The evolutionary origination of a novel expression pattern through an extreme heterochronic shift

Evolutionary mechanisms modulating the mammalian skull development

Mammals possess impressive craniofacial variation that mirrors their adaptation to diverse ecological niches, feeding behaviour, physiology and overall lifestyle. The spectrum of craniofacial geometries is established mainly during embryonic development. The formation of the head represents a sequence of events regulated on genomic, molecular, cellular and tissue level, with each step taking place under tight spatio-temporal control. Even minor variations in timing, position or concentration of the molecular drivers and the resulting events can affect the final shape, size and position of the skeletal elements and the geometry of the head. Our knowledge of craniofacial development increased substantially in the last decades, mainly due to research using conventional vertebrate model organisms. However, how developmental differences in head formation arise specifically within mammals remains largely unexplored. This review highlights three evolutionary mechanisms acknowledged to modify ontogenesis: heterochrony, heterotopy and heterometry. We present recent research that links changes in developmental timing, spatial organization or gene expression levels to the acquisition of species-specific skull morphologies. We highlight how these evolutionary modifications occur on the level of the genes, molecules and cellular processes, and alter conserved developmental programmes to generate a broad spectrum of skull shapes characteristic of the class Mammalia. This article is part of the theme issue 'The mammalian skull: development, structure and function'.

Transcriptional Landscapes of Divergent Sporophyte Development in Two Mosses, Physcomitrium (Physcomitrella) patens and Funaria hygrometrica

Understanding the molecular basis of morphological shifts is a fundamental question of evolutionary biology. New morphologies may arise through the birth/death of genes (gene gain/loss) or by reutilizing existing gene sets. Yet, the relative contribution of these two processes to radical morphological shifts is still poorly understood. Here, we use the model system of two mosses, Funaria hygrometrica and Physcomitrium (Physcomitrella) patens, to investigate the molecular mechanisms underlying contrasting sporophyte architectures. We used comparative analysis of time-series expression data for four stages of sporophyte development in both species to address this question in detail. We found that large-scale differences in sporophytic architecture are mainly governed by orthologous (i.e., shared) genes frequently experiencing temporal gene expression shifts between the two species. While the absolute number of species-specific genes expressed during sporophyte development is somewhat smaller, we observed a significant increase of their proportion in preferentially sporophyte expressed genes, suggesting a fundamental role in the sporophyte phase. However, further functional studies are necessary to determine their contribution to diverging sporophyte morphologies. Our results add to the growing set of studies suggesting that radical changes in morphology may rely on the heterochronic expression of conserved regulators.

The developmental and genetic trajectory of coloration in the guppy (Poecilia reticulata)

Examining the association between trait variation and development is crucial for understanding the evolution of phenotypic differences. Male guppy ornamental caudal fin coloration is one trait that shows a striking degree of variation within and between guppy populations. Males initially have no caudal fin coloration, then gradually develop it as they reach sexual maturity. For males, there is a trade-off between female preference for caudal fin coloration and increased visibility to predators. This trade-off may reach unique endpoints in males from different predation regimes. Caudal fin coloration includes black melanin, orange/yellow pteridines or carotenoids, and shimmering iridescence. This study examined the phenotypic trajectory and genetics associated with color development. We found that black coloration always developed first, followed by orange/yellow, then iridescence. The ordering and timing of color appearance was the same regardless of predation regime. The increased expression of melanin synthesis genes correlated well with the visual appearance of black coloration, but there was no correlation between carotenoids or pteridine synthesis gene expression and the appearance of orange/yellow. The lack of orange/yellow coloration in earlier male caudal fin developmental stages may be due to reduced expression of genes underlying the development of orange/yellow xanthophores.

The Times They Are A-Changin': Heterochrony in Plant Development and Evolution

Alterations in the timing of developmental programs during evolution, that lead to changes in the shape, or size of organs, are known as heterochrony. Heterochrony has been widely studied in animals, but has often been neglected in plants. During plant evolution, heterochronic shifts have played a key role in the origin and diversification of leaves, roots, flowers, and fruits. Heterochrony that results in a juvenile or simpler outcome is known as paedomorphosis, while an adult or more complex outcome is called peramorphosis. Mechanisms that alter developmental timing at the cellular level affect cell proliferation or differentiation, while those acting at the tissue or organismal level change endogenous aging pathways, morphogen signaling, and metabolism. We believe that wider consideration of heterochrony in the context of evolution will contribute to a better understanding of plant development.

Enhancer evolution and the origins of morphological novelty

A central goal of evolutionary biology is to understand the genetic origin of morphological novelties-i.e. anatomical structures unique to a taxonomic group. Elaboration of morphology during development depends on networks of regulatory genes that activate patterned gene expression through transcriptional enhancer regions. We summarize recent case studies and genome-wide investigations that have uncovered diverse mechanisms though which new enhancers arise. We also discuss how these enhancer-originating mechanisms have clarified the history of genetic networks underlying diversification of genital structures in flies, limbs and neural crest in chordates, and plant leaves. These studies have identified enhancers that were pivotal for morphological divergence and highlighted how novel genetic networks shaping form emerged from pre-existing ones.