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Background: Alternative patterns of secondary growth in stems of Nyctaginaceae is present in all growth habits of the family and have been known for a long time. However, the interpretation of types of cambial variants have been controversial, given that different authors have given them different developmental interpretations. The different growth habits coupled with an enormous stem anatomical diversity offers the unique opportunity to investigate the evolution of complex developments, to address how these anatomies shifted within habits, and how the acquisition of novel cambial variants and habit transitions impacted the diversification of the family.

Methods: We integrated developmental data with a phylogenetic framework to investigate the diversity and evolution of stem anatomy in Nyctaginaceae using phylogenetic comparative methods, reconstructing ancestral states, and examining whether anatomical shifts correspond to species diversification rate shifts in the family.

Results: Two types of cambial variants, interxylary phloem and successive cambia, were recorded in Nyctaginaceae, which result from four different ontogenies. These ontogenetic trajectories depart from two distinct primary vascular structures (regular or polycyclic eustele) yet, they contain shared developmental stages which generate stem morphologies with deconstructed boundaries of morphological categories (continuum morphology). Unlike our a priori hypotheses, interxylary phloem is reconstructed as the ancestral character for the family, with three ontogenies characterized as successive cambia evolving in few taxa. Cambial variants are not contingent on habits, and their transitions are independent from species diversification.

Conclusions: Our findings suggest that multiple developmental mechanisms, such as heterochrony and heterotopy, generate the transitions between interxylary phloem and successive cambia. Intermediate between these two extremes are present in Nyctaginaceae, suggesting a continuum morphology across the family as a generator of anatomical diversity.

Background: An outstanding question in evolutionary biology is how genetic interactions defining novel traits evolve. They may evolve either by de novo assembly of previously non-interacting genes or by en bloc co-option of interactions from other functions. We tested these hypotheses in the context of a novel phenotype-Lamiales flower monosymmetry-defined by a developmental program that relies on regulatory interaction among CYCLOIDEA, RADIALIS, DIVARICATA, and DRIF gene products. In Antirrhinum majus (snapdragon), representing Lamiales, we tested whether components of this program likely function beyond their previously known role in petal and stamen development. In Solanum lycopersicum (tomato), representing Solanales which diverged from Lamiales before the origin of Lamiales floral monosymmetry, we additionally tested for regulatory interactions in this program.

Results: We found that RADIALIS, DIVARICATA, and DRIF are expressed in snapdragon ovaries and developing fruit, similar to their homologs during tomato fruit development. In addition, we found that a tomato CYCLOIDEA ortholog positively regulates a tomato RADIALIS ortholog.

Conclusion: Our results provide preliminary support to the hypothesis that the developmental program defining floral monosymmetry in Lamiales was co-opted en bloc from a function in carpel development. This expands our understanding of novel trait evolution facilitated by co-option of existing regulatory interactions.

Background: The LEAFY (LFY) transcription factors are present in algae and across land plants. The available expression and functional data of these genes in embryophytes suggest that LFY genes control a plethora of processes including the first zygotic cell division in bryophytes, shoot cell divisions of the gametophyte and sporophyte in ferns, cone differentiation in gymnosperms and floral meristem identity in flowering plants. However, their putative plesiomorphic role in plant reproductive transition in vascular plants remains untested.

Results: We perform Maximum Likelihood (ML) phylogenetic analyses for the LFY gene lineage in embryophytes with expanded sampling in lycophytes and ferns. We recover the previously identified seed plant duplication that results in LEAFY and NEEDLY paralogs. In addition, we recover multiple species-specific duplications in ferns and lycophytes and large-scale duplications possibly correlated with the occurrence of whole genome duplication (WGD) events in Equisetales and Salviniales. To test putative roles in diverse ferns and lycophytes we perform LFY expression analyses in Adiantum raddianum, Equisetum giganteum and Selaginella moellendorffii. Our results show that LFY genes are active in vegetative and reproductive tissues, with higher expression in early fertile developmental stages and during sporangia differentiation.

Conclusions: Our data point to previously unrecognized roles of LFY genes in sporangia differentiation in lycophytes and ferns and suggests that functions linked to reproductive structure development are not exclusive to seed plant LFY homologs.

Background: Understanding the relationship between macroevolutionary diversity and variation in organism development is an important goal of evolutionary biology. Variation in the morphology of several plant and animal lineages is attributed to pedomorphosis, a case of heterochrony, where an ancestral juvenile shape is retained in an adult descendant. Pedomorphosis facilitated morphological adaptation in different plant lineages, but its cellular and molecular basis needs further exploration. Plant development differs from animal development in that cells are enclosed by cell walls and do not migrate. Moreover, in many plant lineages, the differentiated epidermis of leaves, and leaf-derived structures, such as petals, limits organ growth. We, therefore, proposed that pedomorphosis in leaves, and in leaf-derived structures, results from delayed differentiation of epidermal cells with respect to reproductive maturity. This idea was explored for petal evolution, given the importance of corolla morphology for angiosperm reproductive success.

Results: By comparing cell morphology and transcriptional profiles between 5 mm flower buds and mature flowers of an entomophile and an ornitophile Loasoideae species (a lineage that experienced transitions from bee- to hummingbird-pollination), we show that evolution of pedomorphic petals of the ornithophile species likely involved delayed differentiation of epidermal cells with respect to flower maturity. We also found that developmental mechanisms other than pedomorphosis might have contributed to evolution of corolla morphology.

Conclusions: Our results highlight a need for considering alternatives to the flower-centric perspective when studying the origin of variation in flower morphology, as this can be generated by developmental processes that are also shared with leaves.

