Colony integration and the expression of the Hox gene, Cnox-2, in Hydractinia symbiolongicarpus (Cnidaria: Hydrozoa).

The stolonal mat is an anatomical feature correlated with increased colonial integration in several lineages of the cnidarian class Hydrozoa. Cnox-2 is a Hox gene known to be expressed in the body column of the cnidarian polyp. We report the pattern of Cnox-2 expression in both the stolonal mat and free stolons of the hydroid Hydractinia symbiolongicarpus. The gene is found to have high levels of expression in the mat similar to that found in the basal portion of the polyp, but it is not detectably expressed in those regions of free stolons where polyps are budded. These findings suggest that the stolonal mat arose via an expansion of the basal ectoderm of the polyp.

Expression of a Hox gene, Cnox-2, and the division of labor in a colonial hydroid.

We report the isolation and expression of the Hox gene, Cnox-2, in Hydractinia symbiolongicarpus, a hydrozoan displaying division of labor. We found different patterns of aboral-to-oral Cnox-2 expression among polyp polymorphs, and we show that experimental conversion of one polyp type to another is accompanied by concordant alteration in Cnox-2 expression. Our results are consistent with the suggestion that polyp polymorphism, characteristic of hydractiniid hydroids, arose via evolutionary modification of proportioning of head to body column.

Dynamics of Gastrovascular Circulation in the Hydrozoan Podocoryne carnea: the One-Polyp Case.

Time-lapse video microscopy and image analysis algorithms were used to generate high-resolution time series of the length and volume of a single hydrozoan polyp before and after feeding. A polyp of Podocoryne carnea prior to feeding is effectively static in length and volume. At 20{deg}C, feeding elicits 8-millihertz (mHz) oscillations in polyp length and volume. A polyp connected to a colony by a single stolon displayed an abrupt transition from low-amplitude, 8-mHz oscillations to large-amplitude, 6-mHz oscillations at 1.5-2 h after feeding. The transition was preceded by a substantial decrease in polyp volume and increase in length which coincided with the export of food items from the digestive cavity of the polyp into the colonial gastrovascular system. In contrast, 8-mHz oscillations of a polyp isolated from a colony continued for 12.7 h after feeding, at which time particulates from the digestive cavity were exported into the hydrorhiza and a 4-mHz subharmonic became briefly dominant. Regular oscillatory behavior was terminated by regurgitation at comparable intervals post-feeding in coupled and isolated polyps. These observations are compatible with the hypothesis that the presence of nutrients in the digestive cavity induces polyp oscillations and that release of nutrients into the gastrovascular system similarly induces unfed polyps to oscillate, thereby distributing the contents of the fed polyp throughout the colony.

Over one-half billion years of head conservation? Expression of an ems class gene in Hydractinia symbiolongicarpus (Cnidaria: Hydrozoa).

We report the isolation of an empty spiracles class homeodomain-containing gene, Cn-ems, from the hydrozoan Hydractinia symbiolongicarpus, the first gene of this class characterized in a lower metazoan. Cn-ems was found to be expressed in the head of gastrozooids, specifically in endodermal epithelial cells of the taeniolae of the hypostome. Cn-ems is not expressed in gonozooids, which lack taeniolae. Experimental conversion of the posterior region of the planula larva into head structures up-regulates expression of the gene. These findings establish that the association of ems-class genes with head structures preceded the evolution of bilateral symmetry.

Transmission genetics of allorecognition in Hydractinia symbiolongicarpus (Cnidaria:Hydrozoa).

Allorecognition is ubiquitous, or nearly so, amongst colonial invertebrates. Despite the prominent role that such phenomena have played both in evolutionary theory and in on the origin of the vertebrate immune system, unambiguous data on the transmission genetics of fusibility (i.e., the ability of two individuals to fuse upon tissue contact) is lacking for any metazoan outside of the phylum Chordata. We have developed lines of the hydroid Hydractinia symbiolongicarpus (Phylum Cnidaria) inbred for fusibility and here report results of breeding experiments establishing that fusibility segregates as expected for a single locus with codominantly expressed alleles, with one shared allele producing a fusible phenotype. Surveys of fusibility in field populations and additional breeding experiments indicate the presence of an extensive allele series.

Class-level relationships in the phylum Cnidaria: molecular and morphological evidence.

The evolutionary history of cnidarian life cycles has been debated since the 1880s, with different hypotheses favored even by current textbooks. Contributing to the disagreement is the fact that the systematic relationships of the four cnidarian classes have received relatively little examination using modern systematic methods. Here we present analyses of class-level relationships based on 18S ribosomal DNA (rDNA) sequence, mitochondrial 16S rDNA sequence, mitochondrial genome structure, and morphological characters. DNA sequences were aligned using a repeatable parsimony-based approach incorporating a range of alignment parameters. Analyses of individual data sets and of all data combined are unanimous in grouping the classes possessing a medusa stage, leaving the holobenthic Anthozoa basal within the phylum.

HOM/Hox-type homeoboxes from Stylaria lacustris (Annelida: Oligochaeta).

