Group living, competition, and the evolution of cooperation in a sessile invertebrate.

Competition and cooperation are thought to represent the opposite extremes of organism interactions. I here show that the formation of aggregations in a sessile organism requires cooperation between individuals and that the gregarious pattern of habitat selection generating these aggregations is a response to a density dependence in the outcome of interference competition.

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.

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.

Competitive intransitivity and size-frequency distributions of interacting populations.

The competitive ranking of three marine sessile organisms was determined from overgrowth observations and found to be intrasitive. Overgrowth results were dependent upon relative vertical relief which, in turn, was related to colony area. Discriminant function analysis demonstrates that competitive rankings can be largely predicted on the basis of knowledge of the size-frequency distribution of interacting populations. Because size increases with age for these species, size-dependent competition implies age-dependent competition. The role of age structure in determining the results of inter-specific interactions has been curiously neglected, particularly as it relates to competitive ability, yet the age dependence of competitive ability suggests (i) the occurrence of intransitivity in purely exploitative systems, (ii) a mechanism for frequency-dependent behavior, (iii) the production of limit cycles, and (iv) the possibility of size refugia.

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.

Somatic cell parasitism and the evolution of somatic tissue compatibility.

Selection pressures proposed to account for the convergent evolution of self/not-self recognition systems in lower organisms include defense against microbial parasites and somatic cell variants. Direct support for the existence of somatic cell parasites in natural populations has been lacking. I here report the occurrence of a somatic cell parasite in the cellular slime mold Dictyostelium mucoroides and discuss the implications of this phenomenon to the evolution of mechanisms of somatic tissue compatibility.

Evolution, development, and the units of selection.

The "Modern Synthesis" forms the foundation of current evolutionary theory. It is based on variation among individuals within populations. Variations within individuals are believed to hold no phylogenetic significance because such variation cannot be transmitted to the germ line (i.e., Weismann's doctrine). Weismann's doctrine, however, does not apply to protists, fungi, or plants and is an entirely unsupported assumption for 19 phyla of animals. This fact requires that the Modern Synthesis be reexamined and modified.

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.

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.

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.

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.

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.

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.

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.

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.

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.