Nodal Points In Regulatory Networks And The Evolution Of Character Number And Pattern

In the examples above of pigmentation evolution in vertebrates and insects, genes such as MC1R, bab, or yellow were frequently involved in phenotypic divergence, but there were other genetic routes to similar phenotypic changes. This illustrates that an apparently simple developmental readout (such as melanic pigmentation) can be modified by a variety of mechanisms. Other characters, however, may be governed by genetic regulatory architectures that may have fewer or greater possible genetic...

Conserved developmental functions rebuilding the bilaterian ancestor from phylogenetic inference

Although the conservation of biochemical functionality may simply reflect evolutionary constraint on protein sequence evolution, similarities in the developmental function of homologous genes raise the possibility of deeper evolutionary significance. For example, how should we interpret the striking similarity in the developmental function of Hox genes in regulating regional identities along the A P axis of both mice and Drosophila One possibility is that this similarity is mere coincidence...

The Genetic Toolkit

Animal genomes contain thousands of genes. Many of these genes encode proteins that function in essential processes in all cells in the body (for example, metabolism, biosynthesis of macromolecules) and are often referred to as housekeeping genes. Other genes encode proteins that carry out specialized functions in particular cells or tissues within the body (for example, oxygen transport, immune defense) or, to extend the housekeeping metaphor, in specific rooms in the house. But here we are...

How do Hox domains shift during evolution

Each of the bilaterian phyla that have metameric body organization exhibits relative shifts in Hox boundaries between lineages. These differences evolved through changes in the regulation of Hox gene expression during the radiation of arthropod, vertebrate, and annelid body plans. Mechanistically, shifts in Hox expression domains could result from changes in the expression or activity of the proteins that regulate the expression of Hox genes. Alternatively, changes could evolve within the...

Placodes neural crest and jaws rise of the predators

One of the main vertebrate novelties was the elaboration of the vertebrate head, with an array of sense organs and features involved in an active predatory lifestyle. These structures are derived from two embryonic cell populations, cranial placodes and the neural crest, which arose near the base of the vertebrate lineage. Both of these pluripotent cell types undergo epithelial to mesenchymal transitions and have the capacity to migrate extensively within the body to form a variety of...

Evolution Of Radical Body Plan Changes

Morphological novelty also encompasses the evolution of novel body organizations. Radical reorganization of body plans can include both the appearance of new structures and the loss of ancestral characters. The first two examples we discuss here have lost one or more key morphological features characteristic of their clade. The other examples illustrate the appearance of new characters and rearrangements of older features resulting in dramatically different body organization. As with the other...

Animal Origins And The Fossil Record

The fossil record is our primary window into the history of life. It provides many kinds of information that cannot be inferred from living animals. Fossils give us pictures of extinct forms that may be ancestors of modern animals, provide minimal estimates of the time of origin or divergence of particular groups, reveal episodes of extinctions and radiations, and, in favorable circumstances, offer detailed accounts of the evolution of important structures. The search for the origins of modern...

Evolution Of Animal Color Patterns

Mammalian coat and bird plumage colors Some of the most striking and best understood examples of phenotypic divergence are the color patterns of vertebrates. Mammalian coat, bird plumage, and fish scale coloration schemes are wonderfully diverse. Much progress has been made in understanding the genetic control of color formation and of differences within and between species. The most widespread pigment in the animal kingdom is melanin. It occurs in various chemical forms that when polymerized...

General Features Of Animal Design And Diversity

One of the most outstanding features of animal design, particularly of larger bilaterians, is their construction from repeating structures (or modules). The segments of arthropods and annelids and the vertebrae (and associated processes) of vertebrates are the basic units of body plan organization in these phyla (Fig. 1.5a-c). Similarly, many body parts such as the insect wing (Fig. 1.5d) and the tetrapod hand (Fig. 1.5e) are composed of repeated structures. An important trend in the...

