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M Q Martindale

Publications and source records attributed to M Q Martindale.

At least 19 recordsLinked to original sources
PubMed2001

Multiple inductive signals are involved in the development of the ctenophore Mnemiopsis leidyi.

Ctenophores possess eight longitudinally arrayed rows of comb plate cilia. Previous intracellular cell lineage analysis has shown that these comb rows are derived from two embryonic lineages, both daughters of the four e(1) micromeres (e(11) and e(12)) and a single daughter of the four m(1) micromeres (the m(12) micromeres). Although isolated e(1) micromeres will spontaneously generate comb plates, cell deletion experiments have shown that no comb plates appear during embryogenesis following the removal of e(1) descendents. Thus, the m(1) lineage requires the inductive interaction of the e(1) lineage to contribute to comb plate formation. Here we show that, although m(12) cells are normally the only m(1) derivatives to contribute to comb plate formation, m(11) cells are capable of generating comb plates in the absence m(12) cells. The reason that m(11) cells do not normally make comb rows may be attributable either to their more remote location relative to critical signaling centers (e.g., e(1) descendants) or to inhibitory signals that may be provided by other nearby cells such as sister cells m(12). In addition, we show that the signals provided by the e(1) lineage are not sufficient for m(1)-derived comb plate formation. Signals provided by endomesodermal progeny of either the E or the M lineages (the 3E or 2M macromeres) are also required.

Animals↗
PubMed2001

The spatial and temporal expression of Ch-en, the engrailed gene in the polychaete Chaetopterus, does not support a role in body axis segmentation.

We are interested in understanding whether the annelids and arthropods shared a common segmented ancestor and have approached this question by characterizing the expression pattern of the segment polarity gene engrailed (en) in a basal annelid, the polychaete Chaetopterus. We have isolated an en gene, Ch-en, from a Chaetopterus cDNA library. Genomic Southern blotting suggests that this is the only en class gene in this animal. The predicted protein sequence of the 1.2-kb cDNA clone contains all five domains characteristic of en proteins in other taxa, including the en class homeobox. Whole-mount in situ hybridization reveals that Ch-en is expressed throughout larval life in a complex spatial and temporal pattern. The Ch-en transcript is initially detected in a small number of neurons associated with the apical organ and in the posterior portion of the prototrochophore. At later stages, Ch-en is expressed in distinct patterns in the three segmented body regions (A, B, and C) of Chaetopterus. In all segments, Ch-en is expressed in a small set of segmentally iterated cells in the CNS. In the A region, Ch-en is also expressed in a small group of mesodermal cells at the base of the chaetal sacs. In the B region, Ch-en is initially expressed broadly in the mesoderm that then resolves into one band/segment coincident with morphological segmentation. The mesodermal expression in the B region is located in the anterior region of each segment, as defined by the position of ganglia in the ventral nerve cord, and is involved in the morphogenesis of segment-specific feeding structures late in larval life. We observe banded mesodermal and ectodermal staining in an anterior-posterior sequence in the C region. We do not observe a segment polarity pattern of expression of Ch-en in the ectoderm, as is observed in arthropods.

Amino Acid Sequence↗
PubMed2000

Regulation and regeneration in the ctenophore Mnemiopsis leidyi.

Lobate ctenophores (tentaculates) generally exhibit a remarkable ability to regenerate missing structures as adults. On the other hand, their embryos exhibit a highly mosaic behavior when cut into halves or when specific cells are ablated. These deficient embryos do not exhibit embryonic regulation, and generate incomplete adult body plans. Under certain conditions, however, these deficient animals are subsequently able to replace the missing structures during the adult phase in a process referred to as "post-regeneration." We have determined that successful post-regeneration can be predicted on the basis of a modified polar coordinate model, and the rules of intercalary regeneration, as defined by French et al. (V. French, P. J. Bryant, and S. V. Bryant, 1976, Science 193, 969-981.) The model makes certain assumptions about the organization of the ctenophore body plan that fit well with what we have determined on the basis of cell lineage fates maps, and their twofold rotational ("biradial") symmetry. The results suggest that cells composing the ctenophore adult body plan possess positional information, which is utilized to reconstruct the adult body plan. More specifically, we have found that the progeny of three specific cell lineages are required to support post-regeneration of the comb rows (the e(1), e(2), and m(1) micromeres). Furthermore, post-regeneration of the comb rows involves a suite of cell-cell inductive interactions, which are similar to those that take place during their embryonic formation. The significance of these findings is discussed in terms of the organization of the ctenophore body plan, and the mechanisms involved in cell fate specification. This situation is also contrasted with that of the atentaculate ctenophores, which are unable to undergo post-regeneration.

