Evolution of Methyltransferase-Like (METTL) Proteins in Metazoa: A Complex Gene Family Involved in Epitranscriptomic Regulation and Other Epigenetic Processes.

The methyltransferase-like (METTL) proteins constitute a family of seven-beta-strand methyltransferases with S-adenosyl methionine-binding domains that modify DNA, RNA, and proteins. Methylation by METTL proteins contributes to the epigenetic, and in the case of RNA modifications, epitranscriptomic regulation of a variety of biological processes. Despite their functional importance, most investigations of the substrates and functions of METTLs within metazoans have been restricted to model vertebrate taxa. In the present work, we explore the evolutionary mechanisms driving the diversification and functional differentiation of 33 individual METTL proteins across Metazoa. Our results show that METTLs are nearly ubiquitous across the animal kingdom, with most having arisen early in metazoan evolution (i.e., occur in basal metazoan phyla). Individual METTL lineages each originated from single independent ancestors, constituting monophyletic clades, which suggests that each METTL was subject to strong selective constraints driving its structural and/or functional specialization. Interestingly, a similar process did not extend to the differentiation of nucleoside-modifying and protein-modifying METTLs (i.e., each METTL type did not form a unique monophyletic clade). The members of these two types of METTLs also exhibited differences in their rates of evolution. Overall, we provide evidence that the long-term evolution of METTL family members was driven by strong purifying selection, which in combination with adaptive selection episodes, led to the functional specialization of individual METTL lineages. This work contributes useful information regarding the evolution of a gene family that fulfills a variety of epigenetic functions, and can have profound influences on molecular processes and phenotypic traits.

Gene expression patterns of red sea urchins (Mesocentrotus franciscanus) exposed to different combinations of temperature and pCO2 during early development.

BACKGROUND: The red sea urchin Mesocentrotus franciscanus is an ecologically important kelp forest herbivore and an economically valuable wild fishery species. To examine how M. franciscanus responds to its environment on a molecular level, differences in gene expression patterns were observed in embryos raised under combinations of two temperatures (13 °C or 17 °C) and two pCO2 levels (475 µatm or 1050 µatm). These combinations mimic various present-day conditions measured during and between upwelling events in the highly dynamic California Current System with the exception of the 17 °C and 1050 µatm combination, which does not currently occur. However, as ocean warming and acidification continues, warmer temperatures and higher pCO2 conditions are expected to increase in frequency and to occur simultaneously. The transcriptomic responses of the embryos were assessed at two developmental stages (gastrula and prism) in light of previously described plasticity in body size and thermotolerance under these temperature and pCO2 treatments. RESULTS: Although transcriptomic patterns primarily varied by developmental stage, there were pronounced differences in gene expression as a result of the treatment conditions. Temperature and pCO2 treatments led to the differential expression of genes related to the cellular stress response, transmembrane transport, metabolic processes, and the regulation of gene expression. At each developmental stage, temperature contributed significantly to the observed variance in gene expression, which was also correlated to the phenotypic attributes of the embryos. On the other hand, the transcriptomic response to pCO2 was relatively muted, particularly at the prism stage. CONCLUSIONS: M. franciscanus exhibited transcriptomic plasticity under different temperatures, indicating their capacity for a molecular-level response that may facilitate red sea urchins facing ocean warming as climate change continues. In contrast, the lack of a robust transcriptomic response, in combination with observations of decreased body size, under elevated pCO2 levels suggest that this species may be negatively affected by ocean acidification. High present-day pCO2 conditions that occur due to coastal upwelling may already be influencing populations of M. franciscanus.

Ocean acidification promotes broad transcriptomic responses in marine metazoans: a literature survey.

For nearly a decade, the metazoan-focused research community has explored the impacts of ocean acidification (OA) on marine animals, noting that changes in ocean chemistry can impact calcification, metabolism, acid-base regulation, stress response and behavior in organisms that hold high ecological and economic value. Because OA interacts with several key physiological processes in marine organisms, transcriptomics has become a widely-used method to characterize whole organism responses on a molecular level as well as inform mechanisms that explain changes in phenotypes observed in response to OA. In the past decade, there has been a notable rise in studies that examine transcriptomic responses to OA in marine metazoans, and here we attempt to summarize key findings across these studies. We find that organisms vary dramatically in their transcriptomic responses to pH although common patterns are often observed, including shifts in acid-base ion regulation, metabolic processes, calcification and stress response mechanisms. We also see a rise in transcriptomic studies examining organismal response to OA in a multi-stressor context, often reporting synergistic effects of OA and temperature. In addition, there is an increase in studies that use transcriptomics to examine the evolutionary potential of organisms to adapt to OA conditions in the future through population and transgenerational experiments. Overall, the literature reveals complex organismal responses to OA, in which some organisms will face more dramatic consequences than others. This will have wide-reaching impacts on ocean communities and ecosystems as a whole.

Transcriptional profiles of early stage red sea urchins (Mesocentrotus franciscanus) reveal differential regulation of gene expression across development.

The red sea urchin, Mesocentrotus franciscanus, is an ecologically important kelp forest species that also serves as a valuable fisheries resource. In this study, we have assembled and annotated a developmental transcriptome for M. franciscanus that represents eggs and six stages of early development (8- to 16-cell, morula, hatched blastula, early gastrula, prism and early pluteus). Characterization of the transcriptome revealed distinct patterns of gene expression that corresponded to major developmental and morphological processes. In addition, the period during which maternally-controlled transcription was terminated and the zygotic genome was activated, the maternal-to-zygotic transition (MZT), was found to begin during early cleavage and persist through the hatched blastula stage, an observation that is similar to the timing of the MZT in other sea urchin species. The presented developmental transcriptome will serve as a useful resource for investigating, in both an ecological and fisheries context, how the early developmental stages of this species respond to environmental stressors.

