A dual ribosomal system in the zebrafish soma and germline

Reviewer #1 (Public review):

In all animals, the fertilized egg is transcriptionally silent, and thus early embryonic development relies on maternally deposited factors. A key mode of regulation is translational control to produce the proteins needed by the developing embryo. In zebrafish as well as other animals, distinct ribosomes, those coming from the maternal pool (maternal ribosomes produced in the germ line/oocytes), and those produced from new transcription after genome activation (somatic ribosomes). In zebrafish, the maternal pool consists of a "maternal" rRNA produced from rDNA on chromosome 4, that has previously been shown to be amplified or expressed specifically in the germ line and in oocytes. The observed sex-specific expression of m-rDNA has led to models that it is involved in sex differentiation and/or maternal control of early embryonic development, both as mediators of translation and as a source of raw materials needed to produce new ribosomes. The work to date in the field indicates that maternal and somatic ribosomes are distinct in their expression profiles but whether they have unique, or gene-specific activities awaits determining if somatic rDNA can functionally replace m-rDNA.

In this manuscript, the authors investigated the expression profiles, protein composition, and ability of maternal and somatic ribosome components to interact with one another and their association with polysomes. This study reports sequence differences between maternal and somatic ribosomal components as well as proteomics and structural analysis of ribosome composition in oocytes and early development. This analysis shows that ribosome subunit composition changes over developmental time but did not uncover evidence suggesting maternal or somatic ribosome-specific ribosomal protein paralog use. The key findings of this work are:
(1) Observation of hybrid ribosomes composed of subunits of maternal and somatic origin in the embryo.
(2) Detection of both maternal and somatic ribosomes in polysomes, indicating maternal and somatic ribosomes both support translation in the embryos and may not be functionally unique.
(3) Persistent expression of m-rRNA in germ cells, suggesting m-ribosomes, as the main ribosome type present, are important for translation in germ cells. The question of ribosome heterogeneity and the function of maternal versus somatic rDNA and ribosomes is of great interest to the broader scientific community. Overall, the manuscript is clearly written and the solid data provided support the main ideas and conclusions.

Specific points are detailed below.

(1) In Figure 1D the m-rRNA abundance goes down at 3dpf, then up again while the s-rRNA steadily increases and peaks at 3dpf then drops thereafter. As presented in the graph it is unclear if this up-then-down trend is consistently observed or not. There are bars on the graph for m-rRNA but not for s-rRNA, thus it is unclear how many times this experiment was performed for the s-rRNA or how variable the results were from sample to sample. Beyond this technical point, if the pattern is consistent, this is an interesting observation as it would signal either a shift in rDNA transcription to silence the somatic locus and/or post-transcriptional targeted degradation of the somatic rRNA in germ cells.

(2) Although qualified by the authors to some extent, the conclusion regarding maternal ribosomes and specificity related to the translation of germ line-specific transcripts is potentially confusing or misleading. Since the maternal form appears to be the only or predominant form of ribosomes in the germ cells at this stage, these would be the only ribosomes available for translation in germ cells. So, any RNA being translated in the germ cells, even RNAs that are not specifically expressed in the germline would be "enriched in association with" and translated by the maternal ribosomes in germ cells. Additional supporting evidence would be required to support the conclusion that the maternal ribosomes are specifically dedicated to the translation of germ cell-specific RNAs, like nanos3, rather than just general translation in germ cells. Consistent with a more general role for the maternal ribosomes in translation in germ cells, differential codon use has been previously documented for the RNAs produced in oocytes (aka maternal RNAs) (for example Bazzini et al EMBO 2016; Mishima and Tomari Mol Cell 2016), and tRNA genes were recently reported by Wilson and Postlethwait to reside along with the maternal 5S genes and maternal-specific spliceosome components in the region of chromosome 4 that is differentially activated in oocytes and testis (region 2 coding genes are silenced in the ovary but maternal ribosome-related genes are expressed in the ovary; region 4 contains the maternal 45S gene). Further, some of the authors of this manuscript undergo a shift in tRNA repertoire and a change in iso-decoder expression at the onset of gastrulation (Rappol et al, Nucleic Acids Research 2024). Technical limitations pose challenges to definitively testing the hypothesis, but it would be helpful to place the findings here in the context of the published work.