Animals↗
PubMed2000

Conservation of the sequence and temporal expression of let-7 heterochronic regulatory RNA.

Two small RNAs regulate the timing of Caenorhabditis elegans development. Transition from the first to the second larval stage fates requires the 22-nucleotide lin-4 RNA, and transition from late larval to adult cell fates requires the 21-nucleotide let-7 RNA. The lin-4 and let-7 RNA genes are not homologous to each other, but are each complementary to sequences in the 3' untranslated regions of a set of protein-coding target genes that are normally negatively regulated by the RNAs. Here we have detected let-7 RNAs of approximately 21 nucleotides in samples from a wide range of animal species, including vertebrate, ascidian, hemichordate, mollusc, annelid and arthropod, but not in RNAs from several cnidarian and poriferan species, Saccharomyces cerevisiae, Escherichia coli or Arabidopsis. We did not detect lin-4 RNA in these species. We found that let-7 temporal regulation is also conserved: let-7 RNA expression is first detected at late larval stages in C. elegans and Drosophila, at 48 hours after fertilization in zebrafish, and in adult stages of annelids and molluscs. The let-7 regulatory RNA may control late temporal transitions during development across animal phylogeny.

Adult↗
PubMed2000

Combined-method phylogenetic analysis of Hox and ParaHox genes of the metazoa.

The clustered Hox genes show a conserved role in patterning the body axis of bilaterian metazoans. Increasingly, a broader phylogenetic sampling of non-model system organisms is being examined to detect a correlation, if any, between Hox gene evolution, and body plan innovations. To assess how Hox gene expression and function evolve with changing cluster arrangements, we must be able to reliably assign gene orthologies between Hox genes. Recent evidence suggests that a four-gene proto-Hox cluster duplicated to form the precursor of the present cluster and an additional sister-cluster, the ParaHox group. Here, phylogenetic methods are used to determine Hox-gene orthologies and to infer probable clustering events leading to the current bilaterian Hox complement. This analysis supports the ParaHox hypothesis and gives first confirmation that ind (intermediate neuroblasts defective) is an anterior ParaHox ortholog from protostomes. This analysis supports a proto-Hox cluster of four genes in which the central-class member of the ParaHox cluster may have been lost. It is also proposed here that ancestral diploblasts had central-class members of both Hox and ParaHox clusters. Primitive Hox gene ancestors are estimated by phylogenetic methods and found to have no strong affinity to any particular class of extant Hox members.

Amino Acid Sequence↗
PubMed2000

The unique developmental program of the acoel flatworm, Neochildia fusca.

Acoel embryos exhibit a unique form of development that some investigators argue is related to that found in polyclad turbellarians and coelomate spiralians, which display typical quartet spiral cleavage. We generated the first cell-lineage fate map for an acoel flatworm, Neochildia fusca, using modern intracellular lineage tracers to assess the degree of similarity between these distinct developmental programs. N. fusca develops via a "duet" cleavage pattern in which second cleavage occurs in a leiotropically oblique plane relative to the animal-vegetal axis. At the four-cell stage, the plane of first cleavage corresponds to the plane of bilateral symmetry. All remaining cleavages are symmetrical across the sagittal plane. No ectomesoderm is formed; the first three micromere duets generate only ectodermal derivatives. Endomesoderm, including the complex assemblage of circular, longitudinal, and oblique muscle fibers, as well as the peripheral and central parenchyma, is generated by both third duet macromeres. The cleavage pattern, fate map, and origins of mesoderm in N. fusca share little similarity to that exhibited by other spiralians, including the Platyhelminthes (e.g., polyclad turbellarians). These findings are considered in light of the possible evolutionary origins of the acoel duet cleavage program versus the more typical quartet spiral cleavage program. Finally, an understanding of the cell-lineage fate map allows us to interpret the results of earlier cell deletion studies examining the specification of cell fates within these embryos and reveals the existence of cell-cell inductive interactions in these embryos.

