Results 1 - 10
of
69
Six major steps in animal evolution: are we derived sponge larvae
- Evol Dev
"... SUMMARY A review of the old and new literature on animal morphology/embryology and molecular studies has led me to the following scenario for the early evolution of themetazoans. The metazoan ancestor, ‘‘choanoblastaea,’ ’ was a pelagic sphere consisting of choanocytes. The evolution ofmulticellular ..."
Abstract
-
Cited by 30 (1 self)
- Add to MetaCart
SUMMARY A review of the old and new literature on animal morphology/embryology and molecular studies has led me to the following scenario for the early evolution of themetazoans. The metazoan ancestor, ‘‘choanoblastaea,’ ’ was a pelagic sphere consisting of choanocytes. The evolution ofmulticellularity enabled division of labor between cells, and an ‘‘advanced choanoblastaea’ ’ consisted of choanocytes and nonfeeding cells. Polarity became established, and an adult, sessile stage developed. Choanocytes of the upper side became arranged in a groove with the cilia pumping water along the groove. Cells overarched the groove so that a choanocyte chamber was formed, establishing the body plan of an adult sponge; the pelagic larval stage was retained but became lecithotrophic. The sponges radiated into monophyletic Silicea, Calcarea, and Homoscleromorpha. Homoscleromorph larvae show cell layers resembling true, sealed epithelia. A homoscleromorph-like larva developed an archenteron, and the sealed epithelium made extracellular digestion possible in this isolated space. This larva became sexually mature, and the adult sponge-stage was abandoned in an extreme progenesis. This eumetazoan ancestor, ‘‘gastraea,’ ’ corresponds to Haeckel’s gastraea. Trichoplax represents this stage, but with the blastopore spread out so that the endoderm has become the underside of the creeping animal. Another lineage developed a nervous system; this ‘‘neurogastraea’ ’ is the ancestor of the Neuralia. Cnidarians have retained this organization, whereas the Triploblastica (Ctenophora1Bilateria), have developed the mesoderm. The bilaterians developed bilaterality in a primitive form in the Acoelomorpha and in an advanced form with tubular gut and long Hox cluster in the Eubilateria (Protostomia1Deuterostomia). It is indicated that the major evolutionary steps are the result of suites of existing genes becoming co-opted into new networks that specify new structures. The evolution of the eumetazoan ancestor from a progenetic homoscleromorph larva implies that we, as well as all the other eumetazoans, are derived sponge larvae.
Evolutionary Patterns of Non-Coding RNAs
, 2005
"... A plethora of new functions of non-coding RNAs have been discovered in past few years. In fact, RNA is emerging as the central player in cellular regulation, taking on active roles in multiple regulatory layers from transcription, RNA maturation, and RNA modification to translational regulation. Ne ..."
Abstract
-
Cited by 22 (7 self)
- Add to MetaCart
A plethora of new functions of non-coding RNAs have been discovered in past few years. In fact, RNA is emerging as the central player in cellular regulation, taking on active roles in multiple regulatory layers from transcription, RNA maturation, and RNA modification to translational regulation. Nevertheless, very little is known about the evolution of this “Modern RNA World ” and its components. In this contribution we attempt to provide at least a cursory overview of the diversity of non-coding RNAs and functional RNA motifs in non-translated regions of regular messenger RNAs (mRNAs) with an emphasis on evolutionary questions. This survey is complemented by an in-depth analysis of examples from different classes of RNAs focusing mostly on their evolution in the vertebrate lineage. We present a survey of Y RNA genes in vertebrates, studies of the molecular evolution of the U7 snRNA, the snoRNAs E1/U17, E2, and E3, the Y RNA family, the let-7 microRNA family, and the mRNA-like evf-1 gene. We furthermore discuss the statistical distribution
Phylogenetic Context and Basal Metazoan Model Systems 1
"... SYNOPSIS. In comparative studies using model organisms, extant taxa are often referred to as basal. The term suggests that such taxa are descendants of lineages that diverged early in the history of some larger taxon. By this usage, the basal metazoans comprise just four phyla (Placozoa, Porifera, C ..."
