Establishment of a New Model Sea Anemone for Comparative Studies on Cnidarian-Algal Symbiosis.

Frequent coral bleaching has drawn attention to the mechanisms of coral dinoflagellate endosymbiosis. Owing to the difficulty of rearing corals in the laboratory, model symbiosis systems are desired. The sea anemone Exaiptasia diaphana, hosting clade B1 of the genus Breviolum, has long been studied as a model system; however, a single species is insufficient for comparative studies and thus provides only limited resources for symbiosis research, especially regarding the specificity of host-symbiont associations. We established a clonal strain of the sea anemone Anthopleura atodai, whose symbiont was identified as a novel subclade of Symbiodinium (clade A) using a novel feeding method. We also developed a method to efficiently bleach various sea anemone species using a quinoclamine-based herbicide. Bleached A. atodai polyps were vital and able to reproduce asexually, exhibiting no signs of harmful effects of the drug treatment. Pilot studies have suggested that host-symbiont specificity is influenced by multiple steps differently in A. atodai and E. diaphana. RNAseq analyses of A. atodai showed that multiple NPC2 genes were expressed in the symbiotic state, which have been suggested to function in the transport of sterols from symbionts to host cells. These results reveal the usefulness of A. atodai in comparative studies of cnidarian-algal symbiosis.

Heterogeneity in maternal mRNAs within clutches of eggs in response to thermal stress during the embryonic stage.

BACKGROUND: The origin of variation is of central interest in evolutionary biology. Maternal mRNAs govern early embryogenesis in many animal species, and we investigated the possibility that heterogeneity in maternal mRNA provisioning of eggs can be modulated by environmental stimuli. RESULTS: We employed two sibling species of the ascidian Ciona, called here types A and B, that are adapted to different temperature regimes and can be hybridized. Previous study showed that hybrids using type B eggs had higher susceptibility to thermal stress than hybrids using type A eggs. We conducted transcriptome analyses of multiple single eggs from crosses using eggs of the different species to compare the effects of maternal thermal stress on heterogeneity in egg provisioning, and followed the effects across generations. We found overall decreases of heterogeneity of egg maternal mRNAs associated with maternal thermal stress. When the eggs produced by the F1 AB generation were crossed with type B sperm and the progeny ('ABB' generation) reared unstressed until maturation, the overall heterogeneity of the eggs produced was greater in a clutch from an individual with a heat-stressed mother compared to one from a non-heat-stressed mother. By examining individual genes, we found no consistent overall effect of thermal stress on heterogeneity of expression in genes involved in developmental buffering. In contrast, heterogeneity of expression in signaling molecules was directly affected by thermal stress. CONCLUSIONS: Due to the absence of batch replicates and variation in the number of reads obtained, our conclusions are very limited. However, contrary to the predictions of bet-hedging, the results suggest that maternal thermal stress at the embryo stage is associated with reduced heterogeneity of maternal mRNA provision in the eggs subsequently produced by the stressed individual, but there is then a large increase in heterogeneity in eggs of the next generation, although itself unstressed. Despite its limitations, our study presents a proof of concept, identifying a model system, experimental approach and analytical techniques capable of providing a significant advance in understanding the impact of maternal environment on developmental heterogeneity.

Chromosomal Inversion Polymorphisms in Two Sympatric Ascidian Lineages.

Chromosomal rearrangements can reduce fitness of heterozygotes and can thereby prevent gene flow. Therefore, such rearrangements can play a role in local adaptation and speciation. In particular, inversions are considered to be a major potential cause for chromosomal speciation. There are two closely related, partially sympatric lineages of ascidians in the genus Ciona, which we call type-A and type-B animals in the present study. Although these invertebrate chordates are largely isolated reproductively, hybrids can be found in wild populations, suggesting incomplete prezygotic barriers. Although the genome of type-A animals has been decoded and widely used, the genome for type-B animals has not been decoded at the chromosomal level. In the present study, we sequenced the genomes of two type-B individuals from different sides of the English Channel (in the zone of sympatry with type-A individuals) and compared them at the chromosomal level with the type-A genome. Although the overall structures were well conserved between type A and type B, chromosomal alignments revealed many inversions differentiating these two types of Ciona; it is probable that the frequent inversions have contributed to separation between these two lineages. In addition, comparisons of the genomes between the two type-B individuals revealed that type B had high rates of inversion polymorphisms and nucleotide polymorphisms, and thus type B might be in the process of differentiation into multiple new types or species. Our results suggest an important role of inversions in chromosomal speciation of these broadcasting spawners.