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1. Examining developmental plasticity in the skeletal system through a sensitive developmental windows framework

   https://doi.org/10.1002/ajpa.24338  

   McPherson, Cait B. ( June 2021 , American Journal of Physical Anthropology)

   null (Ed.)

   Developmental plasticity facilitates energetically costly but potentially fitness-enhancing adjustments to phenotypic trajectories in response to environmental stressors, and thus may significantly impact patterns of growth, morbidity, and mortality over the life course. Ongoing research into epigenetics and developmental biology indicate that the timing of stress exposures is a key factor when assessing their impact on developmental processes. Specifically, stress experienced within sensitive developmental windows (SDWs), discrete developmental periods characterized by heightened energy requirements and rapid growth, may alter the pace and tempo of growth in ways that significantly influence phenotypic development over both the short and long term. In human skeletal biology, efforts to assess how developmental environments shape health outcomes over the life course could be enhanced by incorporating the SDW concept into existing methodological approaches. The goal of this article is to outline an interpretive framework for identifying and interpreting evidence of developmental stress in the skeletal system using the SDW concept. This framework provides guidance for the identification of elements most likely to capture evidence of stress most relevant to a study's core research questions, the interpretation of developmental stress exhibited by those elements, and the relationship of skeletal indicators of stress to the demographic patterning of morbidity and mortality. Use of the SDW concept in skeletal biology has the potential to enrich traditional approaches to addressing developmental origins of health and disease hypotheses, by targeting periods in which individuals are most susceptible to stress and thus most likely to exhibit plasticity in response. 

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2. Disentangling the developmental origins of a novel phenotype: enhancement versus reversal of environmentally induced gene expression

   https://doi.org/10.1098/rspb.2022.1764  

   Levis, Nicholas A. ; McKay, Daniel J. ; Pfennig, David W. ( October 2022 , Proceedings of the Royal Society B: Biological Sciences)

   Increasing evidence suggests that many novel traits might have originated via plasticity-led evolution (PLE). Yet, little is known of the developmental processes that underpin PLE, especially in its early stages. One such process is ‘phenotypic accommodation’, which occurs when, in response to a change in the environment, an organism experiences adjustments across variable parts of its phenotype that improve its fitness. Here, we asked if environmentally induced changes in gene expression are enhanced or reversed during phenotypic accommodation of a novel, complex phenotype in spadefoot toad tadpoles ( Spea multiplicata ). More genes than expected were affected by both the environment and phenotypic accommodation in the liver and brain. However, although phenotypic accommodation primarily reversed environmentally induced changes in gene expression in liver tissue, it enhanced these changes in brain tissue. Thus, depending on the tissue, phenotypic accommodation may either minimize functional disruption via reversal of gene expression patterns or promote novelty via enhancement of existing expression patterns. Our study thereby provides insights into the developmental origins of a novel phenotype and the incipient stages of PLE. 

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3. Developmental bias in the evolution and plasticity of beetle horn shape

   https://doi.org/10.1098/rspb.2022.1441  

   Rohner, Patrick T. ; Hu, Yonggang ; Moczek, Armin P. ( September 2022 , Proceedings of the Royal Society B: Biological Sciences)

   The degree to which developmental systems bias the phenotypic effects of environmental and genetic variation, and how these biases affect evolution, is subject to much debate. Here, we assess whether developmental variability in beetle horn shape aligns with the phenotypic effects of plasticity and evolutionary divergence, yielding three salient results. First, we find that most pathways previously shown to regulate horn length also affect shape. Second, we find that the phenotypic effects of manipulating divergent developmental pathways are correlated with each other as well as multivariate fluctuating asymmetry—a measure of developmental variability. Third, these effects further aligned with thermal plasticity, population differences and macroevolutionary divergence between sister taxa and more distantly related species. Collectively, our results support the hypothesis that changes in horn shape—whether brought about by environmentally plastic responses, functional manipulations or evolutionary divergences—converge along ‘developmental lines of least resistance’, i.e. are biased by the developmental system underpinning horn shape. 