Diversification of insect wing morphology

Four Winged Insect Ancestor

Although all modern winged insects bear two pairs of wings, many structural, functional, and morphological differences exist between forewings and hindwings both within and between species (Fig. 5.12). For example, the wings of dragonflies (Odonata) appear rather similar, but the forewings of beetles (Coleoptera) have been modified into hardened coverings that protect the hindwings. Butterfly (Lepidoptera) forewings and hindwings are often of similar size, but have evolved different shapes and...

Evolution of the notochord

The presence of a notochord unites the chordates and is the distinguishing feature that inspired their name. The notochord is a stiff, axial rod of cells that represents the functional precursor of the vertebral column in basal chordates. It acts as an organizer for the early development of the CNS and adjacent axial mesoderm. Efforts to determine the origin of the notochord have often focused on the urochordates because of their relatively simple body plan and their basal position within the...

What Is Morphological Novelty

The history of evolutionary biology is replete with operational definitions of novelty. For our purposes, which are primarily to understand the developmental and genetic basis of the evolution of novel animal forms and patterns, a novelty is defined as a structure or pattern element, or even an entire body plan, that has a new adaptive function. This chapter focuses on the best examples of morphological novelty for which developmental genetic knowledge has been elucidated. We do not address...

The evolution of insect wings and spider spinneretsfrom ancestral gills

Arthropods were among the first metazoans to live on land. The transition from an aquatic to terrestrial environment required a number of changes in their morphology and physiology. Some of these changes represent the evolution of novel structures that led to adaptive radiations of specific arthropod lineages. For example, the evolution of insect wings is recognized as the key innovation behind the evolutionary success of the insects, which account for more than 75 of all known metazoan...

The origin and evolution of avian feathers

Feather Evolution From Scale

True powered flight has arisen three independent times in vertebrate evolutionary history encompassing changes in the morphology and function of forelimb structures in the pterosaur, bird, and bat lineages. In all three cases, the leading edge of the wing is stabilized by the forelimb skeletal elements. But, in contrast to the pterosaurs and bats, which use a membranous lifting surface, the majority of the flight surface in birds consists of feathers. Feather-like structures have been found on...

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Shrimp Artemia, is quite different from Drosophila and other insects. In Artemia, the anterior boundary of Ubx falls at the anterior of the thorax, at the transition from the gnathal head segments which bear feeding appendages to the thoracic segments which bear swimming appendages . Thus the expression of this Hox gene marks a transition in appendage morphology along the Artemia anteroposterior axis, but Ubx is expressed at a more anterior position relative to the insects. Other, more derived...

The Vertebrate Body Plan

The development of the vertebrate body plan has long been a focus of experimental embryology. Many fundamental concepts such as organizers, fields, and morphogens were derived first from observations of vertebrate embryos. The major features of adult vertebrate morphology, including segmented vertebral columns, paired appendages, and skulls, have undergone considerable evolutionary diversification. Therefore, we will focus on the developmental genetics of these major features here, and consider...

Diversification of Insect Segmental Morphology Wings and Legs

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Fins to limbs paired appendages and the tetrapod hand

Tetrapod Limb

The adaptive evolution of vertebrates capable of surviving in aquatic, terrestrial, and aerial environments involved the acquisition and modification of paired pectoral and pelvic appendages. These limbs boosted vertebrates' maneuverability and speed in water and later were used as the primary means of locomotion on land. The early history of paired Evolution of vertebrate paired appendages Vertebrate paired appendages evolved in a series of steps. Paired appendages are first encountered in the...

Vertebrate Hox Expression Domains and Axial Patterning

Belting H-G, Shashikant CS, Ruddle FH. Modification of expression and cis-regulation of Hoxc8 in the evolution of diverged axial morphology. Proc Natl Acad Sci USA 1998 95 2355-2360. Burke AC, Nelson CE, Morgan BA, Tabin C. Hox genes and the evolution of vertebrate axial morphology. Development 1995 121 333-346. Cohn MJ, Tickle C. Developmental basis of limblessness and axial patterning in snakes. Nature 1999 399 474-479. Gaunt SJ. Evolutionary shifts of vertebrate structures and Hox expression...