Diverse regulatory mechanisms enable precise spatio-temporal control of gene expression across developmental stages, tissues, and sexes, contributing to the proper development of the organism. Evolutionary divergence leads to species-specific gene expression patterns, even in preserved developmental structures, due to regulatory changes that can disproportionately influence subsets of developmental genetic networks. Here, we quantify the evolution of sex-biased and tissue-biased transcriptomes from two tissue types (gonad and soma) for each of two sexes (male and female) from two of the closest known sister species of Caenorhabditis nematodes (C. remanei and C. latens). Differential gene expression and co-expression network analyses identify gene sets with distinct transcriptomic profiles, revealing widespread divergence between these morphologically and developmentally cryptic sister species. The transcriptomic divergence occurs despite most genes showing conserved expression across tissues and sexes. These observations implicate shared selection pressures related to tissue and sex differences as outweighing species-specific selection and developmental system drift in shaping overall transcriptome profiles. Although developmentally plastic tissue-biased expression profiles are mostly conserved between species, we find that sex-biased genes, particularly male-biased genes, contribute disproportionately to species-differences in gene expression, consistent with a disproportionate role for male-biased selection driving gene expression divergence.

Acquisition of air-breathing organs was one of the key events in the diversification of arthropods. Among terrestrial arthropods, isopod crustaceans have evolved a unique air-breathing structure called the pleopodal lung, which is located in their abdominal appendages (pleopods), while retaining pleopodal gills. These lungs offer an intriguing model for studying the evolution of respiratory organs during arthropod terrestrialization. However, the molecular mechanisms underlying lung function or development in isopods remain poorly understood. In this study, we conducted comparative transcriptomic analyzes using the common rough woodlouse, Porcellio scaber, in which pleopods with and without lungs are adjacent to each other. The results revealed distinct gene expression profiles linked to the structure and function of pleopods, including genes involved in morphogenesis. In particular, candidate lung development regulatory genes that were expressed specifically in the exopods of the second pleopods during the manca 1 stage were identified. Transcriptome analysis and immunohistochemistry suggested that the Hox gene abdominal-A is involved in lung formation. However, the two genes previously implicated in respiratory organ formation in pancrustaceans, trachealess and ventral veins lacking, did not show lung-related expression. Our comparison of gene expression patterns between exopods with and without lungs suggest that the function of gas exchange in the pleopodal lungs may be influenced by structural differences resulting from changes in developmental processes. Overall, this study provides essential insights into the molecular mechanisms underlying pleopodal lung development and sets the foundation for future evolutionary research.

The early development of Cryptoheros spilurus, a substrate-breeding Middle American cichlid, was studied from hatching to 16 days post-hatching (dph), to document for the first time, the sequence of key ontogenetic changes. Eggs, deposited on rocky substrates, measured 1.65 ± 0.05 mm in diameter, with asynchronous hatching occurring at 52-54 h post-fertilization. Hatchlings (TL = 4.739 ± 0.27 mm) showed a large yolk sacs, finfold, straight notochord, and undeveloped eyes. Scanning electron microscopy revealed early differentiation of structures, including cement glands, olfactory pits, and optic primordia. Cement glands, previously reported in other cichlids, were documented here in their full developmental chronology, including their regression by 7 dph. Cranial development proceeded rapidly, with pigmentation and eye formation initiating by 1 dph and oral cavity, dentition, and taste buds forming by 6 dph. Fin development followed a sequential pattern: early pectoral fin formation supported initial mobility, while caudal, dorsal, anal, and pelvic fins emerged progressively, with full formation completed by 16 dph. Pigmentation evolved from a ventral melanophore stripe to a distinct species-specific pattern involving xanthophores and iridophores. By 16 dph, C. spilurus had completed metamorphosis (TL = 13.168 ± 0.55 mm). Allometric analysis revealed biphasic growth trajectories. Structures involved in feeding and sensory input, such as head length, snout length, and gape size, exhibited prolonged positive allometry, while trunk and tail traits showed delayed or negative allometry. These patterns reflect functional prioritization during the shift to active foraging. This study highlights C. spilurus as a valuable model for examining heterochrony, morphological modularity, and ecological adaptation during early development. Our findings provide essential baseline data for future comparative work on developmental plasticity and diversification in Neotropical cichlids.