The polymerase chain reaction was used to survey the oligochaete worm Stylaria lacustris for HOM/Hox type homeobox genes. Twelve fragments were identified and in most cases were assignable to cognate groups on the basis of amino acid similarity. Stylaria appears to have three labial-type genes, a HOX B3 cognate, several genes similar to Antennapedia, and cognates of Abdominal-B and CHox7.

A PCR-based survey of homeobox genes in Ctenodrilus serratus (Annelida: Polychaeta).

The polychaete worm Ctenodrilus serratus was surveyed for the presence of HOM/HOX and engrailed-type homeobox genes using PCR with degenerate primers. Sixteen unique homeobox fragments were found in surveys of genomic and cDNA with three different primer sets. For three fragments, RACE was employed to obtain additional homeobox sequence and the 3' flanking region. Nine HOM/HOX-type fragments were identified, including putative representatives of the Hox1/lab, Hox2/pb, Hox3, Hox4/Dfd, and Antp/Lox5 cognate groups. Two additional Antp-like fragments could not be assigned specific orthology. Presence of an ortholog of leech Lox2 in addition to a Ubx/abdA-like gene suggests that independent duplications of a single precursor occurred in the annelid and arthropod lineages. No representative of the Hox9/AbdB group was identified. Our results are consistent with a hypothesis of a single HOM/HOX cluster in Ctenodrilus as extensive as that seen in strongly tagmatized arthropods, suggesting that the primitive role of these genes even in overtly metameric animals was something other than specification of overt segmental differentiation. The primers used also detected representatives of six other homeobox classes or families: Xlox (XlHbox8/HTr-A2), Ovx (Chox7), caudal, Prh (proline-rich homeobox), NEC (ceh-9/Tghbox5), and engrailed.

What would be conserved if "the tape were played twice"?

We develop an abstract chemistry, implemented in a lambda-calculus-based modeling platform, and argue that the following features are generic to this particular abstraction of chemistry; hence, they would be expected to reappear if "the tape were run twice": (i) hypercycles of self-reproducing objects arise; (ii) if self-replication is inhibited, self-maintaining organizations arise; and (iii) self-maintaining organizations, once established, can combine into higher-order self-maintaining organizations.

HOM/Hox type homeoboxes in the chelicerate Limulus polyphemus.

The polymerase chain reaction was used to survey HOM/Hox type homeobox genes in the North American horseshoe crab, Limulus polyphemus. The survey reveals 28 different homeobox fragments possessing 44-80% nucleotide sequence similarity with HOM-C sequences in Drosophila and Hox B sequences in mouse. Limulus has one to four representatives of HOM/Hox cognates when fragments are assigned according to amino acid identity. A parsimony analysis including the Limulus sequences and homologous mouse Hox B and Drosophila HOM-C sequences reveals several instances of monophyletic groupings of Limulus homeoboxes with a Drosophila or mouse gene as the sister branch. Under the current model of HOM/Hox cluster evolution, these results suggest that Limulus has as many as four HOM/Hox type clusters. This is the first evidence suggesting the presence of multiple clusters in a nonvertebrate taxon. Putative homologs of mouse Hoxb-3 in Limulus suggest that the gene was present in the common ancestor of arthropods and vertebrates and has been lost in Drosophila.

Determination of paternity in dragonflies by Random Amplified Polymorphic DNA fingerprinting.

We used Random Amplified Polymorphic DNA (RAPD) fingerprinting to address issues of paternity in two odonate species. Amplification artifacts of RAPD markers were controlled by assessing paternity patterns relative to the banding patterns generated by quantitative mixtures of DNA from putative parents ('synthetic offspring'). In the aeshnid dragonfly Anax parthenope, for which the mating histories of both males and females were unknown, we found strong evidence for complete paternity success for the contact guarding male. In the highly polygamous libellulid dragonfly Orthetrum coerulescens, we detected and quantified mixed paternity in sequentially produced offspring clutches and demonstrated that fertilization success is correlated with the duration of copulation. Our results suggest that RAPD fingerprinting is suitable to address issues of paternity in systems which are genetically uncharacterized and produce large offspring clutches.

Class-level relationships in the phylum Cnidaria: evidence from mitochondrial genome structure.

The phylogenetic relationships of the Recent cnidarian classes remain one of the classic problems in invertebrate zoology. We survey the structure of the mitochondrial genome in representatives of the four extant cnidarian classes and in the phylum Ctenophora. We find that all anthozoan species tested possess mtDNA in the form of circular molecules, whereas all scyphozoan, cubozoan, and hydrozoan species tested display mtDNA in the form of linear molecules. Because ctenophore and all other known metazoan mtDNA is circular, the shared occurrence of linear mtDNA in three of the four cnidarian classes suggests a basal position for the Anthozoa within the phylum.

Treatment with 2,4-dinitrophenol mimics ontogenetic and phylogenetic changes in a hydractiniid hydroid.

