A multi-locus time-calibrated phylogeny of the siphonous green algae

The siphonous green algae are an assemblage of seaweeds that consist of a single giant cell. They comprise two sister orders, the Bryopsidales and Dasycladales. We infer the phylogenetic relationships among the siphonous green algae based on a five-locus data matrix and analyze temporal aspects of their diversification using relaxed molecular clock methods calibrated with the fossil record. The multi-locus approach resolves much of the previous phylogenetic uncertainty, but the radiation of families belonging to the core Halimedineae remains unresolved. In the Bryopsidales, three main clades were inferred, two of which correspond to previously described suborders (Bryopsidineae and Halimedineae) and a third lineage that contains only the limestone-boring genus Ostreobium. Relaxed molecular clock models indicate a Neoproterozoic origin of the siphonous green algae and a Paleozoic diversification of the orders into their families. The inferred node ages are used to resolve conflicting hypotheses about species ages in the tropical marine alga Halimeda.     

EVOLUTIONARY STRATEGIES IN A TROPICAL BARRIER REEF SYSTEM: FUNCTIONAL-FORM GROUPS OF MARINE MACROALGAE1

Predictions of an evolutionary model were examined for 43 tropical macroalgae using a functional-form group approach. The ranking from high to low primary producers (Sheet- and Filamentous-Groups > Coarsely Branched- and Thick Leathery-Groups > Jointed Calcareous- and Crustose-Groups), and data from the literature, support the hypothesis that persistent forms which allocate resources for environmental resistance, interference competition or antiherbivory defenses do so at the cost of lower primary production rates. The results for percent thallus lost to fish grazing over a 24 h period support the hypothesis that members of the Thick Leathery-, Jointed Calcareous- and Crustose-Groups have evolved antipredator defenses, with a tendency for decreasing herbivore resistance toward the Sheet- and Filamentous-Groups. The most heavily-calcified species (e.g. crustose corallines) ranked among the most grazer resistant as did the thick rubbery or leathery species. The ranking of functional-form group means for resistance to predation was as follows: Filamentous-Group (62% lost-24 h^?1), Sheet-Group (42%), Coarsely Branched-Group (33%), Jointed Calcareous-Group (10%), Thick Leathery-Group (7%) and Crustose-Group (0%), in accordance with the hypothesis. The algal groups generally showed an increase in mean penetration toughness from filaments (<200 g-cm^?2 to shear thallus) to sheets (216 g·cm²), coarsely branched forms (328 g·cm^?2) and thick leathery species (1800 g·cm^?2) in agreement with the predictions of the model. Contrary to earlier findings, there was no consistent gradation between the first four groups (i.e. fleshy algae) based on calorific values. However, in partial support of the functional-form model, a seven-fold difference was noted when the mean for these groups (1.7 kcal·g^?1) was compared with that of the Jointed Calcareous- and Crustose-Groups (0.2 kcal·g^?1. The functional-form group approach appears to have powerful capabilities in that it can indicate important morphological-metabolic-ecological interactions in a given community, where the macroalgae are known, without the need to examine each population in detail and without being constrained to a specific habitat or geographical region.     

BLOOM OF THE GIANT ANADYOMENE GIGANTODICTYON SP. NOV. (ANADYOMENACEAE,CLADOPHORALES) FROM THE OUTER SLOPE (25–50 m) OF THE BELIZE BARRIER REEF1

A giant form of Anadyomene, most similar to Anadyomene pavonina (J. Agardh) Wille, a rare and diminutive alga endemic to Florida, appeared as up to 10 m long net‐like strands covering 10%–80% of a 0.5 km region of the 25–50 m deep Belizean outer reef slope where none had been present up to 12 months earlier. This new species, described herein as Anadyomene gigantodictyon Littler et D. S. Littler, is characterized by a unistratose blade or cluster of blades formed by the polychotomous branching of uniseriate veins, with the interstices, or spaces between the veins, completely or partially filled with cells that are smaller than those of the veins, with cylindrical to ovate cells. The cells at mid‐blade are 1.7–2.0 mm in length and 0.2–0.3 mm diameter; interstitial cells are parallel and not juxtaposed. All cells are joined in one plane and form species‐specific, fan‐shaped patterns with secondary interstitial cells loosely or tightly woven.     

Fixation times and probabilities for an independent loci model in genetics

Assuming that two genetic loci evolve independently and that there is no mutation and selection, formulae are obtained for mean first and final fixation times and first fixation probabilities. These results, together with known results for the case of complete linkage, give some indication of the effect of linkage on these quantities.