Embryos are vulnerable. Rapid development decreases the period of vulnerability. Parents' protections also decrease vulnerability and may decrease selection for rapid development. A previous study showed that invertebrate embryos with more protection had slower early cell cycles. The slowing varied greatly among species. Hypotheses for the slowing include genetic drift and selection for developmental improvements. Here, published data on teleost fish indicated that (1) guarded and hidden embryos exhibit a similar pattern of varied slowing and (2) the pattern of slowing is similar for early cell cycles (mostly dependent on times for DNA replication and cell division) and somite formation (which also involves transcription and cell signaling). Times for early cell cycles and somite formation were more uniformly fast for teleosts with scattered nonadhesive eggs than for those with guarded or hidden eggs. Some species with adhesive eggs that were not reported to be guarded or hidden also developed slowly, as expected if parents select safe sites for egg attachment. Slower development is expected to increase bias against evolutionary reversals to less protection of embryos. Differences in egg size did not account for slower development of protected embryos. Slow development increased age at hatching but did not account for all the increase in age at hatching of protected embryos. Greater protection of embryos was associated with an evolutionary slowing of developmental processes as simple as early cell multiplication and complex as somite formation, in fish with disparate protections of embryos, in habitats ranging from the ocean to temporary ponds.

A fundamental focus of evolutionary developmental biology is uncovering the genetic mechanisms responsible for the gain and loss of characters. One approach to this question is to investigate changes in the coordinated expression of a group of genes important for the development of a character of interest (a gene regulatory network). Here we consider the possibility that modifications to the wing gene regulatory network (wGRN), as defined by work primarily done in Drosophila melanogaster, were involved in the evolution of wing dimorphisms of the pea aphid (Acyrthosiphon pisum). We hypothesize that this may have occurred via changes in expression levels or by duplication followed by divergence of wGRN components. To test this, we annotated members of the wGRN in the pea aphid genome and assessed their expression levels in first and third nymphal instars of winged and wingless morphs of males and asexual females. We find that only 2 of the 32 assessed genes exhibit morph-biased expression. We also find that three wing genes (apterous (ap), warts (wts), and decapentaplegic (dpp)) have undergone gene duplication. In each case, the resulting paralogs show signs consistent with functional divergence, exhibiting either sex-, morph-, or stage-specific expression. Two gene duplicates, wts2 and dpp3, are of particular interest with respect to wing dimorphism, as they exhibit male morph-specific isoforms and wingless male-biased expression, respectively. These gene expression results provide an important first step toward identifying members of the pea aphid wGRN that may play a causative role in differentiating winged from wingless morphs. These findings supplement our understanding of trends in developmental gene network evolution, such as side-stepping pleiotropic constraint via duplication and sub-functionalization, underlying the emergence of novel phenotypes.

Telomeres (repeat-DNA-protein structures primarily located at the ends of chromosomes) protect coding DNA against attacks by reactive molecules and the cells' own DNA repair systems. If that capacity is costly, but enhances an individual's viability, we might expect to see natural selection acting on telomere length: that is, individuals with optimal telomere lengths should have higher lifetime reproductive success than conspecifics with shorter or longer telomeres. Some recent studies on humans broadly support that prediction, but no data are available for free-ranging ectothermic vertebrates that, unlike mammals, can facultatively adjust telomere length during an individual's lifetime. In our decade-long study of a natural population of sand lizards (Lacerta agilis), including measurement of 2736 telomeres across > 1700 hatchling lizards and their > 500 parents, but with a very high hatchling mortality reducing later-life sample sizes, we found that lifespan, lifetime reproductive success and offspring recruitment rate were highest for hatchlings with "average-length" telomeres. Hatchlings with shorter-than-average telomeres elongated their telomeres during juvenile life, attaining the population-average telomere length by the time of sexual maturity; but that compensatory telomere growth was associated with lower body condition.

During their development, fish pass through a series of developmental processes advancing, for example, their physiological and locomotive abilities. In particular, many marine fish larvae often hatch at an (semi-) embryonic developmental stage, and existential processes, such as digestion and respiration, and structures, such as muscle and skeleton, must form and/or change during the larval development. In this study, we analyzed the gene expression of factors associated with myogenesis, skeletogenesis, and growth within the different larval life stages of Atlantic herring. We evaluated these results in relation to length and stage ratio and performed histological analysis of cross-sections of herring larvae in different stages. Overall, the length per stage ratios showed that there are two major growth periods in larval herring development, the first occurring during the dorsal fin development phase and the second during the transition from caudal fin to pelvic fin development phase. This is consistent with the histological analysis, as an increase in muscle fibers was observed in both phases. The gene expression data also showed that factors responsible for muscle cell lineage determination and fiber development were highest before a period with increased growth. Combining our results with other studies on skeletogenesis, organogenesis, and the development of neural and sensory systems in herring, it becomes evident that other energetically costly developmental processes tend to occur in periods when growth is less prominent. It can therefore be concluded that growth and developmental priority periods alternate during larval development.

