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Progress in evolutionary developmental biology (evo‐devo) has deepened our understanding of how intrinsic properties of embryogenesis, along with natural selection and population genetics, shape phenotypic diversity. A focal point of recent empirical and theoretical research is the idea that highly developmentally stable phenotypes are more conserved in evolution. Previously, we demonstrated that in Japanese medaka (Oryzias latipes), embryonic stages and genes with high stability, estimated through whole‐embryo RNA‐seq, are highly conserved in subsequent generations. However, the precise origin of the stability of gene expression levels evaluated at the whole‐embryo level remained unclear. Such stability could be attributed to two distinct sources: stable intracellular expression levels or spatially stable expression patterns. Here we demonstrate that stability observed in whole‐embryo RNA‐seq can be attributed to stability at the cellular level (low variability in gene expression at the cellular levels). We quantified the intercellular variations in expression levels and spatial gene expression patterns for seven key genes involved in patterning dorsoventral and rostrocaudal regions during early development in medaka. We evaluated intracellular variability by counting transcripts and found its significant correlation with variation observed in whole‐embryo RNA‐seq data. Conversely, variation in spatial gene expression patterns, assessed through intraindividual left–right asymmetry, showed no correlation. Given the previously reported correlation between stability and conservation of expression levels throughout embryogenesis, our findings suggest a potential general trend: the stability or instability of developmental systems—and the consequent evolutionary diversity—may be primarily anchored in intrinsic fundamental elements such as the variability of intracellular states. Research highlights: We investigated the precise origins of developmental stability that was previously determined by whole‐embryo RNA‐seq, given its significance in indicating the potential for phenotypic conservation during evolution.Our findings suggest that the developmental stability can be primarily attributed to stability in gene expression at the cellular level, rather than stability in spatial gene expression patterns.The result provides further insight into how embryogenesis introduces bias in phenotypic evolution, and thus paves the way for a deeper understanding of developmental stability that would enable the prediction of phenotypic evolution. [ABSTRACT FROM AUTHOR]

Maternal microchimerism (MMc) is the phenomenon that a low number of cells from the mother persists within her progeny. Despite their regular presence in mammalian pregnancies, the overall cell type repertoire and roles of maternal cells, especially after birth, remain unclear. By using transgenic mouse strains and human umbilical blood samples, recent studies have for the first time characterized and quantified MMc cell type repertoires in offspring, identified the cross‐generational influence on fetal immunity, and determined possible factors that affect their presence in offspring. This review summarizes new findings, especially on the maternal cell type repertoires and their potential role in utero, in postnatal life, and long after birth. [ABSTRACT FROM AUTHOR]

To understand Haeckel's idea of recapitulation with modern evolutionary biology, one has to realize how evolutionarily conserved embryonic stages appear sequentially in developmental processes as chains of causality. Whether the idea of evolution was accepted or not, Haeckel and von Baer commonly saw an importance of a particularly conserved mid‐embryonic stage in biphasic development of metazoans, the phylotype, that defines an animal phylum as the developmental source of a basic body plan. In an evolutionary context, the phylotypic stage was once understood by Haeckel to reflect the common ancestor of animal phyla, which went through hypermorphosis independently into various phyla. Recent comprehensive molecular studies, however, accumulated data to refute this idea. The conserved embryonic pattern does not reflect an ancestral adult morphology but appears to have arisen primarily as an embodiment of developmental constraints established through evolutionary processes. How the developmental burden results in a nested series of constraints will solve the recapitulative tendency of developmental programs. Research Highlights: This review is primarily focused on the possible evolutionary mechanism that resulted in a recapitulation‐like sequence of developmental processes. Theory of developmental burden by Riedl (1978) may explain why evolutionarily new traits tend to appear after the establishment of ancestral patterns in development. This ancestral pattern, however, does not resemble adult forms of ancestors, but more likely reflect ancestral developmental constraints, which is consistent with recent transcriptome data. [ABSTRACT FROM AUTHOR]

The notochord functions primarily as a supporting tissue to maintain the anteroposterior axis of primitive chordates, a function that is replaced entirely by the vertebral column in many vertebrates. The notochord still appears during vertebrate embryogenesis and plays a crucial role in the developmental pattern formation of surrounding structures, such as the somites and neural tube, providing the basis for the vertebrate body plan. The indispensable role of the notochord has often been referred to as the developmental burden and used to explain the evolutionary conservation of notochord; however, the existence of this burden has not been successfully exemplified so far. Since the adaptive value of target tissues appears to result in the evolutionary conservation of upstream structures through the developmental burden, we performed comparative gene expression profiling of the notochord, somites, and neural tube during the mid‐embryonic stages in turtles and chicken to measure their evolutionary conservation. When compared with the somites and neural tube, overall gene expression profiles in the notochord showed significantly lower or merely comparable levels of conservation. However, genes involved in inductive signalings, such as the sonic hedgehog (Shh) cascade and the formation of functional primary cilia, showed relatively higher levels of conservation in all the three structures analyzed. Collectively, these results suggest that shh signals are critical as the inductive source and receiving structures, possibly constituting the inter‐dependencies of developmental burden. Research Highlights: Potential evolutionary effects toward notochord by developmental burden was evaluated by Laser Micro Dissection RNAseq (LMDseq). Notochord was less conserved than neural tube and somites; however, genes in sonic hedgehog (shh) signaling cascade was found to be evolutionarily conserved (not only in notochord but also in somites and neural tube). These results suggest that Shh signals are critical as the inductive source and receiving structures, possibly constituting the inter‐dependencies of developmental burden. Further studies that directly measure the burden required to verify the hypothesis are awaited. [ABSTRACT FROM AUTHOR]