(3) "An alternate and non-exclusive hypothesis is that the maternal rDNA locus may be involved in PGC fate and sex determination in zebrafish." It would be helpful to further discuss the published evidence supporting this hypothesis. In accord with a potential role for m-rDNA in ovary differentiation, differential methylation of m-rDNA has been previously reported, with high methylation in testis and low methylation in ovaries. Further, several groups have shown that treating fish with broad inhibitors of methyltransferases causes testis-biased differentiation of the gonad. Finally, Moser et al (Philosophical Transactions of the Royal Society B 2024) recently published work in which CRISPR-Cas9 was used to target the 45S m-rDNA promoter and interfere with its expression. The mutants with these promoter mutations developed as fertile males, consistent with a role for m-rDNA in ovary differentiation. A recent paper from Moser et. al. (Philosophical Transactions of the Royal Society B 2024) showing that disrupting the m-rDNA locus leads to male-only development should be discussed. This paper does not exclude the possibility of a maternal role for the ribosomes since only one female was recovered among the 45S-m-rDNA mutants. The expression data in Figure 1D of this manuscript showing that m-rRNA levels go down and then up in PGCs indicates the PGCs are making their own m-rRNA. This observation together with the recovery of fertile males reported in the Moser et al study (Philosophical Transactions of the Royal Society B 2024) doesn't seem to support a requirement for m-rDNA in PGC fate or germ cell-specific translation, at least in testis, since the mutant males produce sperm and are fertile.

(4) Although the rationale for examining rRNAs in adult tumors, cultured zebrafish cell lines, and during fin regeneration is clear based on the published literature showing elevated embryonic rRNAs, this line of investigation doesn't add much to this study and is a bit of a distraction. That said, the observation that in contrast to published work, neither the maternal (early embryo) nor the specific rRNAs examined are unregulated in these contexts is important and warrants communication with the research community.

(5) The numbers of embryos and stages are not consistently stated in the manuscript. For example, in the "Isolation of zebrafish ribosome." and "isolation of monosomes" sections of the methods, the stage and number of embryos used for the IPs are not clearly stated in the methods. These important details should be stated throughout the manuscript so that others can perform future studies in a manner that will facilitate comparisons.

(6) The terminology used for the RiboFLAG experiments is potentially confusing or misleading. Specifically, different terms are used to describe the source of the ribosomes (Figure 5, Figure S7, Figure S8 and in the text). For example, "transmission" is used to describe "maternal transmission" for Mat-RiboFLAG, and "paternal transmission" is used for Som-RiboFLAG, and in Figure 5 and Figure S8 "maternally provided" and "paternally provided" are used. However, these terms may be confusing or unintentionally misleading because transmission and provided refer to two different things. In the case of Mat-RiboFLAG, the terms refer to the maternal Rpl10-FLAG ribosomes, which the progeny receive from their mother independent of whether or not they express the transgene. On the other hand, for Som-RiboFLAG, the terms refer to the transgene rather than the Rpl10-FLAG ribosomes that will be produced by the embryo using the transgene they inherited from their father. Consider instead sticking to "maternal" and "somatic", or alternatively "zygotic expression" and "maternal expression" or "zygotic ribosomes" and "maternal ribosomes".

Reviewer #2 (Public review):

Summary:

The study expands previous knowledge on the dual ribosome system in zebrafish by demonstrating the expression of maternal ribosomes in the primordial germ cells as well as the formation of hybrid ribosomes combining subunits of maternal and somatic ribosomes. Although the distinction between the two types is clear at the rRNA level, this is not paralleled at the protein level. An attempt to associate the expression of germ-line-specific transcripts to maternal ribosomes remains inconclusive. Thus, evidence for the functional specialisation of ribosomes in this system is still lacking.

Strengths:

The experiments are well-conducted and the main conclusions are well-supported.

Weaknesses:

The attempt to take advantage of the system to provide an example of functional ribosome specialisation is justified and the expression of maternal-type ribosomes in the germ line may still be key to understanding the expression of classes of mRNA. However, an alternative possibility related to genome evolution and sex determination is equally relevant.

Assessment following the structure of the manuscript:

Shah et al.: "A dual ribosomal system in zebrafish soma and germline"

The zebrafish dual ribosome system is attractive because it offers a favourable setting to look for ribosome specialization and my impression is that this is exactly what the authors set out to do rather than to try to understand why zebrafish have this unusual setup. If this is correct, the title and the abstract should better reflect the authors' aim and main results. The title suggests to the non-specialist that the dual ribosome system is a novel find which obviously is not the case.

I was a bit confused when reading the introduction. In the first paragraph, it was unclear to me if the degradation of maternal ribosomes is an active process different from normal turnover. I also found the third paragraph slightly out of tune with the discussion section. The dual ribosome setting at the level of ribosomal RNA genes represents an extreme case of sequence heterogeneity and appears to be sporadic in nature in that it only is reported from Plasmodium and zebrafish. The Xenopus example is 5S rRNA (as also mentioned in the discussion section), and the Drosophila example is protein composition, only. If a broader view of ribosome types is intended, there will be more examples, e.g. Trypanosomes that express different stage-dependent ribosomes at the level of rRNA modifications. The occurrence of dual ribosomes in fish should be placed in context with insight from other fish genomes, e.g. Medaka, which has only one type of ribosomes. Also, the duality in zebrafish is not restricted to ribosomes, but also comprises two types of spliceosomes. These observations suggest that the phenomenon should be investigated in the context of genome evolution. This is appropriately brought up in the discussion section, but I believe it would serve the reading of the manuscript if this was made clear from the beginning. With respect to the structural aspects, I am puzzled why one of the few other papers studying this system, Ramachandran et al. RNA 2020 (PMID: 32912962) is not referenced. This paper is focused on ribose methylation of the two types of ribosomal RNA and should be relevant to several aspects of the present study.