Segmentation is one of the most striking features of bilaterians, and understanding its mechanisms provides insights into the evolution of body plans. In annelids, segmentation occurs at different developmental stages through a variety of processes, yet the molecular pathways remain underexplored. Aiming to compare segmentation patterns in ontogeny and phylogeny, we analysed the expression of Avi-en (homologous to engrailed) and Avi-wnt1 in the nereidid polychaete Alitta virens. Using in situ hybridization, immunofluorescence, and cell proliferation assays, we mapped the spatiotemporal expression of these genes across embryonic, larval, and postlarval stages. We found that Avi-en was expressed in solid lateral domains early in the unsegmented protrochophore stage and progressed through a metameric pattern, while Avi-wnt1 expression appeared later, also aligning with segmental boundaries. At the nectochaete stage, the posterior domain of Avi-en expression in the growth zone expanded and split into two due to increased cell proliferation. The postlarval segment primordium then developed progressively, culminating in the activation of Avi-wnt1 at the posterior border. According to available published data, the revealed pattern of gradual segment formation is unique to nereidids. The observed divergence in gene expression and cell proliferation across annelids suggests that segmentation in bilaterians did not arise from a common ancestral mechanism. Our study enhances future progress in understanding the evolution of body patterning by providing a foundation for future comparisons.

Modularity is an inherent property of organismal design where organisms are subdivided into quasi-independent units. Studies have shown that patterns of modularity can dramatically shift during the ontogenies of direct developing organisms. However, it is unclear how modularity patterns shift in organisms that undergo metamorphosis which is a dynamic period of development where organisms undergo morphological changes in response to ecological or physiological cues. Here we examined the ontogenetic modularity of the skull in the Red Sea anemonefish, Amphiprion bicinctus, across four larval stages, preflexion, flexion, postflexion, and metamorphosis, using micro-CT scanning and 3D geometric morphometrics to assess ontogenetic allometry and modularity. We hypothesized that skull modularity reorganizes during developmental transitions, and that oral jaw elements exhibit strong allometric growth and integration linked to functional demands during larval stages. The mandible, premaxilla, and maxilla exhibited strong size-shape relationships while the lower pharyngeal jaw and parasphenoid were isometric. Morphological disparity peaked at preflexion, suggesting high developmental plasticity early in ontogeny. We tested modularity hypotheses based on developmental and functional interactions and found that the developmental hypotheses were favored across most stages. However, during flexion, a stage characterized by structural reorganization, support shifted to functional hypotheses and then reverted back to developmental hypothesis. This suggests that modular reorganization coincides with key functional transitions. Across all stages, the premaxilla and mandible remained highly integrated, underscoring the need for coordinated oral jaw development during early feeding. Our findings reveal how allometry, modularity, and integration interact during early skull development in a species undergoing rapid developmental and ecological transitions and highlight the functional importance of oral jaw coordination during early feeding.

Evolutionary novelties often arise through complex interactions among genetic, developmental, and ecological processes, yet their origins remain poorly understood. Here, we investigate the germline capsule in Camponotus (Carpenter ants) as a case of an evolutionary novelty. Using an integrated framework combining transcriptomic, morphological, and comparative developmental approaches, we characterize its molecular signatures, cellular architecture, and ontogeny. We show that germline gene-expressing cells adjacent to bacteriocytes fuse to form a multinucleated germline capsule, which subsequently contributes to the presumptive gonads, as revealed by label tracing. Despite harboring endosymbiotic bacteria like bacteriocytes, germline capsules exhibit distinct gene expression profiles. Furthermore, their phenotypic variation is developmentally modulated by bacterial presence. By examining the expression profile of germ-line specific gene (oskar) across multiple Camponotus species, we test the germline function of the capsule and its evolutionary conservation. Based on these findings, we propose a model in which the germline capsule evolved through cell fusion events enabled by developmental plasticity and shaped by interactions between host germline determinants and endosymbiotic bacteria. This study illustrates how integrating molecular, developmental, and ecological perspectives can illuminate the mechanisms underlying evolutionary innovation.