Seasonal transcriptomes of the Antarctic pteropod, Limacina helicina antarctica.

High latitude seas will be among the first marine systems to be impacted by ocean acidification (OA). Previous research studying the effects of OA on the pteropod, Limacina helicina antarctica, has led this species to be identified as a sentinel organism for OA in polar oceans. Here, we present gene expression data on L. h. antarctica, collected in situ during the seasonal transition from early spring to early summer. Our findings suggest that after over-wintering under seasonal sea ice, pteropods progress toward full maturity in the early summer when food becomes increasingly available. This progression is highlighted by a dramatic shift in gene expression that supports the development of cytoskeletal structures, membrane ion transportation, and metabolically important enzymes associated with glycolysis. In addition, we observed signs of defense of genomic integrity and maturation as evidenced by an up-regulation of genes involved in DNA replication, DNA repair, and gametogenesis. These data contribute to a broader understanding of the life-cycle dynamics for L. h. antarctica and provide key insights into the transcriptomic signals of pteropod maturation and growth during this key seasonal transition.

Transcriptomics reveal transgenerational effects in purple sea urchin embryos: Adult acclimation to upwelling conditions alters the response of their progeny to differential pCO2 levels.

Understanding the mechanisms with which organisms can respond to a rapidly changing ocean is an important research priority in marine sciences, especially in the light of recent predictions regarding the pace of ocean change in the coming decades. Transgenerational effects, in which the experience of the parental generation can shape the phenotype of their offspring, may serve as such a mechanism. In this study, adult purple sea urchins, Strongylocentrotus purpuratus, were conditioned to regionally and ecologically relevant pCO2 levels and temperatures representative of upwelling (colder temperature and high pCO2 ) and nonupwelling (average temperature and low pCO2 ) conditions typical of coastal upwelling regions in the California Current System. Following 4.5 months of conditioning, adults were spawned and offspring were raised under either high or low pCO2 levels, to examine the role of maternal effects. Using RNA-seq and comparative transcriptomics, our results indicate that differential conditioning of the adults had an effect on the gene expression patterns of the progeny during the gastrula stage of early development. For example, maternal conditioning under upwelling conditions intensified the transcriptomic response of the progeny when they were raised under high versus low pCO2 conditions. Additionally, mothers that experienced upwelling conditions produced larger progeny. The overall findings of this study are complex, but do suggest that transgenerational plasticity in situ could act as an important mechanism by which populations might keep pace with rapid environmental change.

Additive effects of pCO2 and temperature on respiration rates of the Antarctic pteropod Limacina helicina antarctica.

The Antarctic pteropod, Limacina helicina antarctica, is a dominant member of the zooplankton in the Ross Sea and supports the vast diversity of marine megafauna that designates this region as an internationally protected area. Here, we observed the response of respiration rate to abiotic stressors associated with global change-environmentally relevant temperature treatments (-0.8°C, 4°C) and pH treatments reflecting current-day and future modeled extremes (8.2, 7.95 and 7.7 pH at -0.8°C; 8.11, 7.95 and 7.7 pH at 4°C). Sampling repeatedly over a 14-day period in laboratory experiments and using microplate respirometry techniques, we found that the metabolic rate of juvenile pteropods increased in response to low-pH exposure (pH 7.7) at -0.8°C, a near-ambient temperature. Similarly, metabolic rate increased when pteropods were exposed simultaneously to multiple stressors: lowered pH conditions (pH 7.7) and a high temperature (4°C). Overall, the results showed that pCO2 and temperature interact additively to affect metabolic rates in pteropods. Furthermore, we found that L. h. antarctica can tolerate acute exposure to temperatures far beyond its maximal habitat temperature. Overall, L. h. antarctica appears to be susceptible to pH and temperature stress, two abiotic stressors which are expected to be especially deleterious for ectothermic marine metazoans in polar seas.

Phylogenetic and transcriptomic analyses reveal the evolution of bioluminescence and light detection in marine deep-sea shrimps of the family Oplophoridae (Crustacea: Decapoda).

Bioluminescence is essential to the survival of many organisms, particularly in the deep sea where light is limited. Shrimp of the family Oplophoridae exhibit a remarkable mechanism of bioluminescence in the form of a secretion used for predatory defense. Three of the ten genera possess an additional mode of bioluminescence in the form of light-emitting organs called photophores. Phylogenetic analyses can be useful for tracing the evolution of bioluminescence, however, the few studies that have attempted to reconcile the relationships within Oplophoridae have generated trees with low-resolution. We present the most comprehensive phylogeny of Oplophoridae to date, with 90% genera coverage using seven genes (mitochondrial and nuclear) across 30 oplophorid species. We use our resulting topology to trace the evolution of bioluminescence within Oplophoridae. Previous studies have suggested that oplophorid visual systems may be tuned to differentiate the separate modes of bioluminescence. While all oplophorid shrimp possess a visual pigment sensitive to blue-green light, only those bearing photophores have an additional pigment sensitive to near-ultraviolet light. We attempt to characterize opsins, visual pigment proteins essential to light detection, in two photophore-bearing species (Systellaspis debilis and Oplophorus gracilirostris) and make inferences regarding their function and evolutionary significance.