Animals↗
PubMed2000

Expression patterns of anterior Hox genes in the polychaete Chaetopterus: correlation with morphological boundaries.

Expression patterns for five Hox genes were examined by whole-mount in situ hybridization in larvae of Chaetopterus, a polychaete annelid with a tagmatized axial body plan. Phylogenetic analysis demonstrates that these genes are orthologs of the Drosophila genes labial, proboscipedia, zen, Deformed, and Sex combs reduced and are termed CH-Hox1, CH-Hox2, CH-Hox3, CH-Hox4, and CH-Hox5, respectively. Expression studies reveal a biphasic expression pattern. In early larval stages, well before any indications of segmental organization exist, a novel pattern of expression in bilateral posterior proliferating cell populations, corresponding to the teloblasts, was detected for each of the genes, with CH-Hox1 and CH-Hox2 expressed before the remaining three. In middle larval stages, all five genes are expressed in bilateral strips along the ventral midline, corresponding with the developing ventral nerve cord. In addition, CH-Hox1 and CH-Hox2 show strong expression at the foregut-midgut boundary. By late larval stages the expression is generally confined to the ventral CNS and ectoderm of the anterior parapodia. Anterior boundaries of expression are "colinear," at later larval stages, with CH-Hox2 expressed most rostrally, in the first segment, and anterior expression boundaries for CH-Hox1, CH-Hox3, CH-Hox4, and CH-Hox5 in segments 2, 3, 4, and 5, respectively. Like vertebrates and spiders, but unlike insects, CH-Hox3 participates in this colinear axial expression pattern. CH-Hox1 and CH-Hox2 have distinct posterior boundaries of expression in the ninth segment, which corresponds to a major morphological boundary, and may reflect a reorganization of Hox gene regulation related to the evolutionary reorganization of the Chaetopterus body plan.

Amino Acid Sequence↗
PubMed1999

Intracellular fate mapping in a basal metazoan, the ctenophore Mnemiopsis leidyi, reveals the origins of mesoderm and the existence of indeterminate cell lineages.

Ctenophores are marine invertebrates that develop rapidly and directly into juvenile adults. They are likely to be the simplest metazoans possessing definitive muscle cells and are possibly the sister group to the Bilateria. All ctenophore embryos display a highly stereotyped, phylum-specific pattern of development in which every cell can be identified by its lineage history. We generated a cell lineage fate map for Mnemiopsis leidyi by injecting fluorescent lineage tracers into individual blastomeres up through the 60-cell stage. The adult ctenophore body plan is composed of four nearly identical quadrants organized along the oral-aboral axis. Each of the four quadrants is derived largely from one cell of the four-cell-stage embryo. At the eight-cell stage each quadrant contains a single E ("end") and M ("middle") blastomere. Subsequently, micromeres are formed first at the aboral pole and later at the oral pole. The ctene rows, apical organ, and tentacle apparatus are complex structures that are generated by both E and M blastomere lineages from all four quadrants. All muscle cells are derived from micromeres born at the oral pole of endomesodermal precursors (2M and 3E macromeres). While the development of the four quadrants is similar, diagonally opposed quadrants share more similarities than adjacent quadrants. Adult ctenophores possess two diagonally opposed endodermal anal canals that open at the base of the apical organ. These two structures are derived from the two diagonally opposed 2M/ macromeres. The two opposing 2M/ macromeres generated a unique set of circumpharyngeal muscle cells, but do not contribute to the anal canals. No other lineages displayed such diagonal asymmetries. Clones from each blastomere yielded regular, but not completely invariant patterns of descendents. Ectodermal descendents normally, but not always, remained within their corresponding quadrants. On the other hand, endodermal and mesodermal progeny dispersed throughout the body. The variability in the exact complements of adult structures, along with previously published cell deletion experiments, demonstrates that cell interactions are required for normal cell fate determination. Ctenophore embryos, like those of many bilaterian phyla (e.g., spiralians, nematodes, and echinoids), display a highly stereotyped cleavage program in which some, but not all, blastomeres are determined at the time of their birth. The results suggest that mesodermal tissues originally evolved from endoderm tissue.