The manuscript reports three novel and important findings. First, the maternal-type ribosomes are expressed in PGCs, where they furthermore are shown to translate germ line-specific transcripts, and in the male germ line. Regardless, the authors wisely decide to maintain the classical terminology of maternal and somatic ribosomes. Second, both types of ribosomes are polysome-associated and thus translationally active at 24 hpf when they are found in equal amounts. An elaborate experiment shows that hybrid ribosomes are formed at this stage. Finally, a RIP experiment fails to show selectivity in ribosomal recruitment of a germ line-specific mRNA based on the nanos3 3´-UTR. There are several other results, but these are mainly confirmatory or negative, albeit of good quality and important to communicate.

The part of the study that describes differences in protein composition is a bit difficult to follow, partly because of the complexity of the results, and partly because of the disappointment that no parallel changes in proteins to the clear differences in rRNA were observed. Except for the discussion of eS8 in relation to subunit bridging, it is purely descriptive. There is quite a literature on paralog expression (e.g. in yeast and humans) and perhaps it would be possible to relate to the literature in a way that could provide more meaning to the observations. From the M&M section, it appears that the proteomics data were already published in the Leesch and Lorenzo-Orts et al. paper (Nature 2023). They are here found in Table S1 which is presented in a minimal fashion, from which it is time-consuming to extract meaningful information, e.g. on how stringently the ribosomes were prepared.

The hybrid-ribosome observation is convincing, but additional information on the choice of cycloheximide concentration would be helpful to rule out other interpretations.

The experiment on translation of primordial germ cell-specific transcripts by maternal ribosomes is a key experiment. Unfortunately, the experiment failed to show selectivity compared to somatic ribosomes, and in my reading, the promise in the abstract of "preferential association" is not quite justified. More importantly, this experiment is not exhaustive, and a more elaborate discussion on the limitations of the experiment and other approaches would be helpful.

The discussion section is interesting. Importantly, the authors make the non-specialist aware of the peculiarities of laboratory strains of zebrafish with respect to the lack of sex chromosomes and a possible connection between the rDNA locus and sex determination. This information is critical to include in a journal that has a broad readership. I was unable to follow the argument about the 3´half of 5.8S "to play a role" in ribosome degradation based on Locati et al., 2018 (which is missing from the reference list) and "serve as a target for degradation of maternal ribosomes". Kinetic effects on the degradation pattern of rRNA are frequently observed and difficult to interpret.

Reviewer #3 (Public review):

Summary:

Ribosomes are generally considered homogeneous complexes with no inherent role in regulating translation. However, recent studies have found heterogeneity in the composition of ribosome accessory factors, proteins, and ribosomal RNA. Moreover, there is evidence that district ribosomal isoforms are produced at different developmental stages in Xenopus, Drosophila, and zebrafish. In Drosophila, germline-derived ribosomes have a different protein composition to those produced by somatic cell types. In zebrafish, germline vs. somatic ribosomes have been shown to incorporate distinct rRNA isoforms. However, the functional significance of ribosome heterogeneity is not known.

The manuscript by Shah et al., uses the power of the zebrafish to test the hypothesis that maternal ribosome isoforms have a distinct function relative to ribosome isoforms produced by somatic cells after the maternal-to-zygotic transition (MTZ). They confirm previous findings that all maternal rRNA are derived from the maternal-specific rRNA locus on Chromosome 4. Additionally, proteomic analysis showed that maternal and somatic ribosomes also differ in protein composition. Using ribosome tagging experiments they showed that maternally derived subunits can form functional heteroduplexes (hybrids) with somatic-derived subunits. Finally, they show that maternal-derived ribosomes continue to be expressed in germ cells where they preferentially associate with the maternally derived and germline localized nanos3 mRNA. This suggests a possible role of maternal ribosomes in germ cell-specific translational regulation.

Strengths:

The authors use the experimental power of zebrafish to test the hypothesis that maternal and somatic-derived ribosomes have distinct functions. They use state-of-the art proteomics, molecular modeling, and transgenesis techniques. For the most part, the data presented is clear and supports their conclusions.

Weaknesses:

Using pulldown experiments they show that maternal ribosomes associate with the PGC-enriched nanos3 RNA, suggesting a role for the maternal isoform in germline-specific translation. However, they acknowledge that the level of enrichment is similar to the level of maternal vs. somatic isoforms that localize to PGCs. The nanos3 mRNA is unique in that it is actively degraded in somatic cells shortly after MTZ so is never present in cells that express the somatic isoforms. Therefore, the association of nanos3 with maternal ribosomes shows that these ribosomes can associate with germline-specific RNAs, but does not provide compelling evidence for a maternal isoform-specific role in translational regulation.