Abstract
-
Cited by 13 (6 self)
- Add to MetaCart
(Show Context)
SYNOPSIS. In comparative studies using model organisms, extant taxa are often referred to as basal. The term suggests that such taxa are descendants of lineages that diverged early in the history of some larger taxon. By this usage, the basal metazoans comprise just four phyla (Placozoa, Porifera, Cnidaria, and Ctenophora) and the large clade Bilateria. We advise against this practice because basal refers to a region at the base or root of a phylogenetic tree. Thus, referring to an extant taxon or species as basal, or as more basal than another, can be misleading. While much progress has been made toward understanding some of the phylogenetic relationships within these groups, the relationships among them are still largely not known with certainty. Thus, sound inferences from comparative studies of model organisms demand continued illumination of phylogeny. Hypotheses about the mechanisms underlying metazoan evolution can be drawn from the study of model organisms in Cnidaria, Ctenophora, Placozoa, and Porifera, but it is clear that these model organisms are likely to be derived in many respects. Therefore, testing these hypotheses requires the study of yet additional model organisms. The most effective tests are those that investigate model organisms with phylogenetic positions among two sister groups comprising a larger clade of interest.
The segmented Urbilateria: a testable scenario
- Int. Comp. Biol
, 2003
"... SYNOPSIS. The idea that the last common ancestor of bilaterian animals (Urbilateria) was segmented has been raised recently on evidence coming from comparative molecular embryology. Leaving aside the com-plex debate on the value of genetic evidence, the morphological and developmental evidence in fa ..."
Abstract
-
Cited by 12 (0 self)
- Add to MetaCart
(Show Context)
SYNOPSIS. The idea that the last common ancestor of bilaterian animals (Urbilateria) was segmented has been raised recently on evidence coming from comparative molecular embryology. Leaving aside the com-plex debate on the value of genetic evidence, the morphological and developmental evidence in favor of a segmented Urbilateria are discussed in the light of the emerging molecular phylogeny of metazoans. Applying a cladistic character optimization procedure to the question of segmentation is vastly complicated by the problem of defining without ambiguity what segmentation is and to what taxa this definition applies. An ancestral segmentation might have undergone many complex derivations in each different phylum, thus rendering the cladistics approaches problematic. Taking the most general definitions of coelom and seg-mentation however, some remarkably similar patterns are found across the bilaterian tree in the way seg-ments are formed by the posterior addition of mesodermal segments or somites. Postulating that these striking similarities in mesodermal patterns are ancestral, a scenario for the diversification of bilaterians from a metameric ancestor is presented. Several types of evolutionary mechanisms (specialization, tagmosis, progenesis) operating on a segmented ancestral body plan would explain the rapid emergence of body plans during the Cambrian. We finally propose to test this hypothesis by comparing genes involved in mesodermal segmentation.
Segmentation: mono- or polyphyletic
- International Journal of Developmental Biology
, 2003
"... ABSTRACT Understanding the evolutionary origins of segmented body plans in the metazoa has been a long-standing fascination for scientists. Competing hypotheses explaining the presence of distinct segmented taxa range from the suggestion that all segmentation in the metazoa is homolo-gous to the pro ..."
Abstract
-
Cited by 7 (0 self)
- Add to MetaCart
ABSTRACT Understanding the evolutionary origins of segmented body plans in the metazoa has been a long-standing fascination for scientists. Competing hypotheses explaining the presence of distinct segmented taxa range from the suggestion that all segmentation in the metazoa is homolo-gous to the proposal that segmentation arose independently many times, even within an individual clade or species. A major new source of information regarding the extent of homology vs. homoplasy of segmentation in recent years has been an examination of the extent to which molecular mecha-nisms underlying the segmentation process are conserved, the rationale being that a shared history will be apparent by the presence of common molecular components of a developmental program that give rise to a segmented body plan. There has been substantial progress recently in understanding the molecular mechanisms underlying the segmentation process in many groups, specifically within the three overtly segmented phyla: Annelida, Arthropoda and Chordata. This review will discuss what we currently know about the segmentation process in each group and how our understanding of the development of segmented structures in distinct taxa have influenced the hypotheses explaining the presence of a segmented body plan in the metazoa.
The tommotiid Camenella reticulosa from the early Cambrian of South Australia: Morphology, scleritome reconstruction, and phylogeny
, 2009
"... The tommotiid Camenella reticulosa is redescribed based on new collections of well preserved sclerites from the ..."