Clonal plants, fungi, and invertebrate animals often display both intra- and interspecific heterochronic variation in the relative proportions of ramets (e.g., polyps, plantlets) and stolons (or rhizomes, hyphae). Treatment of developing colonies of Podocoryne carnea, a hydractiniid hydroid, with dilute solutions of 2,4-dinitrophenol (DNP), an uncoupler of oxidative phosphorylation, accelerates the usual ontogenetic trajectory of polyp and stolon production. This morphological heterochrony can be related to the hydrodynamic behavior of the gastrovascular system under the conditions of "loose-coupling" of oxidative phosphorylation produced by DNP. In its normal ontogeny, Hydractinia symbiolongicarpus, a closely related hydractiniid hydroid, not only shows morphological heterochrony similar to that induced in P. carnea by DNP, but also shows a pattern of gastrovascular flow similar to that observed in P. carnea under treatment with DNP. These results support hypotheses linking metabolic activity to heterochronic variation and provide a potentially widely applicable experimental basis for the study of such variation.

Evolution of king crabs from hermit crab ancestors.

King crabs (Family Lithodidae) are among the world's largest arthropods, having a crab-like morphology and a strongly calcified exoskeleton. The hermit crabs, by contrast, have depended on gastropod shells for protection for over 150 million years. Shell-living has constrained the morphological evolution of hermit crabs by requiring a decalcified asymmetrical abdomen capable of coiling into gastropod shells and by preventing crabs from growing past the size of the largest available shells. Whereas reduction in shell-living and acquisition of a crab-like morphology (carcinization) has taken place independently in several hermit crab lineages, and most dramatically in king crabs, the rate at which this process has occurred was entirely unknown. We present molecular evidence that king crabs are not only descended from hermit crabs, but are nested within the hermit crab genus Pagurus. We estimate that loss of the shell-living habit and the complete carcinization of king crabs has taken between 13 and 25 million years.

Homeoboxes in cnidarians.

Homeoboxes have previously been documented from various phyla of triploblastic, coelomate and pseudocoelomate, animals. We report here the first homeoboxes from cnidarians, a phylum of diploblastic organisms thought to occupy a near-basal position in metazoan phylogeny. We have sequenced three partial (77 bp) fragments of Antennapedia (Antp) class homeoboxes from the hydroids Hydractinia symbiolongicarpus and Eleutheria dichotoma. A pair of fragments, Cnox-2-Hs and Cnox-2-Ed, from the two species differ in nucleotide sequences but have identical derived amino acid sequences. A gene tree produced by parsimony analysis shows that these two fragments cluster within the Antp homeobox lineage. The third fragment, Cnox-1, clusters as a sister group of the other Antp class homeoboxes.

MOLECULAR AND GEOLOGIC EVIDENCE OF SHARED HISTORY BETWEEN HERMIT CRABS AND THE SYMBIOTIC GENUS HYDRACTINIA.

The paleobiogeographic histories of three North Atlantic hermit crab lineages were compared with a single-copy DNA-DNA hybridization phylogeny of their symbiotic hydroid genus Hydractinia to test hypotheses of shared history between these host and symbiont lineages. A survey of the geologic literature suggests that two vicariance events in the Quaternary are responsible for existing range disjunctions of the host hermit crab lineages. The Hydractinia phylogeny revealed two distinct clades, one with a primarily northern and the other with a primarily southern distribution. In two of three cases, hydroids associated with closely related hermits on both sides of the range disjunction appear as sister taxa in the phylogeny. A linear scaling between a measure of hydroid sequence divergence and independent geologic estimates of the timing of the vicariant events believed to have established the hermit crab range disjunctions is consistent with the claim of temporal coincidence of cladogenic and vicariance events. These findings provide evidence for shared history of symbiotic associations in two of the three cases.

Shape Variation in Hydractiniid Hydroids.

Colonies of hydractiniid hydroids consist of feeding polyps connected by a common gastrovascular system. The gastrovascular system consists of stolons, which enclose gastrovascular canals. Stolons may be fused into a stolonal mat or extend from the periphery of the colony. Hydractinia forms a stolonal mat early in colony development; Podocoryne, on the other hand, does not. To facilitate comparisons of these taxa, we propose a simple shape metric, perimeter/{radical}area, and show that this measure: (1) correlates closely with relative amounts of peripheral stolon and stolonal mat structures in Hydractinia, (2) permits analyses of within- and between-species variation of growth morphology in Podocoryne and Hydractinia, and (3) allows quantitative analysis of breeding studies of Hydractinia, both before and after stolonal mat formation in the progeny.

Competition within and between encrusting clonal invertebrates.

Colonies of encrusting marine invertebrates are tractable models for the study of competition, because of the relative ease with which observations can be made on the frequency and outcome of overgrowth interactions. Studies of intraspecific competition have found that competition is predicated upon a genetically controlled recognition event, which results in either fusion or rejection. Data are rapidly accumulating in two model systems showing that fusion is associated with somatic cell parasitism and that rejection is associated with overgrowth. Thus, encounters between conspecifics define a choice: to compete at the level of the cell lineage or to compete at the level of the colony. Fusion-rejection genes act to control the units (or targets) of selection.