The nucleotide sequence of blue-green algae phenylalanine-tRNA and the evolutionary origin of chloroplasts.

Phenylalanine tRNA from the blue-green alga, Agmenellum quadruplicatum, has been purified to homogeneity. The nucleotide sequence of this tRNA was determined to be: (see tests) Comparisons of the sequence and the modified nucleosides of this tRNA with those of other tRNAPhes thus far sequenced, indicate that this blue green algal tRNAPhe is typically prokaryotic and closely resembles the chloroplast tRNAPhes of higher plants and Euglena. The significance of this observation to the evolutionary origin of chloroplasts is discussed.     

The evolutionary origin and diversification of feathers

Progress on the evolutionary origin and diversification of feathers has been hampered by conceptual problems and by the lack of plesiomorphic feather fossils. Recently, both of these limitations have been overcome by the proposal of the developmental theory of the origin of feathers, and the discovery of primitive feather fossils on nonavian theropod dinosaurs. The conceptual problems of previous theories of the origin of feathers are reviewed, and the alternative developmental theory is presented and discussed. The developmental theory proposes that feathers evolved through a series of evolutionary novelties in developmental mechanisms of the follicle and feather germ. The discovery of primitive and derived fossil feathers on a diversity of coelurosaurian theropod dinosaurs documents that feathers evolved and diversified in nonavian theropods before the origin of birds and before the origin of flight. The morphologies of these primitive feathers are congruent with the predictions of the developmental theory. Alternatives to the theropod origin of feathers are critique and rejected. Hypotheses for the initial function of feathers are reviewed. The aerodynamic theory of feather origins is falsified, but many other functions remain developmentally and phylogenetically plausible. Whatever their function, feathers evolved by selection for a follicle that would grow an emergent tubular appendage. Feathers are inherently tubular structures. The homology of feathers and scales is weakly supported. Feathers are composed of a suite of evolutionary novelties that evolved by the duplication, hierarchical organization, interaction, dissociation, and differentiation of morphological modules. The unique capacity for modular subdivision of the tubular feather follicle and germ has fostered the evolution of numerous innovations that characterize feathers. The evolution of feather keratin and the molecular basis of feather development are also discussed.     

The evolutionary origin of feathers.

Previous theories relating the origin of feathers to flight or to heat conservation are considered to be inadequate. There is need for a model of feather evolution that gives attention to the function and adaptive advantage of intermediate structures. The present model attempts to reveal and to deal with, the spectrum of complex questions that must be considered. In several genera of modern lizards, scales are elongated in warm climates. It is argued that these scales act as small shields to solar radiation. Experiments are reported that tend to confirm this. Using lizards as a conceptual model, it is argued that feathers likewise arose as adaptations to intense solar radiation. Elongated scales are assumed to have subdivided into finely branched structures that produced a heat-shield, flexible as well as long and broad. Associated muscles had the function of allowing the organism fine control over rates of heat gain and loss: the specialized scales or early feathers could be moved to allow basking in cool weather or protection in hot weather. Subdivision of the scales also allowed a close fit between the elements of the insulative integument. There would have been mechanical and thermal advantages to having branches that interlocked into a pennaceous structure early in evolution, so the first feathers may have been pennaceous. A versatile insulation of movable, branched scales would have been a preadaptation for endothermy. As birds took to the air they faced cooling problems despite their insulative covering because of high convective heat loss. Short glides may have initially been advantageous in cooling an animal under heat stress, but at some point the problem may have shifted from one of heat exclusion to one of heat retention. Endothermy probably evolved in conjunction with flight. If so, it is an unnecessary assumption to postulate that the climate cooled and made endothermy advantageous. The development of feathers is complex and a model is proposed that gives attention to the fundamental problems of deriving a branched structure with a cylindrical base from an elongated scale.     

The evolutionary origin of orphan genes

Gene evolution has long been thought to be primarily driven by duplication and rearrangement mechanisms. However, every evolutionary lineage harbours orphan genes that lack homologues in other lineages and whose evolutionary origin is only poorly understood. Orphan genes might arise from duplication and rearrangement processes followed by fast divergence; however, de novo evolution out of non-coding genomic regions is emerging as an important additional mechanism. This process appears to provide raw material continuously for the evolution of new gene functions, which can become relevant for lineage-specific adaptations.     