The carpel is a fascinating structure that plays a critical role in flowering plant reproduction and contributed greatly to the evolutionary success and diversification of flowering plants. The remarkable feature of the carpel is that it is a closed structure that envelopes the ovules and after fertilization develops into the fruit which protects, helps disperse, and supports seed development into a new plant. Nearly all plant-based foods are either derived from a flowering plant or are a direct product of the carpel. Given its importance it's no surprise that plant and evolutionary biologists have been trying to explain the origin of the carpel for a long time. Before carpel evolution seeds were produced on open leaf-like structures that are exposed to the environment. When the carpel evolved in the stem lineage of flowering plants, seeds became protected within its closed structure. The evolutionary transition from that open precursor to the closed carpel remains one of the greatest mysteries of plant evolution. In recent years, we have begun to complete a picture of what the first carpels might have looked like. On the other hand, there are still many gaps in our understanding of what the precursor of the carpel looked like and what changes to its developmental mechanisms allowed for this evolutionary transition. This review aims to present an overview of existing theories of carpel evolution with a particular emphasis on those that account for the structures that preceded the carpel and/or present testable developmental hypotheses. In the second part insights from the development and evolution of diverse plant organs are gathered to build a developmental hypothesis for the evolutionary transition from a hypothesized laminar open structure to the closed structure of the carpel.

Background: Botryllid ascidians are a group of marine invertebrate chordates that are colonial and grow by repeated rounds of asexual reproduction to form a colony of individual bodies, called zooids, linked by a common vascular network. Two distinct processes are responsible for zooid regeneration. In the first, called blastogenesis, new zooids arise from a region of multipotent epithelium from a pre-existing zooid. In the second, called whole body regeneration (WBR), mobile cells in the vasculature coalesce and are the source of the new zooid. In some botryllid species, blastogenesis and WBR occur concurrently, while in others, blastogenesis is used exclusively for growth, while WBR only occurs following injury or exiting periods of dormancy. In species such as Botrylloides diegensis, injury induced WBR is triggered by the surgical isolation of a small piece of vasculature. However, Botryllus schlosseri has unique requirements that must be met for successful injury induced WBR. Our goal was to understand why there would be different requirements between these two species.

Results: While WBR in B. diegensis was robust, we found that in B. schlosseri, new zooid growth following injury is unlikely due to circulatory cells, but instead a result of ectopic development of tissues leftover from the blastogenic process. These tissues could be whole, damaged, or partially resorbed developing zooids, and we defined the minimal amount of vascular biomass to support ectopic regeneration. We did find a common theme between the two species: a competitive process exists which results in only a single zooid reaching maturity following injury. We utilized this phenomenon and found that competition is reversible and mediated by circulating factors and/or cells.

Conclusions: We propose that WBR does not occur in B. schlosseri and that the unique requirements defined in other studies only serve to increase the chances of ectopic development. This is likely a response to injury as we have discovered a vascular-based reversible competitive mechanism which ensures that only a single zooid completes development. This competition has been described in other species, but the unique response of B. schlosseri to injury provides a new model to study resource allocation and competition within an individual.

The Wnt genes represent a large family of secreted glycoprotein ligands that date back to early animal evolution. Multiple duplication events generated a set of 13 Wnt families of which 12 are preserved in protostomes. Embryonic Wnt expression patterns (Wnt-patterning) are complex, representing the plentitude of functions these genes play during development. Here, we comprehensively investigated the embryonic expression patterns of Wnt genes from three species of spiders covering both main groups of true spiders, Haplogynae and Entelegynae, a mygalomorph species (tarantula), as well as a distantly related chelicerate outgroup species, the harvestman Phalangium opilio. All spiders possess the same ten classes of Wnt genes, but retained partially different sets of duplicated Wnt genes after whole genome duplication, some of which representing impressive examples of sub- and neo-functionalization. The harvestman, however, possesses a more complete set of 11 Wnt genes but with no duplicates. Our comprehensive data-analysis suggests a high degree of complexity and evolutionary flexibility of Wnt-patterning likely providing a firm network of mutational protection. We discuss the new data on Wnt gene expression in terms of their potential function in segmentation, posterior elongation, and appendage development and critically review previous research on these topics. We conclude that earlier research may have suffered from the absence of comprehensive gene expression data leading to partial misconceptions about the roles of Wnt genes in development and evolution.

Background: Despite the conserved pattern of the cell-fate map among spiralians, bivalves display several modified characteristics during their early development, including early specification of the D blastomere by the cytoplasmic content, as well as the distinctive fate of the 2d blastomere. However, it is unclear what changes in gene regulatory mechanisms led to such changes in cell specification patterns. Spiralian-TALE (SPILE) genes are a group of spiralian-specific transcription factors that play a role in specifying blastomere cell fates during early development in limpets. We hypothesised that the expansion of SPILE gene repertoires influenced the evolution of the specification pattern of blastomere cell fates.

Results: We performed a transcriptome analysis of early development in the purplish bifurcate mussel and identified 13 SPILE genes. Phylogenetic analysis of the SPILE gene in molluscs suggested that duplications of SPILE genes occurred in the bivalve lineage. We examined the expression patterns of the SPILE gene in mussels and found that some SPILE genes were expressed in quartet-specific patterns, as observed in limpets. Furthermore, we found that several SPILE genes that had undergone gene duplication were specifically expressed in the D quadrant, C and D quadrants or the 2d blastomere. These expression patterns were distinct from the expression patterns of SPILE in their limpet counterparts.

Conclusions: These results suggest that, in addition to their ancestral role in quartet specification, certain SPILE genes in mussels contribute to the specification of the C and D quadrants. We suggest that the expansion of SPILE genes in the bivalve lineage contributed to the evolution of a unique cell fate specification pattern in bivalves.