The First De Novo Transcriptome Assembly From the Gonads of a Scleractinian Coral Euphyllia Ancora: Molecular Mechanisms Underlying Scleractinian Gametogenesis

Abstract

Background: Sexual reproduction of scleractinians has captured the attention of researchers and the general public for decades. Although extensive ecological data has been acquired, underlying molecular and cellular mechanisms remain largely unknown. In this study, to better understand mechanisms underlying gametogenesis, we isolated ovaries and testes at different developmental phases from a gonochoric coral, Euphyllia ancora, and adopted a transcriptomic approach to reveal sex- and phase-specific gene expression profiles. In particular, we explored genes associated with oocyte development and maturation, spermiogenesis, sperm motility / capacitation, and fertilization.Results: 1.6 billion raw reads were obtained from 24 gonadal samples. De novo assembly of trimmed reads, and elimination of contigs derived from symbiotic algae (Symbiodiniaceae) and other organisms yielded a reference E. ancora gonadal transcriptome of 35,802 contigs. Differential gene expression analysis of 4 developmental phases of ovaries and testes identified 2,023 and 678 differentially expressed genes in oogenesis and spermatogenesis, respectively. In premature/mature ovaries, 631 genes were specifically upregulated, with 538 in mature testes. Upregulated genes included those involved in gametogenesis, gamete maturation, sperm motility / capacitation, and fertilization in other metazoans, including humans. Meanwhile, a large number of genes without homology to sequences in the SWISS-PROT database were also observed among upregulated genes in premature / mature ovaries and mature testes.Conclusions: Our findings show that scleractinian gametogenesis shares many molecular characteristics with that of other metazoans, but it also possesses unique characteristics developed during cnidarian and/or scleractinian evolution. To the best of our knowledge, this study is the first to create a gonadal transcriptome assembly from any scleractinian. This study and associated datasets provide a foundation for future studies regarding gametogenesis and differences between male and female colonies from molecular and cellular perspectives. Furthermore, our transcriptome assembly will be a useful reference for future development of sex-specific and/or stage-specific germ cell markers that can be used in coral aquaculture and ecological studies.

Transcriptome analysis provides a blueprint of coral egg and sperm functions

“…Gamete survival in the water column is a period of a few hours during which they must abide environmental challenges such as predation, infection by microbial pathogens, and fluctuations in temperature and pH, before taking part in the fertilization process (Baird et al, 2009) and larval recruitment. Despite the significance of gamete survival in the marine environment and its effect on overall recruitment potential in broadcast-spawning corals, few studies have focused primarily on egg and sperm cells and only one recent study has investigated gamete transcriptomics in the context of gametogenesis (Chiu et al, 2020). Moreover, investigation of the potential impacts of bleaching on reproduction has not shown significant differences in egg quality or spawning potential between bleached and non-bleached M. capitata (Cox, 2007), despite the potential for parental investment in egg algal symbiont populations through vertical transmission.…”