The evolutionary origin of peroxisomes: an ER-peroxisome connection

The peroxisome is an essential eukaryotic organelle, crucial for lipid metabolism and free radical detoxification, development, differentiation, and morphogenesis from yeasts to humans. Loss of peroxisomes invariably leads to fatal peroxisome biogenesis disorders in man. The evolutionary origin of peroxisomes remains unsolved; proposals for either a symbiogenetic or cellular membrane invagination event are unconclusive. To address this question, we have probed with a peroxisomal proteome, an "ensemble" of 19 representative eukaryotic complete genomes. Molecular phylogenetic and sequence comparison tools allowed us to identify four proteins as peroxisomal markers for unequivocal in silico peroxisome detection. We have then detected the Apicomplexa phylum as the first group of organisms devoid of peroxisomes, in the presence of mitochondria. Finally, we deliver evidence against a prokaryotic ancestor of peroxisomes: (1) the peroxisomal membrane is composed of purely eukaryotic bricks and is thus useful to trace the eukaryotes in their evolutionary paths and (2) the peroxisomal matrix protein import system shares mechanistic similarities with the endoplasmic reticulum/proteasome degradation process, indicating a common evolutionary history.     

The origin and early evolutionary history of amniotes

Recent phylogenetic analyses of Paleozoic tetrapods have yielded startling new insights into the origin and early evolutionary history of amniotes. The origin of this successful group involves evolutionary innovations that are associated with the development of the cleidoic egg and related reproductive strategies, and are therefore not represented directly in the fossil record. Despite this obvious difficulty, recent studies have been able to distinguish Paleozoic amniotes from their anamniotic tetrapod relatives to determine major patterns of interrelationships.     

The evolutionary origin of animal cellulose synthase

Urochordates are the only animals that produce cellulose, a polysaccharide existing primarily in the extracellular matrices of plant, algal, and bacterial cells. Here we report a Ciona intestinalis homolog of cellulose synthase, which is the core catalytic subunit of multi-enzyme complexes where cellulose biosynthesis occurs. The Ciona cellulose synthase gene, Ci-CesA, is a fusion of a cellulose synthase domain and a cellulase (cellulose-hydrolyzing enzyme) domain. Both the domains have no animal homologs in public databases. Exploiting this fusion of atypical genes, we provided evidence of a likely lateral transfer of a bacterial cellulose synthase gene into the urochordate lineage. According to fossil records, this likely lateral acquisition of the cellulose synthase gene may have occurred in the last common ancestor of extant urochordates more than 530 million years ago. Whole-mount in situ hybridization analysis revealed the expression of Ci-CesA in C. intestinalis embryos, and the expression pattern of Ci-CesA was spatiotemporally consistent with observed cellulose synthesis in vivo. We propose here that urochordates may use a laterally acquired homologous gene for an analogous process of cellulose synthesis.Electronic Supplementary Material Supplementary material is available in the online version of this article at Edited by D. Tautz     

The evolutionary gain of spliceosomal introns: sequence and phase preferences

Theories regarding the evolution of spliceosomal introns differ in the extent to which the distribution of introns reflects either a formative role in the evolution of protein-coding genes or the adventitious gain of genetic elements. Here, systematic methods are used to assess the causes of the present-day distribution of introns in 10 families of eukaryotic protein-coding genes comprising 1,868 introns in 488 distinct alignment positions. The history of intron evolution inferred using a probabilistic model that allows ancestral inheritance of introns, gain of introns, and loss of introns reveals that the vast majority of introns in these eukaryotic gene families were not inherited from the most recent common ancestral genes, but were gained subsequently. Furthermore, among inferred events of intron gain that meet strict criteria of reliability, the distribution of sites of gain with respect to reading-frame phase shows a 5:3:2 ratio of phases 0, 1 and 2, respectively, and exhibits a nucleotide preference for MAG GT (positions -3 to +2 relative to the site of gain). The nucleotide preferences of intron gain may prove to be the ultimate cause for the phase bias. The phase bias of intron gain is sufficient to account quantitatively for the well-known 5:3:2 bias in phase frequencies among extant introns, a conclusion that holds even when taxonomic heterogeneity in phase patterns is considered. Thus, intron gain accounts for the vast majority of extant introns and for the bias toward phase 0 introns that previously was interpreted as evidence for ancient formative introns.     

The evolutionary reorganization of ontogeny and origin of multicellularity

The formation of morphogenetic mechanisms during emergence of multicellularity is discussed in this article.     

The evolutionary origin of chordate segmentation: revisiting the enterocoel theory

One of the definitive characteristics of chordates (cephalochordates, vertebrates) is the somites, which are a series of paraxial mesodermal blocks exhibiting segmentation. The presence of somites in the basal chordate amphioxus and in vertebrates, but not in tunicates (the sister group of vertebrates), suggests that the tunicates lost the somites secondarily. Somites are patterned from anterior to posterior during embryogenesis. How such a segmental pattern evolved from deuterostome ancestors is mysterious. The classic enterocoel theory claims that chordate mesoderm evolved from the ancestral deuterostome mesoderm that organizes the trimeric body parts seen in extant hemichordates. Recent progress in molecular embryology has been tremendous, which has enabled us to test this classic theory. In this review, the history of the study on the evolution of the chordate mesoderm is summarized. This is followed by a review of the current understanding of genetic mapping on anterior/posterior (A/P) mesodermal patterning between chordates (cephalochordates, vertebrates) and a direct developing hemichordate (Saccoglossus kowalevskii). Finally, a possible scenario about the evolution of the chordate mesoderm from deuterostome ancestors is discussed.     