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4. Spatial and temporal variation in phenotypes and fitness in response to developmental thermal environments

   https://doi.org/10.1111/1365-2435.13910  

   Pruett, Jenna E. ; Warner, Daniel A. ( October 2021 , Functional Ecology)

   Phenotypic variation within populations is influenced by the environment via plasticity and natural selection. How phenotypes respond to the environment can vary among traits, populations and life stages in ways that can influence fitness.

   Plastic responses during early development are particularly important because they can affect components of fitness throughout an individual's life. Consequently, how natural selection shapes developmental plasticity could be influenced by fitness consequences across different life stages. Moreover, spatial variation in selection pressures could generate differences in plastic responses among populations.

   To gain insight into sources of variation in phenotypes and survival, we used a laboratory egg incubation experiment using brown anole lizardsAnolis sagreifrom mainland (ancestral) and island (descendent) populations, combined with a mark–release–recapture experiment in the field. Our study was designed to (a) quantify the effects developmental temperature on embryo development and offspring morphology, (b) assess how developmental temperature influences offspring survival across different life stages and (c) quantify how thermal reaction norms vary among ancestral and descendant populations.

   Developmental temperature influenced offspring morphology, but thermal reaction norms of embryos showed little variation among populations. Developmental temperature influenced offspring survival, but the patterns differed between embryo and hatchling stages; the optimal temperature for embryos was about 5℃ lower than that for hatchlings. High temperatures were thermally stressful to embryos, but they reduced incubation duration and led to early hatching. In turn, earlier hatching increased the probability of survival to adulthood. Moreover, the effect of developmental temperature on hatchling survival was most pronounced for offspring that hatched late in the season.

   The difference in optimal developmental temperatures between life stages may be driven by physiological tolerance for embryos and by ecological factors for hatchlings. Moreover, the fitness consequences of the developmental environment depend on the phenology of hatching. Overall, these results highlight how the developmental environment can differentially affect fitness across life stages and show that temporal thermal heterogeneity can influence survival of embryos, but the consequences on post‐hatching stages may vary at different times of the season.

   A freePlain Language Summarycan be found within the Supporting Information of this article.

    

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5. Attempting genetic inference from directional asymmetry during convergent hindlimb reduction in squamates

   https://doi.org/10.1002/ece3.9088  

   Swank, Samantha ; Elazegui, Ethan ; Janidlo, Sophia ; Sanger, Thomas J. ; Bell, Michael A. ; Stuart, Yoel E. ( July 2022 , Ecology and Evolution)

   Loss and reduction in paired appendages are common in vertebrate evolution. How often does such convergent evolution depend on similar developmental and genetic pathways? For example, many populations of the threespine stickleback and ninespine stickleback (Gasterosteidae) have independently evolved pelvic reduction, usually based on independent mutations that caused reducedPitx1expression. ReducedPitx1expression has also been implicated in pelvic reduction in manatees. Thus, hindlimb reduction stemming from reducedPitx1expression has arisen independently in groups that diverged tens to hundreds of millions of years ago, suggesting a potential for repeated use ofPitx1across vertebrates. Notably, hindlimb reduction based on the reduction inPitx1expression produces left‐larger directional asymmetry in the vestiges. We used this phenotypic signature as a genetic proxy, testing for hindlimb directional asymmetry in six genera of squamate reptiles that independently evolved hindlimb reduction and for which genetic and developmental tools are not yet developed:Agamodon anguliceps,Bachia intermedia,Chalcides sepsoides,Indotyphlops braminus,Ophisaurus attenuatuas and O. ventralis, andTeius teyou. Significant asymmetry occurred in one taxon,Chalcides sepsoides, whose left‐side pelvis and femur vestiges were 18% and 64% larger than right‐side vestiges, respectively, suggesting modification inPitx1expression in that species.However, there was either right‐larger asymmetry or no directional asymmetry in the other five taxa, suggesting multiple developmental genetic pathways to hindlimb reduction in squamates and the vertebrates more generally.

    

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