Abstract
-
Cited by 7 (1 self)
- Add to MetaCart
(Show Context)
The tommotiid Camenella reticulosa is redescribed based on new collections of well preserved sclerites from the
Nematode small subunit phylogeny correlates with alignment parameters
- Syst. Biol. 2006
"... Abstract.—The number of nuclear small subunit (SSU) ribosomal RNA (rRNA) sequences for Nematoda has increased dra-matically in recent years, and although their use in constructing phylogenies has also increased, relatively little attention has been given to their alignment. Here we examined the sens ..."
Abstract
-
Cited by 6 (1 self)
- Add to MetaCart
(Show Context)
Abstract.—The number of nuclear small subunit (SSU) ribosomal RNA (rRNA) sequences for Nematoda has increased dra-matically in recent years, and although their use in constructing phylogenies has also increased, relatively little attention has been given to their alignment. Here we examined the sensitivity of the nematode SSU data set to different alignment parame-ters and to the removal of alignment ambiguous regions. Ten alignments were created with CLUSTAL W using different sets of alignment parameters (10 full alignments), and each alignment was examined by eye and alignment ambiguous regions were removed (creating 10 reduced alignments). These alignment ambiguous regions were analyzed as a third type of data set, culled alignments. Maximum parsimony, neighbor-joining, and parsimony bootstrap analyses were performed. The resulting phylogenies were compared to each other by the symmetric difference distance tree comparison metric (SymD). The correlation of the phylogenies with the alignment parameters was tested by comparing matrices from SymD with corre-sponding matrices of Manhattan distances representing the alignment parameters. Differences among individual parsimony trees from the full alignments were frequently correlated with the differences among alignment parameters (580/1000 tests), as were trees from the culled alignments (403/1000 tests). Differences among individual parsimony trees from the reduced alignments were less frequently correlated with the differences among alignment parameters (230/1000 tests). Differences among majority-rule consensus trees (50%) from the parsimony analysis of the full alignments were significantly correlated with the differences among alignment parameters, whereas consensus trees from the reduced and culled analyses were not
Global regulation of Hox gene expression in C. elegans by a SAM domain protein
- Dev. Cell
, 2003
"... remodeling is not blocked, positive factors are present in many cells that will initiate transcription outside the normal expression domains. ..."
Abstract
-
Cited by 5 (1 self)
- Add to MetaCart
(Show Context)
remodeling is not blocked, positive factors are present in many cells that will initiate transcription outside the normal expression domains.
Trilobite tagmosis and body patterning from morphological and developmental perspectives
- Integr. Comp. Biol
"... SYNOPSIS. The Trilobita were characterized by a cephalic region in which the biomineralized exoskeleton showed relatively high morphological differentiation among a taxonomically stable set of well defined seg-ments, and an ontogenetically and taxonomically dynamic trunk region in which both exoskel ..."
Abstract
-
Cited by 5 (1 self)
- Add to MetaCart
(Show Context)
SYNOPSIS. The Trilobita were characterized by a cephalic region in which the biomineralized exoskeleton showed relatively high morphological differentiation among a taxonomically stable set of well defined seg-ments, and an ontogenetically and taxonomically dynamic trunk region in which both exoskeletal segments and ventral appendages were similar in overall form. Ventral appendages were homonomous biramous limbs throughout both the cephalon and trunk, except for the most anterior appendage pair that was antenniform, preoral, and uniramous, and a posteriormost pair of antenniform cerci, known only in one species. In some clades trunk exoskeletal segments were divided into two batches. In some, but not all, of these clades the boundary between batches coincided with the boundary between the thorax and the adult pygidium. The repeated differentiation of the trunk into two batches of segments from the homonomous trunk condition indicates an evolutionary trend in aspects of body patterning regulation that was achieved independently in several trilobite clades. The phylogenetic placement of trilobites and congruence of broad patterns of tag-mosis with those seen among extant arthropods suggest that the expression domains of trilobite cephalic Hox genes may have overlapped in a manner similar to that seen among extant arachnates. This, coupled with the fact that trilobites likely possessed ten Hox genes, presents one alternative to a recent model in which Hox gene distribution in trilobites was equated to eight putative divisions of the trilobite body plan.
On the origin of the chordate central nervous system: expression of onecut in the sea urchin embryo, Evol
- Development
, 2004
"... the sea urchin embryo ..."
(Show Context)