The evolutionary origin of eukaryotic transmembrane signal transduction

1. A comparison was made of transmembrane signal transduction mechanisms in different eukaryotes and prokaryotes. 2. Much attention was given to eukaryotic microbes and their signal transduction mechanisms, since these organisms are intermediate in complexity between animals, plants and bacteria. 3. Signal transduction mechanisms in eukaryotic microbes, however, do not appear to be intermediate between those in animals, plants and bacteria, but show features characteristic of the higher eukaryotes. 4. These similarities include the regulation of receptor function, adenylate cyclase activity, the presence of a phosphatidylinositol cycle and of GTP-binding regulatory proteins. 5. It is proposed that the signal transduction systems known to operate in present-day eukaryotes evolved in the earliest eukaryotic cells.     

The evolutionary origin of proinsulin: Amino acid sequence homology with the trypsin-related serine proteases detected and evaluated by new statistical methods

Proinsulins and pancreatic serine proteases were analyzed for possible amino acid sequence similarity, using an adapted version of the nucleotide sequence alignment technique of Sankoff (1972). The technique allowed us to determine simultaneously the statistical significance of both the sequence alignment and the number of gaps necessary to achieve that alignment. In the course of this work, it was realized that a rigorous analysis required non-parametric statistics.For the B-chain (amino-terminal) of insulin a highly significant gap-free sequence alignment with the serine proteases was found. For the A-chain (carboxy-terminal) of insulin a sequence alignment of modest statistical significance with two gaps could be obtained, while the search for a corresponding alignment for the C-peptide remained unsuccessful. Presumably the rapid evolution of the C-peptide has obscured its origin. Reconstruction of ancestral sequences was of no help. In contrast to the amino acid sequences, three-dimensional structures of the two protein families are quite different.Considering current histophysiological understanding of ontogeny and phylogeny of exocrine and endocrine pancreas, the observed sequence similarity of proinsulins and serine proteases was interpreted to mean that the two protein families have diverged from a common genetic ancestor. Moreoever, from the organismic distribution of these proteins it was concluded that at least one serine protease existed first, and that proinsulin was generated after duplication of a serine protease gene and subsequent drastic modification, such as a large deletion. Thus proinsulin, basically an anabolic hormone, is derived from a serine protease, an enzyme involved in digestion. This constitutes a refinement of a similar proposal by Steiner et al. (1973).The emergence of proinsulin seems to have occurred after coelenterates diverged, and possibly before most other major animal phyla diverged from the line leading to vertebrates, i.e. 520 to 700 million years ago. The evolution of proinsulin seems to have paralleled the evolution of endocrine cells. Homology of the secreted products of endocrine and exocrine cells was most readily reconciled with a common embryological and phylogenetic origin of the two cell types, as considered by Pictet & Rutter (1972).     

The evolutionary origin of nodal-related genes in teleosts

Because of an extra whole-genome duplication, zebrafish and other teleosts have two copies of genes that are present in a single copy in tetrapod genomes. Some zebrafish genes, however, are present in triplicate. For example, the nodal-related genes encode secreted proteins of the transforming growth factor beta superfamily that are required in all vertebrates to induce the mesoderm and endoderm, pattern all three germ layers, and establish the left-right axis. Zebrafish have three nodal-related genes, called ndr1/squint, ndr2/cyclops, and ndr3/southpaw. As part of an analysis of enhancer elements controlling zebrafish nodal-related gene expression, we analyzed the nodal loci in the sequenced genomes of five teleost species and four tetrapod species. Each teleost genome contains three nodal-related genes, indicating that squint, cyclops, and southpaw orthologues were present early in the teleost lineage. The genes flanking the nodal-related genes are also conserved, demonstrating a high degree of conserved synteny. Although we found little homology outside of the coding sequences in this region, pufferfish enhancer sequences work in zebrafish embryos to drive reporter gene expression in the squint expression pattern. This indicates a high degree of functional conservation of enhancer elements within the teleosts. We conclude that the ancestral squint and cyclops genes arose during the teleost-specific whole-genome duplication event and that southpaw emerged from a subsequent duplication event involving ancestral squint.