Animals↗
PubMed1999

Larval ontogenetic stages of Chaetopterus: developmental heterochrony in the evolution of chaetopterid polychaetes.

Seven post-gastrulation larval stages are described for the sedentary polychaete Chaetopterus. Analysis of larval anatomy and morphology through ontogeny reveals significant differences in the temporal sequence of segmentation, and in the character of segments formed, from the typical embryological pattern described for other polychaete families, such as nereidids or spionids. When compared in alternative phylogenetic schemes, these differences represent significant developmental heterochrony, among other evolutionary transitions, which has arisen in the chaetopterid lineage. The heterochrony is correlated with the extreme morphological regionalization along the anterior-posterior body axis, a feature that is also characteristic of chaetopterids.

Animals↗
PubMed1998

The cell lineage of a polyclad turbellarian embryo reveals close similarity to coelomate spiralians.

Recent molecular evidence suggests the turbellarian Platyhelminthes may represent the extant basal members of the Spiralia and therefore probably exhibit ancient features of the spiralian developmental program. The stereotypic quartet spiral cleavage pattern of the polyclad turbellarian embryo, among other features, indicates that this group may be closely related to the ancestral flatworm; however, polyclad embryos have been the subject of few experimental studies. Here we report the results of a cell lineage analysis of the embryo of the polyclad Hoploplana inquilina based on microinjection of DiI into cleavage-stage blastomeres following formation of each of the four quartets of micromeres. The first quartet gives rise to most of the lateral and anterior ectoderm of the Müller's larva; the second quartet forms largely dorsal and ventral ectoderm as well as the circular muscles; the third quartet forms only small clones of ectoderm; and only the 4d cell of the fourth quartet contributes to larval structure, forming the longitudinal muscles, mesenchyme, and probably endoderm. Our results demonstrate a striking similarity between the cell lineages of polyclad and higher spiralian embryos, in which the four quadrants also bear the same relationships to the larval axes and give rise to comparable larval structures, including derivation of mesoderm from both ectodermal (2b) and endodermal precursors (4d).

Animals↗
PubMed1998

Conservation of the spiralian developmental program: cell lineage of the nemertean, Cerebratulus lacteus.

Lineage tracers were injected into individual blastomeres in embryos of the indirect-developing nemertean Cerebratulus lacteus through the formation of the fourth quartet of micromeres. Subsequent development was followed to the formation of feeding pilidium larvae to establish their ultimate fates. Results showed that these blastomeres have unique fates, and their clones give rise to highly predictable regions of the larval body. As in other spiralians, four discrete cell quadrants can be identified. For the most part, their identities are homologous to the typical spiralian A, B, C, and D cell quadrants. In some respects their fates differ from the typical spiralian fate map; however, these can be understood in terms of simple modifications of the early cleavage program. Unlike most spiralians, the first quartet micromeres in the eight-celled embryo are larger than the corresponding vegetal macromeres, and generate most of the larval ectoderm. All four of these micromeres contribute to the apical organ and generate four bilaterally situated domains of ectoderm, where the progeny of the 1a and 1d micromeres lie to the left of the median plane while those of 1b and 1c lie to the right. Unlike the progeny of the first quartet, those of the second quartet are situated in left (2a), ventral (2b), right (2c), and dorsal (2d) positions. The third quartet micromeres generate clones situated in a bilaterally symmetrical fashion similar to those of the first quartet. The alternating axial relationships exhibited by successive micromere quartets are a characteristic of spiralian development. Unlike other spiralian larvae possessing a ciliary band, the pilidium larval ciliary band is formed by all blastomeres of the first and second micromere quartets, as well as 3c and 3d. Ectomesoderm is derived from two blastomeres (3a and 3b), which give rise to the extensive array of the larval muscle cells. C. lacteus also possesses a true mesentoblast (4d) which gives rise to a pair of mesodermal bandlets, and scattered mesenchymal cells. The dual origin of the mesoderm, as both ectomesoderm and endomesoderm, appears to be a condition present in all spiralians. The gut is formed by all the fourth quartet micromeres as well as the vegetal macromeres (4A, 4B, 4C, 4D). Despite differences in the determination of axial properties and some modifications in quadrant fates, nemerteans appear to be constructed on the typical spiralian developmental platform.

