred amphibian model for the EDSP Tier 1 and Tier 2 tests. Currently, the Tier 2 amphibian test focuses on reproductive development and makes assessments proximal to the adverse outcome. This work provides a foundation for subsequent investigations of chemical-induced perturbations on the molecular level earlier in development to assess pathways proximal to the molecular initiating event (MIE). The ultimate goal of linking the MIE to an adverse effect on the reproductive organ, if achieved, will aide in elucidation of adverse outcome pathways and have potential for cross-species extrapolation and population modeling.
Description:
African clawed frog Xenopus sp. has been used extensively for developmental biology and toxicology research. Xenopus (Silurana) tropicalis has been coveted more recently for genomics research because its diploid genome has been sequenced. Amid concerns of environmental pollutants disrupting endocrine system function in humans and wildlife, toxicology research has led to a focus on linking molecular initiating events to population-level effects. Since proper reproductive development is both inherent to endocrine function and essential for persistent populations, efforts to better understand reproductive development at the molecular level in a cross-species context are warranted. To that end, constitutive gene expression was measured in ovary and testis tissues by cDNA microarray throughout early differentiation. Ovary and testis transcriptomes were assessed at Niewkoop and Faber (NF) stage 58 (pro-metamorphosis), NF66 (completion of metamorphosis), one week (1WPM) and two weeks post-metamorphosis (2WPM). Differential expression analysis revealed a substantial divergence in ovary and testis transcriptomes starting between NF58 and NF66; transcriptomes continued to diverge through 2WPM. Generally, testis-enriched transcripts showed over-expression at a constant level throughout this period of development while ovary-enriched transcripts showed up-regulation at some point within this period of development. Functional analysis of these differentially expressed transcripts allowed linkages to be made between their putative human orthologues and the specific cellular processes unique to differentiating ovary and testis tissues. In ovary tissue, genetic programs are directing germ cells through meiosis to the diplotene stage when many different maternal mRNAs are transcribed and trafficked to particular locations within the oocyte for translation following fertilization. In the testis, there is substantial expression of genes involved in connective tissue development, tubule and blood vessel formation and germ cell support (Leydig and Sertoli cells). This dataset exhibited remarkable consistency with transcript profiles previously described in gonad tissues across species and emphasizes the universal importance of certain transcripts for germ cell development and preparation of these tissues for reproduction. Future toxicological studies can exploit the gender-biased profiles and functional associations of these transcripts to employ biomarkers of gonad phenotype and/or reproductive toxicity potentially linking molecular events to adverse outcomes at the population level