The evolution of the plastic response of organisms to environmental change remains one of the most challenging areas of biological research. Reasons for this include the complex nature of environmental cues and organismal responses, the energetic costs behind phenotypic plasticity performed under different conditions, and the individual capacity to respond, which depends on many developmental factors. A special case is the plastic body size response to temperature, the temperature-size rule (TSR). We used eight experimental populations of the rotifer Lecane inermis and measured body size and population growth rate r over a wide thermal range to investigate (i) the thermal conditions under which rotifers perform the TSR or canalize their body size (= no plasticity) and how this relates to fitness, and (ii) whether this response varies with organismal thermal preferences. We found a relationship between body size and fitness, confirming that the TSR is only performed within a certain thermal range, beyond which body size is canalized. We did not find the expected relationship between the strength of the TSR and the range of thermal tolerance, but our results do not allow us to reject the existence of such a relationship. Furthermore, we found a high repeatability of the parameters informing thermal tolerance compared to previous studies, reflecting a substantial degree of developmental constancy in the context of the organism's preference for temperature. We describe the special case of plasticity versus canalization for body size response to optimal and suboptimal temperatures in organisms that differ in their thermal tolerance.

The biological processes underlying the wide phenotypic mammal diversity are still not thoroughly understood. In this study, we examined how major stages in the life history of the common hippopotamus (Hippopotamus Amphibius) influence its craniomandibular morphology throughout ontogeny. Using geometric morphometrics and life-history meta-analysis correlations, we characterized skulls from 198 individuals spanning 20 developmental stages. The most significant morphological changes were observed during early infancy (0-3 years), coinciding with lactation and weaning, and during puberty (10-15 years), coinciding with reproductive maturation. These findings align with growth patterns typical of social mammals exhibiting high sexual dimorphism. Notably, we identified a pattern previously undocumented in any other vertebrate: cranial morphology stabilizes earlier than the mandibular one. Specifically, late-stage (20-25 years) shape modification in the mandibles indicates progressive reconfiguration of masticatory biomechanics as well as a continuous change of dental occlusion throughout life. This pattern is common in both male and female individuals and may be related to shifts in diet rather than sexual selection. This study provides the most comprehensive ontogenetic dataset for a semi-aquatic, large semigraviportal mammal with a polygynous social structure, offering a valuable foundation for future evolutionary studies based on comparative analyses.

Sulfidic caves are harsh and extreme environments characterized by limited oxygen, low pH, and the presence of hydrogen sulfide. Amazingly, animals can live in sulfidic caves, one such animal being Asellus infernus, a representative of the Asellus aquaticus species complex, originating from Movile Cave and from old wells that represent windows of access to a sulfidic groundwater ecosystem located in southeast Romania. Little previous work has been done on lab-reared populations of A. infernus as they have been historically difficult to raise in the lab. Here, we develop resources for A. infernus, examining questions of timing of morphological differences in cave versus surface individuals, whether the environment (lab-bred vs. wild-caught) influenced size characteristics, and the genes and pathways showing differential expression between cave and surface samples. We found that A. infernus did not develop pigmentation embryonically, and juveniles had increased body length and longer antenna II as compared to surface individuals. Furthermore, we found that some of these measures differed between wild-caught and lab-reared juveniles for a given population, indicating that environmental differences can also influence these size characteristics. In addition, differential expression between cave and surface samples and allele-specific expression studies within F1 hybrids identified multiple genes, including those involved in sulfide metabolism and phototransduction. Strikingly, molecular convergence of genes involved in sulfide detoxification was observed between A. infernus and previous work on a fish that lives in both cave and sulfidic environments, Poecilia mexicana. In sum, we were able to develop embryonic and genomic tools for A. infernus, a model for understanding cave adaptation and adaptation to sulfidic environments.

