Evolution & Development最新文献

A critical issue in evolutionary biology is understanding the relationship between macroevolutionary patterns of diversity and the origin of variation at the organismal level. Among-individual allometry, the relationship between the size and shape of a structure among organisms at a fixed developmental stage, is often similar to evolutionary allometry, the relationship between the size and shape of a structure among populations or species, and the genetic and developmental process that underlie allometric relationships at both levels are thought to influence evolutionary diversification. Metameric organisms present an additional level of allometry: the relationship between the size and shape of structures within individuals. We propose that within-individual allometry is also related to evolutionary diversification among metameric organisms. We explore this idea in temperate deciduous Viburnum (Adoxaceae) species that bear two types of leaves, that is, preformed and neoformed leaves, with contrasting patterns of development. Examination of within-individual, among-individual, among-population, and among-species allometry of leaf shape in both leaf types showed that the slopes of all allometric relationships were significantly different from isometry, and their sign was consistent across allometric hierarchies. Although the allometric slope of preformed leaves was constant across allometry levels, the allometric slope of neoformed leaves became increasingly steeper. We suggest that allometric variation underlying evolutionary diversification in metameric organisms may manifest among individuals and also among their repeated structures. Moreover, structures with contrasting patterns of development within metameric organisms can experience different degrees of developmental constraint, and this can in turn affect morphological diversification.

Stripe patterns are a striking example for a repeatedly evolved color pattern. In the African adaptive radiations of cichlid fishes, stripes evolved several times independently. Previously, it has been suggested that regulatory evolution of a single gene, agouti-related-peptide 2 (agrp2), explains the evolutionary lability of this trait. Here, using a comparative transcriptomic approach, we performed comparisons between (adult) striped and nonstriped cichlid fishes of representatives of Lake Victoria and the two major clades of Lake Malawi (mbuna and non-mbuna lineage). We identify agrp2 to be differentially expressed across all pairwise comparisons, reaffirming its association with stripe pattern divergence. We therefore also provide evidence that agrp2 is associated with the loss of the nonstereotypic oblique stripe of Mylochromis mola. Complementary ontogenetic data give insights into the development of stripe patterns as well as vertical bar patterns that both develop postembryonically. Lastly, using the Lake Victoria species pair Haplochromis sauvagei and Pundamilia nyererei, we investigated the differences between melanic and non-melanic regions to identify additional genes that contribute to the formation of stripes. Expression differences—that most importantly also do not include agrp2—are surprisingly small. This suggests, at least in this species pair, that the stripe phenotype might be caused by a combination of more subtle transcriptomic differences or cellular changes without transcriptional correlates. In summary, our comprehensive analysis highlights the ontogenetic and adult transcriptomic differences between cichlids with different color patterns and serves as a basis for further investigation of the mechanistic underpinnings of their diversification.

There is widespread recognition of the need to increase research opportunities in biomedical science for undergraduate students from underrepresented backgrounds. Here, we describe the implementation of team-based science combined with intensive mentoring to conduct a large-scale project examining the evolution of behavior. This system can be widely applied in other areas of STEM to promote research-intensive opportunities in STEM fields and to promote diversity in science.

Evolution in response to a change in ecology often coincides with various morphological, physiological, and behavioral traits. For most organisms little is known about the genetic and functional relationship between evolutionarily derived traits, representing a critical gap in our understanding of adaptation. The Mexican tetra, Astyanax mexicanus, consists of largely independent populations of fish that inhabit at least 30 caves in Northeast Mexico, and a surface fish population, that inhabit the rivers of Mexico and Southern Texas. The recent application of molecular genetic approaches combined with behavioral phenotyping have established A. mexicanus as a model for studying the evolution of complex traits. Cave populations of A. mexicanus are interfertile with surface populations and have evolved numerous traits including eye degeneration, insomnia, albinism, and enhanced mechanosensory function. The interfertility of different populations from the same species provides a unique opportunity to define the genetic relationship between evolved traits and assess the co-evolution of behavioral and morphological traits with one another. To define the relationships between morphological and behavioral traits, we developed a pipeline to test individual fish for multiple traits. This pipeline confirmed differences in locomotor activity, prey capture, and startle reflex between surface and cavefish populations. To measure the relationship between traits, individual F2 hybrid fish were characterized for locomotor behavior, prey-capture behavior, startle reflex, and morphological attributes. Analysis revealed an association between body length and slower escape reflex, suggesting a trade-off between increased size and predator avoidance in cavefish. Overall, there were few associations between individual behavioral traits, or behavioral and morphological traits, suggesting independent genetic changes underlie the evolution of the measured behavioral and morphological traits. Taken together, this approach provides a novel system to identify genetic underpinnings of naturally occurring variation in morphological and behavioral traits.

The morphology of the mammalian chondrocranium appears to differ significantly from those of other amniotes, since the former possesses uniquely developed brain and cranial sensory organs. In particular, a question has long remained unanswered as to the developmental and evolutionary origins of a cartilaginous nodule called the ala hypochiasmatica. In this study, we investigated the embryonic origin of skeletal elements in the murine orbitotemporal region by combining genetic cell lineage analysis with detailed morphological observation. Our results showed that the mesodermal embryonic environment including the ala hypochiasmatica, which appeared as an isolated mesodermal distribution in the neural crest-derived prechordal region, is formed as a part of the mesoderm that continued from the chordal region during early chondrocranial development. The mesoderm/neural crest cell boundary in the head mesenchyme is modified through development, resulting in the secondary mesodermal expansion to invade into the prechordal region. We thus revealed that the ala hypochiasmatica develops as the frontier of the mesodermal sheet stretched along the cephalic flexure. These results suggest that the mammalian ala hypochiasmatica has evolved from a part of the mesodermal primary cranial wall in ancestral amniotes. In addition, the endoskeletal elements in the orbitotemporal region, such as the orbital cartilage, suprapterygoid articulation of the palatoquadrate, and trabecula, some of which were once believed to represent primitive traits of amniotes and to be lost in the mammalian lineage, have been confirmed to exist in the mammalian cranium. Consequently, the mammalian chondrocranium can now be explained in relation to the pan-amniote cranial configuration.

