Evolution and Development

Hensel, Katrin; Lotan, Tamar; Sanders, Steve M.; Cartwright, Paulyn; Frank, Uri

doi: 10.1111/ede.12089pmid: 25123972

SUMMARY We have studied the evolution of Wnt genes in cnidarians and the expression pattern of all Wnt ligands in the hydrozoan Hydractinia echinata. Current views favor a scenario in which 12 Wnt sub‐families were jointly inherited by cnidarians and bilaterians from their last common ancestor. Our phylogenetic analyses clustered all medusozoan genes in distinct, well‐supported clades, but many orthologous relationships between medusozoan Wnts and anthozoan and bilaterian Wnt genes were poorly supported. Only seven anthozoan genes, Wnt2, Wnt4, Wnt5, Wnt6, Wnt 10, Wnt11, and Wnt16 were recovered with strong support with bilaterian genes and of those, only the Wnt2, Wnt5, Wnt11, and Wnt16 clades also included medusozoan genes. Although medusozoan Wnt8 genes clustered with anthozoan and bilaterian genes, this was not well supported. In situ hybridization studies revealed poor conservation of expression patterns of putative Wnt orthologs within Cnidaria. In polyps, only Wnt1, Wnt3, and Wnt7 were expressed at the same position in the studied cnidarian models Hydra, Hydractinia, and Nematostella. Different expression patterns are consistent with divergent functions. Our data do not fully support previous assertions regarding Wnt gene homology, and suggest a more complex history of Wnt family genes than previously suggested. This includes high rates of sequence divergence and lineage‐specific duplications of Wnt genes within medusozoans, followed by functional divergence over evolutionary time scales.

Liu, Jin; Lemonds, Thomas R.; Popadić, Aleksandar

doi: 10.1111/ede.12090pmid: 25124093

SUMMARY Variations in body pigmentation, encompassing both the range of specific colors as well as the spatial arrangement of those colors, are among the most noticeable and lineage‐specific insect features. However, the genetic mechanisms responsible for generating this diversity are still limited to several model species that are primarily holometabolous insects. To address this lack of knowledge, we utilize Oncopeltus fasciatus, an aposematic hemimetabolous insect, as a new model to study insect pigmentation. First, to determine the genetic regulation of black pigment production in Oncopeltus, we perform an RNAi analysis on three core genes involved in the melanin pathway, tyrosine hydroxylase (TH), dopa decarboxylase (DDC), and laccase 2 (lac2). The black pigmentation is affected in all instances, showing that the black pigments in this species are derived from the melanin pathway. The results of the DDC RNAi are particularly informative because they reveal that it is Dopamine melanin, not DOPA melanin, which is the predominant component of black pigments in Oncopeltus. Second, we test whether pigmentation follows a two‐step model where the spatial pre‐mapping of enzymatic activity is followed by vein‐dependent transportation of melanin substances. We confirm the existence of the first step by observing that premature wings develop black pigmentation when exposed to melanin precursors. In addition, we provide evidence for the second step by showing that wing melanin patterning is disrupted when vein transportation is halted. These findings bring novel insights from a hemimetabolous species and establish a framework for subsequent studies on the mechanisms of pigment production and patterning responsible for variations in insect coloration.