Developmental changes in animals reflect important behavioral, biological, and ecological shifts. Allometric adjustments in arthropods, specifically, are associated with changes in sexual maturity or alterations in life mode. Examining post-embryological allometry of the American horseshoe crab-Limulus polyphemus-here evidences early shifts in prosomal development, later changes in thoracetronic size, and possible modularity across exoskeletal sections. Modifications in prosomal allometry reflect transitions from living above the substrate to primarily burrowing. This change occurs at the 3-4 moult stage and is associated with a 70% mortality rate in both natural settings and under aquaculture conditions. Thoracetron allometry changes record the fusion of opisthosomal tergites into a plate, where tergal development drives shifts in thoracetron morphology. Allometric changes between main body sections present possible evidence for modularity within the horseshoe crab exoskeleton that manifest across moulting events. These allometric shifts reflect the complex evolutionary history of the group, especially changes from surface dwelling and enrollment to burrowing, likely in response to increased predation pressures.

Uncovering mechanisms by which sensory systems evolve is critical for understanding how organisms adapt to a novel environment. Astyanax mexicanus is a species of fish with populations of surface fish, which inhabit rivers and streams, and cavefish, which have adapted to life within caves. Cavefish have evolved sensory system changes relative to their surface fish counterparts, providing an opportunity to investigate mechanisms underlying sensory system evolution. Here, we report the role of the gene retinal homeobox 3 (rx3) in cavefish eye evolution. We generated surface fish with putative loss-of-function mutations in the rx3 gene using CRISPR-Cas9 to determine the role of this gene in eye development in this species. These rx3 mutant surface fish fail to develop eyes, demonstrating that rx3 is required for surface fish eye development. Further, rx3 mutant surface fish exhibit altered behaviors relative to wild-type surface fish, suggesting that the loss of eyes impacts sensory-dependent behaviors. Finally, eye development is altered in cave-surface hybrid fish that inherit the mutant allele of rx3 from surface fish relative to siblings that inherit a wild-type surface fish rx3 allele, suggesting that cis-regulatory variation at the rx3 locus contributes to eye size evolution in cavefish. Together, these findings demonstrate that, as in other species, rx3 is required for eye development in A. mexicanus. Moreover, they suggest that variation at the rx3 locus plays a role in the evolved reduction of eye size in cavefish, shedding light on the genetic mechanisms underlying sensory system evolution in response to extreme environmental changes.

Feathers are the most complex and diverse epidermal appendages found in vertebrates. Their unique hierarchical organization and development is based on a diversity of cell types and morphologies. Despite these presumptive feather cell types being well characterized morphologically, little is known about how gene regulation contributes to their development. Here, we use single cell and single nuclear RNA sequencing with in situ hybridization to identify and characterize cells types in embryonic chicken feathers. We show that the distinct cell morphologies correspond to feather cell types with distinct gene expression profiles. We also describe a previously unidentified cell type, the barb ridge basal epithelium, which appears to play a role alongside the marginal plate in barb ridge differentiation. A cell-cell signaling analysis provides evidence of important roles for the barb ridge basal epithelium and marginal plate signaling to the barb ridge. Furthermore, we analyze RNA velocity trajectories of developing feather cells and find distinct developmental trajectories for epidermal cells that constitute the mature feather and those that function only in feather development. Finally, we produce an evolutionary tree of feather cell types based on transcription factor expression as a test of the prior developmental hypotheses about feather evolution. Our tree is consistent with the developmental model of feather evolution, and sheds light on the influence of ancestral epidermal stratification on feather cell evolution. This transcriptomic approach to studying feather cell types helps lay the ground work for understanding the developmental evolutionary complexity and diversity of feathers.

Cranial and pelvic bones could have evolved in response to each other during human evolutionary history due to the increasingly tight fit between the baby's head and the mother's pelvis during delivery. A recently identified covariation pattern between these sets of bones and stature has shown important evidence of such an evolutionary trade-off, alleviating the chances of obstructed labor. Here, we tested the validity of this covariation pattern in a different sample, from a population with known high rates of C-section. 98 computed tomographies were used to perform statistical covariation tests (two-block partial least squares and ANOVA Procrustes) between pelvic and cranial shape, as well as other anthropometric variables, like stature, body mass, and BMI. Additionally, measurements were taken from cranial and pelvic bones for classic morphometric analyses. The results have shown an important sexual dimorphism in pelvic bones' shape but no correlation between them and stature or cranial size or shape. In terms of size, the sexual dimorphism on the true pelvis was also noticeable. The fact that the results obtained from this sample do not corroborate previous findings suggests the absence of this pattern in some populations. It also suggests that the current ideal rates of C-sections proposed by the World Health Organization might not be considering the existing diversity among human populations that may account for variable levels of difficulties in birth.

