Haploidy-linked cell proliferation defects limit larval growth in Zebrafish

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Abstract

Haploid larvae in non-mammalian vertebrates are lethal with characteristic organ growth retardation collectively called “haploid syndrome.” In contrast to mammals whose haploid intolerance is attributed to imprinting misregulation, the cellular principle of haploidy-linked defects in non-mammalian vertebrates remains unknown. Here, we investigated cellular defects that disrupt the ontogeny of gynogenetic haploid zebrafish larvae. Unlike diploid control, haploid larvae manifested unscheduled cell death at the organogenesis stage, attributed to haploidy-linked p53 upregulation. Moreover, we found that haploid larvae specifically suffered the gradual aggravation of mitotic spindle monopolarization during 1-3 days post fertilization, causing spindle assembly checkpoint-mediated mitotic arrest throughout the entire body. High-resolution imaging revealed that this mitotic defect accompanied the haploidy-linked centrosome loss occurring concomitantly with the gradual decrease in larval cell size. Either resolution of mitotic arrest or depletion of p53 partially improved organ growth in haploid larvae. Based on these results, we propose that haploidy-linked mitotic defects and cell death are parts of critical cellular causes shared among vertebrates that limit the larval growth in the haploid state, contributing to an evolutionary constraint on allowable ploidy status in the vertebrate life cycle.

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    Reply to the reviewers

    Reply to the Reviewers

    We sincerely thank the Referees for providing important and constructive comments. We have addressed their concerns point-by-point as described below.

    Associated to Reviewer#1's comments

    *- Diploid embryos are used as controls. Gynogenetic diploids seem to be better controls to ensure that the observed phenotypes are not related to loss of heterozygosity. To limit the amount of work, the use of gynogenetic diploids could be restricted to spindle polarity and centrosome number experiments. *

    Response 1-1

    __[Experimental plan] __Following the reviewer's suggestion, we will conduct immunostaining of a-tubulin and centrin (for …

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    Referee #3

    Evidence, reproducibility and clarity

    In this study the authors aim to shed light onto the molecular reasons why most animals are restricted to diploid cell generations. In mammals, haploid intolerance has been previously attributed to defects linked to genomic imprinting, but the molecular defects associated with haploidy in non-mammalian species are unknown. To fill these gaps, the authors in this study investigate defects associated with haploidy in zebrafish larvae. They found that haploid larvae show elevated numbers of apoptotic cells that could be partially rescued by inhibition of p53. The also detected many cells with prolonged mitosis reflected by an increase …

  3. Note: This preprint has been reviewed by subject experts for Review Commons. Content has not been altered except for formatting.

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    Referee #2

    Evidence, reproducibility and clarity

    This study examined cell proliferation and death in haploid and diploid zebrafish and attempted to provide insights into cellular mechanisms underlying haploidy-linked defects in non-mammalian vertebrates. While some of the ideas were potentially interesting, the experiments were not rigorous and inadequate data analyses were performed. Major issues include:

    • Lack of proper controls in many experiments. For example, in the experiments where the authors treated haploids with reversine to suppress the SAC, there was no no-treatment control (Fig. 6A-C). In Fig. 6D, when a DMSO control was included, the control fish were from 3 dpf while …
  4. Note: This preprint has been reviewed by subject experts for Review Commons. Content has not been altered except for formatting.

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    Referee #1

    Evidence, reproducibility and clarity

    Summary:

    Yaguchi et al. investigate the causes of the "haploid syndrome" in the zebrafish embryo, the old observation that haploid embryos suffer from severe developmental defects and growth retardation of organs such as the brain and eyes (these defects are not simply a consequence of loss of heterozygosity, as they are rescued by forced diploidization of haploid larvae). Looking at apoptosis and proliferation, the authors show an increase in the number of apoptotic and mitotic cells in haploid larvae. Regarding apoptosis, they show an increase in p53 levels and demonstrate that knockdown of p53 limits apoptosis and leads to some …