Animals↗
PubMed1998

The evolution of the Hox cluster: insights from outgroups.

Two burgeoning research trends are helping to reconstruct the evolution of the Hox cluster with greater detail and clarity. First, Hox genes are being studied in a broader phylogenetic sampling of taxa: the past year has witnessed important new data from teleost fishes, onychophorans, myriapods, polychaetes, glossiphoniid leeches, ribbon worms, and sea anemones. Second, commonly accepted notions of animal relationships are being challenged by alternative phylogenetic hypotheses that are causing us to rethink the evolutionary relationships of important metazoan lineages, especially arthropods, annelids, nematodes, and platyhelminthes.

Animals↗
PubMed1997

Conserved anterior boundaries of Hox gene expression in the central nervous system of the leech Helobdella.

Molecular developmental studies of fly and mouse embryos have shown that the identity of individual body segments is controlled by a suite of homeobox-containing genes called the Hox cluster. To examine the conservation of this patterning mechanism in other segmented phyla, we here describe four Hox gene homologs isolated from glossiphoniid leeches of the genus Helobdella. Based on sequence similarity and phylogenetic analysis, the leech genes Lox7, Lox6, Lox20, and Lox5 are deemed to be orthologs of the Drosophila genes lab, Dfd, Scr, and Antp, respectively. Sequence similarities between Lox5 and Antp outside the homeodomain and phylogenetic reconstructions suggest that the Antennapedia family of Hox genes (as defined by Bürglin, 1994) had already expanded to include at least two discrete Antp and Ubx/abdA precursors prior to the annelid/arthropod divergence. In situ hybridization reveals that the four Lox genes described in this study are all expressed at high levels within the segmented portion of the central nervous system (CNS), with variable levels of expression in the segmental mesoderm. Little or no expression was seen in peripheral ectoderm or endoderm, or in the unsegmented head region (prostomium). Each Lox gene has a distinct anterior expression boundary within one of the four rostral segments, and the anterior-posterior (AP) order of these expression boundaries is identical to that reported for the orthologous Hox gene products in fly and mouse. This finding supports the idea that the process of AP axis differentiation is conserved among the higher metazoan phyla with respect to the regional expression of individual Hox genes along that axis. One unusual feature of leech Hox genes is the observation that some genes are only expressed during later development -- beginning at the time of terminal cell differentiation -- whereas others begin expression at a much earlier stage, and their RNA ceases to be detectable shortly after the onset of expression of the 'late' Hox genes. The functional significance of this temporal disparity is unknown, but it is noteworthy that only the two 'early' Hox genes display high levels of mesodermal expression.

Amino Acid Sequence↗
PubMed1997

The origin and evolution of animal appendages.

Animals have evolved diverse appendages adapted for locomotion, feeding and other functions. The genetics underlying appendage formation are best understood in insects and vertebrates. The expression of the Distal-less (Dll) homeoprotein during arthropod limb outgrowth and of Dll orthologs (Dlx) in fish fin and tetrapod limb buds led us to examine whether expression of this regulatory gene may be a general feature of appendage formation in protostomes and deuterostomes. We find that Dll is expressed along the proximodistal axis of developing polychaete annelid parapodia, onychophoran lobopodia, ascidian ampullae, and even echinoderm tube feet. Dll/Dlx expression in such diverse appendages in these six coelomate phyla could be convergent, but this would have required the independent co-option of Dll/Dlx several times in evolution. It appears more likely that ectodermal Dll/Dlx expression along proximodistal axes originated once in a common ancestor and has been used subsequently to pattern body wall outgrowths in a variety of organisms. We suggest that this pre-Cambrian ancestor of most protostomes and the deuterostomes possessed elements of the genetic machinery for and may have even borne appendages.