Grimbert, Stéphanie; Braendle, Christian

doi: 10.1111/ede.12091pmid: 25143152

SUMMARY Understanding the robustness of developmental systems requires insights into the sensitivity of underlying molecular and cellular parameters to perturbations, and how such sensitivity evolves. We address these issues using vulval cell fate determination—a reproducible and robust patterning process regulated by a cross‐talk of EGF‐Ras‐MAPK and Delta‐Notch pathways. Although the final vulval cell fate pattern is identical in all Caenorhabditis species, the patterning process underlies extensive cryptic genetic variation between and within species. Here, we tested whether this cryptic genetic variation translates into variation in developmental sensitivity to environmental perturbations. We disrupted vulval patterning using thermal perturbations to quantify and compare environmental sensitivity of different system parameters between distinct genotypes of C. elegans and C. briggsae. Thermal perturbations globally debuffered vulval development, triggering diverse pattering variants, whose frequency and spectra were strongly species‐ and genotype‐dependent. This condition‐dependent variation indicates that environmental sensitivity of different system properties, such as vulval competence or vulval induction, is subject to evolutionary change. High temperature induced a genotype‐specific decrease of secondary fate induction and corresponding Notch pathway activity in the C. elegans N2 strain; in contrast, hypoinduction of the primary cell fate was never observed. Vulval precursor cells therefore differ in temperature sensitivity and such cell‐specific sensitivity shows evolutionary variation. We further compared spectra of temperature‐induced vulval variants to the ones induced by mutation accumulation in the same genotypes. In response to either perturbation, we observed similar genotype‐dependence of variant production, allowing identification of distinct system features most sensitive to both mutation and environment. Taken together, we show how sensitivity of system parameters regulating Caenorhabditis vulval development depends on subtle interactions between perturbations and genetic background. Our results imply that cryptic genetic variation may reflect evolutionary variation in developmental robustness, therefore potentially contributing to the maintenance of phenotypic precision when facing perturbations.

Sheil, Christopher A.; Jorgensen, Michael; Tulenko, Frank; Harrington, Sean

doi: 10.1111/ede.12092pmid: 25143258

SUMMARY The evolutionary origin of Lissamphibia likely involved heterochrony, as demonstrated by the biphasic lifestyles of most extant orders, differences between Anura (with tadpole‐to‐froglet metamorphosis) and Urodela (which lack strongly defined metamorphosis), and the appearance of direct development among separate lineages of frogs. Patterns in the timing of appearance of skeletal elements (i.e., ossification sequence data) represent a possible source of information for understanding the origin of Lissamphibia, and with the advent of analytical methods to directly optimize these data onto known phylogenies, there has been a renewed interest in assessing the role of changes in these developmental events. However, little attention has been given to the potential impact of variation in ossification sequence data—this is particularly surprising given that different criteria for collecting these data have been employed. Herein, new and previously published ossification data are compiled and all pairs of data for same‐species comparisons are selected. Analyses are run to assess the impact of using data that were collected by different methodologies: (1) wild‐ versus lab‐raised animals; (2) different criteria for recognizing timing of ossification; and (3) randomly selecting ossification sequences for species from which multiple studies have been published, but for which the data were collected by different criteria. Parsimov‐based genetic inference is utilized to map ossification sequence data onto an existing phylogeny to reconstruct ancestral sequences of ossification and infer instances of heterochrony. All analyses succeeded in optimizing sequence data on internal nodes and instances of heterochrony were identified. However, among all analyses little congruence was found in reconstructed ancestral sequences or among inferred instances of heterochrony. These results indicate a high degree of variation in timing of ossification, and suggest a cautionary note about use of these data, particularly given that in most instances issues associated with the original sources of data (e.g., wild‐ vs. lab‐raised animals; or criteria for identification of earliest ossification) are not addressed. Potential sources of variation in the original data are discussed and may explain the incongruence observed here.

Cvijanović, Milena; Ivanović, Ana; Kalezić, Miloš L.; Zelditch, Miriam L.

doi: 10.1111/ede.12093pmid: 25124217

SUMMARY Comparative studies of ontogenies of closely related species provide insights into the mechanisms responsible for morphological diversification. Using geometric morphometrics, we investigated the ontogenetic dynamics of postlarval skull shape and disparity in three closely related crested newt species. The skull shapes of juveniles just after metamorphosis (hereafter metamorphs) and adult individuals were sampled by landmark configurations that describe the shape of the dorsal and ventral side of the newt skull, and analyzed separately. The three species differ in skull size and shape in metamorphs and adults. The ontogenies of dorsal and ventral skull differ in the orientation but not lengths of the ontogenetic trajectories. The disparity of dorsal skull shape increases over ontogeny, but that of ventral skull shape does not. Thus, modifications of ontogenetic trajectories can, but need not, increase the disparity of shape. In species with biphasic life‐cycles, when ontogenetic trajectories for one stage can be decoupled from those of another, increases and decreases in disparity are feasible, but our results show that they need not occur.

Bolker, J. A.

doi: 10.1111/ede.12094pmid: N/A

Scholtz, Gerhard

doi: 10.1111/ede.12095pmid: N/A