Possible self-fertilization in the brooding coral Acropora (Isopora) brueggemanni

De novo transcriptome assembly from the gonads of a scleractinian coral, Euphyllia ancora: molecular mechanisms underlying scleractinian gametogenesis

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 dinoflagellates (Symbiodiniaceae) and other organisms yielded a reference E. ancora gonadal transcriptome of 35,802 contigs. Analysis of 4 developmental phases identified 2023 genes that were differentially expressed during oogenesis and 678 during spermatogenesis. 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.

Restructuring the Traditional Suborders in the Order Scleractinia Based on Embryogenetic Morphological Characteristics

The order Scleractinia includes two distinct groups, which are termed "complex" and "robust" as indicated by the molecular phylogeny of mitochondrial 16S ribosomal gene sequences. Since this discovery, coral taxonomists have been seeking morphological characters for grouping this deep division in the order Scleractinia. Recently, morphological characteristics during embryogenesis that facilitate grouping the two clades as "complex" and "robust" were reported, thus clarifying a deep division in the Scleractinia. In the present report, I establish two new suborders, Refertina and Vacatina, on the basis of the embryogenetic morphological characteristics, molecular data, and new observations of Tubastraea coccinea and Cyphastrea serailia embryogenesis. In particular, the embryo of T. coccinea has a possible fertilization membrane that was first observed in the phylum Cnidaria. The new suborder Refertina consists of the families that belong to the "complex" clade and have no or little blastocoel. The new suborder Vacatina is composed of the families that fall into the "robust" clade and have an apparent blastocoel.

Reproduction in cultured versus wild coral colonies: fertilization, larval oxygen consumption, and survival

In the late 1990s, the once prolific populations of the coral Acropora intermedia surrounding Okinawa, Japan, dramatically declined because of thermal stress, bleaching caused by heat stress, and consequent mortality. Before the bleaching event, 72 fragments (about 15 cm in length) were collected and transferred to the Okinawa Churami Aquarium. Through growth and repeated fragmentation, these original fragments developed into about 100 colonies that spawned from 1999 to 2009. In this study, we compared gametogenesis, fertilization, survival, and O(2) consumption in cultured and wild colonies of A. intermedia and their offspring. Cultured A. intermedia had larger oocytes and higher fertilization and survival rates than samples from wild colonies. O(2) consumption of cultured embryos was similar to that of wild embryos. These results suggest that cultured A. intermedia and their offspring are as viable as wild colonies. Aquaria can play a role in the conservation of endangered corals, and their cultured colonies could be used to re-establish devastated species on the Okinawa reefs.