Amino Acid Sequence↗
PubMed1997

A survey of homeobox genes in Chaetopterus variopedatus and analysis of polychaete homeodomains.

A survey of genomic DNA from the polychaete Chaetopterus variopedatus was conducted using the polymerase chain reaction. Twelve unique homeobox-containing gene fragments were recovered. Phylogenetic analysis indicates that seven of the fragments are from genes belonging to Hox homeobox classes. Other fragments show orthology with Xlox, caudal, and Prh homeobox classes, with two fragments not definitely assignable to a homeobox class by our analysis. Orthology with gene sequences reported for the polychaete Ctenodrilus serratus, by Dick and Buss (1994), was calculated and indicated that at least eight of the C. variopedatus fragments are homologous to these previously reported sequences. Tabulation of the Hox gene relationships suggest that polychaetes have representative genes of each of the Hox cognate groups except Abd-B. This conclusion further suggests that the Hox cluster in the basal protostome ancestor had already undergone the gene duplications leading to the complete complement of homeotic genes known in Drosophila, with the possible loss of Abd-B in the polychaete lineage.

Amino Acid Sequence↗
PubMed1997

Reassessing embryogenesis in the Ctenophora: the inductive role of e1 micromeres in organizing ctene row formation in the 'mosaic' embryo, Mnemiopsis leidyi.

Ctenophores are a phylum of diploblastic marine animals displaying biradial symmetry organized along an oral-aboral axis. One of the apomorphic sets of adult structures in ctenophores are the eight external comb rows, which run along the oral-aboral axis. Comb rows consist of serial arrays of individual comb plates of cilia, which beat in a coordinated fashion for locomotory behavior. Classical cell lineage experiments using chalk particles indicated that comb rows are derived exclusively from the four e1 micromeres at the 16-cell stage. This conclusion was also supported by the fact that no ctene rows (or their underlying endodermal canals) form when all four e1 micromeres were deleted. We have used intracellular diI cell lineage tracing to determine that, in addition to e1 micromeres, the four m1 micromeres also make significant contributions to the ctene rows. Thus, e1 micromere derivatives not only generate comb plates but are required for ctene row formation by m1 derivatives. These results demonstrate that inductive interactions are an important component of early development in ctenophores and indicate that e1 micromeres influence the development of adjacent cell lineages (both m1 and endodermal lineages) during ctenophore embryogenesis. In addition, intracellular labeling has revealed that there are subtle variations in the composition of clones derived from identified embryonic blastomeres. Together these findings reveal a picture of ctenophore embryogenesis, which is in marked contrast to the former rigid 'mosaic' reputation of ctenophore development, and invite speculation as to the role of the cleavage program in embryonic patterning in the lower Metazoa.

Animals↗
PubMed1997

Homeoboxes in sea anemones (Cnidaria:Anthozoa): a PCR-based survey of Nematostella vectensis and Metridium senile.

Homeobox genes belong to a phylogenetically widespread family of regulatory genes that play important roles in pattern formation and cell-fate specification in several model systems (e.g., Drosophila, mouse, and C. elegans). Although the evolution of many classes of homeobox genes predates the diversification of the Bilateria, comparatively little is known about homeobox genes in outgroups to the Bilateria, such as the Cnidaria. We used the polymerase chain reaction to recover 12 partial homeoboxes from 2 species of sea anemones, Metridium senile and Nematostella vectensis (phylum Cnidaria; class Anthozoa). These homeoboxes appear to represent 9 distinct, mutually paralogous homeobox genes, 5 of which belong to previously identified cnidarian homeobox classes, and 4 of which appear to represent previously unidentified classes. The evolutionary relationships between the homeodomains of sea anemones and of bilaterian animals were assessed through database searches and phylogenetic analyses. As many as 5 of the anemone homeoboxes may belong to the Hox class, which suggests that the Hox gene complement of cnidarians is larger than previously expected. Homologs of the even-skipped gene of Drosophila were also identified in both Metridium and Nematostella.

Amino Acid Sequence↗