The developmental process establishes the foundation upon which natural selection may act. In that same sense, it is inundated with numerous constraints that work to limit the directions in which a phenotype may respond to selective pressures. Extreme phenotypes have been used in the past to identify tradeoffs and constraints and may aid in recognizing how alterations to the Baupläne can influence the trajectories of lineages. The Bramidae, a family of Scombriformes consisting of 20 extant species, are unique in that five species greatly deviate from the stout, ovaloid bodies that typify the bramids. The Ptericlinae, or fanfishes, are instead characterized by relatively elongated body plans and extreme modifications to their medial fins. Here, we explore the development of Bramidae morphologies and examine them through a phylogenetic lens to investigate the concepts of developmental and evolutionary constraints. Contrary to our predictions that the fanfishes had been constrained by inherited properties of an ancestral state, we find that the fanfishes exhibit both increased rates of trait evolution and differ substantially from the other bramids in their developmental trajectories. Conversely, the remaining bramid genera differ little, both among one another and in comparison, to the sister family Caristiidae. In all, our data suggest that the fanfishes have broken constraints, thereby allowing them to mitigate trade-offs on distinctive aspects of morphology.

Directional asymmetry is a systematic difference between the left and right sides for structures with bilateral symmetry or a systematic differentiation among repeated parts for complex symmetry. This study explores factors that produce directional asymmetry in the flower of Iris pumila, a structure with complex symmetry that makes it possible to investigate multiple such factors simultaneously. The shapes and sizes of three types of floral organs, the falls, standards, and style branches, were quantified using the methods of geometric morphometrics. For each flower, this study recorded the compass orientations of floral organs as well as their anatomical orientations relative to the two spathes subtending each flower. To characterize directional asymmetry at the whole-flower level, differences in the average sizes and shapes according to compass orientation and relative orientation were computed, and the left–right asymmetry was also evaluated for each individual organ. No size or shape differences within flowers were found in relation to anatomical position; this may relate to the terminal position of flowers in Iris pumila, suggesting that there may be no adaxial–abaxial polarity, which is very prominent in many other taxa. There was clear directional asymmetry of shape in relation to compass orientation, presumably driven by a consistent environmental gradient such as solar irradiance. There was also clear directional asymmetry between left and right halves of every floral organ, most likely related to the arrangement of organs in the bud. These findings indicate that different factors are acting to produce directional asymmetry at different levels. In conventional analyses not recording flower orientations, these effects would be impossible to disentangle from each other and would probably be included as part of fluctuating asymmetry.

Epibranchial organs (EBOs), found in at least five of the eight otomorphan families, are used to aggregate small prey inside the buccopharyngeal cavity and range in morphological complexity from a singular, small slit on the pharyngeal roof to several, elongated soft tissue tubes. Despite broad phylogenetic representation, little is known about the origin, development, or evolution of EBOs. We hypothesize that both heterochronic and heterotopic changes throughout the evolution of EBOs are at the root of their morphological diversity. Heterochrony is a foundational explanation in developmental studies, however, heterotopy, a developmental change in spatial or topographical relationships, can have even more profound effects on a given structure but has received relatively little attention. Here, we investigate how developmental mechanisms may drive morphological diversity of EBOs within otomorphan fishes. We compare early pharyngeal development in three species, Anchoa mitchilli (Engraulidae) which has the most basic EBO, B. tyrannus (Clupeidae) which has a more complex EBO, and Hypophthalmichthys molitrix (Cyprinidae) which has the most complex EBO yet described. Using branchial arch growth rates and morphological analyses, we illustrate how both heterochronic and heterotopic mechanisms are responsible for some of the phenotypic diversity seen in otomorphan EBOs. Importantly, we also identify conserved developmental patterns that further our understanding of how EBOs may have first originated and evolved across actinopterygian fishes.

Changing the shape of craniofacial bones can profoundly alter ecological function, and understanding how developmental conditions sculpt skeletal phenotypes can provide insight into evolutionary adaptations. Thyroid hormone (TH) stimulates metamorphosis and regulates skeletal morphogenesis across vertebrates. To assess the roles of this hormone in sculpting the craniofacial skeleton of a non-metamorphic vertebrate, we tested zebrafish for developmental periods of TH-induced craniofacial shape change. We analyzed shapes of specific bones that function in prey detection, capture and processing. We quantified these elements from late-larval through adult stages under three developmental TH profiles. Under wild-type conditions, each bone progressively grows allometrically into a mature morphology over the course of postembryonic development. In three of the four bones, TH was required to sculpt an adult shape: hypothyroidism inhibited aspects of shape change, and allowed some components of immature shape to be retained into adulthood. Excess developmental TH stimulated aspects of precocious shape change leading to abnormal morphologies in some bones. Skeletal features with functional importance showed high sensitivities to TH, including the transformator process of the tripus, the mandibular symphysis of the lower jaw, the scutiform lamina of the hyomandibula, and the anterior arm of the pharyngeal jaw. In all, we found that TH is necessary for shaping mature morphology of several essential skeletal elements; this requirement is particularly pronounced during larval development. Altered TH titer leads to abnormal morphologies with likely functional consequences, highlighting the potential of TH and downstream pathways as targets for evolutionary change.