Phenotypic plasticity is an organism's ability to produce a different phenotype in response to nongenetic perturbations such as environmental disturbances. Beneficial phenotypic plasticity can be important in evolution. After an environmental disturbance, it can delay extinction giving opportunity to the appearance of beneficial mutations. In addition, plasticity may also be one of the factors that define the course that evolution takes, for example, through genetic assimilation. This is a process in which a phenotype that initially appears as a plastic response becomes under genetic control. In the end, development of such a phenotype does not require the factor that originally induced it. Here, I use a model of the evolution of gene regulatory networks to study the range of conditions that allow the association between plasticity and the course of evolution. I assayed conditions like the difference between ancestral and optimum phenotypes, the difficulty to build the optimum phenotype, the complexity of the developmental system, mutation rate, strength of plasticity limitations, fitness advantage of the optima, and the similarity between the initially induced phenotype and the optimum. I found that populations that yield a beneficial phenotype through plasticity most often evolve a similar genetically determined phenotype under all the conditions that I assayed. I also identified conditions that facilitate evolution through genetic assimilation. Notwithstanding, even under less favorable circumstances, this form of evolution still confers easier access to a new genetically determined optimum.

Prey attraction is an integral component of the carnivorous syndrome, yet its molecular adaptations have remained largely unexplored. Our study utilized tissue-specific transcriptomic data from the South American marsh pitcher plant, Heliamphora tatei, to explore the molecular and developmental basis of prey attraction. Carnivorous plants often present specialized structures associated with prey attraction and in Heliamphora, that function is carried out by the nectar spoon, a colorful extension of the top of the pitcher that is densely covered in nectaries. Through comparisons of gene expression in the nectar spoon with the rest of the pitcher, we identified a suite of differentially expressed genes that likely contribute to prey attraction, including enzymes involved in volatile synthesis and sugar transporters. We found that one lineage of sugar transporters, the 14a clade of SWEETs (Sugars Will Eventually Be Exported Transporters), is highly upregulated in the nectar spoon and has evolved more rapidly in Sarraceniaceae, consistent with specialization for nectar transport as part of prey attraction. Among the genes related to volatile production, we found several enzymes best known for their role in floral scent. These results suggest that, similar to prey digestion, ancient genes are repurposed for novel functions during the transition to carnivory and may facilitate the repeated convergent origins of carnivory across angiosperms.

Variation in shape and size within a lineage, driven by developmental processes, plays a key role in diversification. Here, we explore the effects of allometry and heterochrony on the skull shape evolution during the post-metamorphic period of species within the Boana faber clade, which vary considerably in body size. We analyzed 61 skulls of specimens belonging to eight species of the Boana faber clade, in addition to two outgroups, through 2D geometric morphometric analyses taken from CT-Scan images. Our results demonstrated that skull shape is considerably impacted by the size, represented by centroid size, and this effect can be observed from ontogenetic and evolutionary perspectives. In this way, we accessed the ontogenetic trajectories of analysed species and, in light of the phylogenetic hypothesis of the clade, we discussed the observed variation based on the concept of heterochrony, suggesting that a peramorphic pattern has evolved in the group.

Two growth modes are recognized in colonial pterobranchs (Graptolithina): monopodial growth and sympodial growth. The earliest colonial Graptolithina likely developed through monopodial growth, a mode of colony formation well-documented in the extant graptolite Rhabdopleura normani. This growth involves a permanent terminal zooid and the sequential budding of additional zooids behind it, as the contractile stalk (gymnocaulus) of this terminal zooid elongates. This process is reflected in specific features of the secreted housing structure, the tubarium. Recently, monopodial growth was identified for the first time in a fossil taxon-the Cambrian dithecodendrid Tarnagraptus-based on tubarium characteristics, as no zooids were preserved. Monopodial growth also appears probable in other Cambrian taxa resembling Tarnagraptus, although evidence remains limited due to fragmentary materials. Sympodial growth, characterized by transient terminal zooids that are sequentially replaced as new buds form, is extensively documented in the fossil record of the Graptolithina. This growth mode characterizes the vast majority of Cambrian to Devonian Dendroidea and Graptoloidea. Phylogenetic evidence suggests sympodial growth evolved from monopodial growth in graptolithines, but the mechanisms underlying this evolutionary transition remain unclear.