Reviewer #3 (Public Review):
This study aims to determine whether the chromosome defects induced by a bacterial endosymbiont in insects in developing embryos are a direct result of paternal chromosome defects from early embryogenesis or due to a second, independent set of defects that arise later: "we addressed whether defects observed in late CI embryos such as chromosome segregation errors and nuclear fallout are the result of first division errors or a second, distinct CI-induced defect."
Using crosses, genetics, and fluorescent microscopy, the study claims that the defects at different embryonic stages are due to independent processes, and this work thus has mechanistic relevance to how bacteria inflict developmental harm to insect embryogenesis. The claim is not well supported by the weight of the evidence in this paper and the literature.
The work is technically sound and proficiently completed to an expert level with appropriate statistics, but it does not provide straight-line evidence to substantiate the primary claim of the paper that later-stage embryos die for different reasons than early-stage embryos. That is no fault of the experimental rigor but rather to the difficulty of directly answering this question. It appears the field has insufficient information on the reductionist, bacterial mechanism that induces embryonic death, namely what acutely is modified by the bacteria to cause embryonic death? As such, the authors hedge that by studying different developmental stages of the embryonic defects, the answer can be surmised. However, a simple explanation for how late and early-stage embryos could die to similar mechanisms is that host cellular conditions are more or less susceptible to the same bacterial-induced change of the insect chromosomes (e.g., new chemical marks on the DNA). It's just not possible to rule this out until the acute mechanism of killing is known. For instance, some embryos may vary in their transcriptomes, proteomes, physiology, etc within a single family of fly offspring, and as such these varying embryos may be more or less susceptible to the same proximal cause of the bacteria-mediated defects. The difference is just when do they take place in development. Without knowing the bacterial mechanism of death (e.g. changes in chemical marks of the fly DNA), the study here can characterize broad strokes of chromatin biology while speculating on the weight of the evidence for whether or not different mechanisms are at play.
To evaluate the primary question of whether or not there are completely separate defects across development, the study shows several pieces of data that offer a finer resolution of the broad defects of embryos that were previously characterized by the literature. The new follow-up details are robustly supported and include percentages of embryos experiencing a defect, nuclear fallout, determination of haploidy/diploid, sequencing depths, Y chromosome tracking, and developmental-staged characterizations of the chromatin defects. However, according to the text, there is effectively a single type of data that speaks to the main question of the paper - whether or not viable embryos that escaped the first mitosis had increased mitotic errors during later developmental stages.
"Therefore, the significant increase in mitotic errors observed in diploid CI-derived embryos relative to wild-type derived embryos demonstrates the existence of a second, CI-induced defect, completely separate from the first division defect." This was already known; later-stage, chromatin defects do occur in a variety of insect species cited in the paper. In effect, the question answers itself because, in order to traverse an early lethal state that does not occur, there must be defects that ensue later in development, several of which have already been characterized, though to a lesser resolution than this study.
Moreover, the study does not link the staged chromatin errors to the CI genes using transgenic tools that are now customary in this field. That work is quite relevant to the conclusion of the paper because the authors speculate in the discussion that additional CI genes may be necessary to explain the later defects in embryogenesis versus the initial defects. This work has been completed to a degree by the papers reporting the initial discovery of the CI genes. CI transgene expression in males causes both 1st mitosis and later chromatin defects, suggesting additional genes are not necessary to explain lethality after the first mitosis. This to me is perhaps the most significant counterpoint of the narrative of the paper's claim because the acute genetic cause of CI can lead to differently timed chromatin errors.
This is solid work and a strong effort to refine the stages and types of embryonic lethality induced by bacteria, however, the claim that there are different acute mechanisms of death during embryogenesis is not well supported.