Recapitulation is a hypothetical concept that assumes embryogenesis of an animal parallels its own phylogenetic history, sequentially developing from more ancestral features to more derived ones. This concept predicts that the earliest developmental stage of various animals should represent the most evolutionarily conserved patterns. Recent transcriptome‐based studies, on the other hand, have reported that mid‐embryonic, organogenetic periods show the highest level of conservation (the developmental hourglass model). This, however, does not rule out the possibility that recapitulation would still be detected after the mid‐embryonic period. In accordance with this, recapitulation‐like morphological features are enriched in late developmental stages. Moreover, our recent chromatin accessibility‐based study provided molecular evidence for recapitulation in the mid‐to‐late embryogenesis of vertebrates, as newly evolved gene regulatory elements tended to be activated at late embryonic stages. In this review, we revisit the recapitulation hypothesis, together with recent molecular‐based studies that support the developmental hourglass model. We contend that the recapitulation hypothesis does not entirely contradict the developmental hourglass model and that these two may even coexist in later embryonic stages of vertebrates. Finally, we review possible mechanisms underlying the recapitulation pattern of chromatin accessibility together with the hourglass‐like evolutionary conservation in vertebrate embryogenesis. Research Highlights: Recapitulation pattern has been reported for chromatin accessibility during the mid‐to‐late embryogenesis.The observed recapitulation pattern and the developmental hourglass model may coexist.The possible evolutionary mechanisms underlying tendencies of embryonic evolution were discussed. [ABSTRACT FROM AUTHOR]

The maxillofacial region is one of the most complex areas in the vertebrate body plan. The homology of the upper jaw bones remain controversial, both between mammals and nonmammalian amniotes and among humans and other mammals, leading to various hypotheses on how this region evolved from ancestral amniotes to humans. As a key mammalian model, the mouse (Mus musculus) is vital for unraveling the evolution and development of the maxillofacial region experimentally. However, limited detailed morphological descriptions of murine cranial development hinder the extrapolation of findings to other species, including humans. Here, we describe the development of the murine face, including the nerves, skeletons, and vasculatures from the pharyngula (9.0 days post-coitum [dpc]) to the late fetal period (18.5 dpc) based on three-dimensional reconstructions of histological sections. The present results confirm that the morphology of the pharyngula stages and developmental process of chondrocranium of mice is highly conserved when compared to nonmammalian tetrapods and humans. We also propose that the Os incisivum, the rostralmost bone in the mammalian upper jaw, consists of septomaxillary and palatine components, supporting our previous hypothesis that the ancestral premaxilla was entirely lost in mammals. The present descriptive study of mice strengthen the anatomical correspondence between mouse and human faces and offers a solid framework for comparative craniofacial studies across vertebrates., (© 2025 The Author(s). Journal of Morphology published by Wiley Periodicals LLC.)

We are naturally chimeras. Apart from our own cells originating from the fertilized egg, placental mammals receive small numbers of maternal cells called maternal microchimerism (MMc) that persist throughout one's whole life. Not only are varying frequencies of MMc cells reported in seemingly contradicting phenomena, including immune tolerance and possible contribution to autoimmune-like disease, but frequencies are observable even among healthy littermates showing varying MMc frequencies and cell type repertoire. These varying differences in MMc frequencies or cell types could be contributing to the diverse phenomena related to MMc. However, factors biasing these MMc differences remain largely unknown. Here, we tested whether immunological activation leads to differing MMc frequencies, based on our recent study that suggests that most maternal cells are immune-related. Unexpectedly, fluorescence-activated cell sorting analysis on the murine spleen, thymus, and liver following maternal immune activation by mid-gestational lipopolysaccharide intraperitoneal injections detected no significant difference in the number, or ratio of, immune-related maternal cells in the tested embryonic organs of healthy offspring. These findings suggest that MMc frequencies remain stable even under immune-activated conditions, implying a possible control system of MMc migration against changes in the immunological conditions., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2024. Published by The Company of Biologists.)

Despite morphological diversification of chordates over 550 million years of evolution, their shared basic anatomical pattern (or 'bodyplan') remains conserved by unknown mechanisms. The developmental hourglass model attributes this to phylum-wide conserved, constrained organogenesis stages that pattern the bodyplan (the phylotype hypothesis); however, there has been no quantitative testing of this idea with a phylum-wide comparison of species. Here, based on data from early-to-late embryonic transcriptomes collected from eight chordates, we suggest that the phylotype hypothesis would be better applied to vertebrates than chordates. Furthermore, we found that vertebrates' conserved mid-embryonic developmental programmes are intensively recruited to other developmental processes, and the degree of the recruitment positively correlates with their evolutionary conservation and essentiality for normal development. Thus, we propose that the intensively recruited genetic system during vertebrates' organogenesis period imposed constraints on its diversification through pleiotropic constraints, which ultimately led to the common anatomical pattern observed in vertebrates.