Sibling species of mutualistic Symbiodinium clade G from bioeroding sponges in the western Pacific and western Atlantic oceans

Dinoflagellates in the genus Symbiodinium associate with a broad array of metazoan and protistian hosts. Symbiodinium‐based symbioses involving bioeroding sponge hosts have received less attention than those involving popular scleractinian hosts. Certain species of common Cliona harbor high densities of an ecologically restricted group of Symbiodinium, referred to as Clade G. Clade G Symbiodinium are also known to form stable and functionally important associations with Foraminifera and black corals (Antipatharia) Analyses of genetic evidence indicate that Clade G likely comprises several distinct species. Here, we use nucleotide sequence data in combination with ecological and geographic attributes to formally describe Symbiodinium endoclionum sp. nov. obtained from the Pacific boring sponge Cliona orientalis and Symbiodinium spongiolum sp. nov. from the congeneric western Atlantic sponge Cliona varians. These species appear to be part of an adaptive radiation comprising lineages of Clade G specialized to the metazoan phyla Porifera and Cnidaria, which began prior to the separation of the Pacific and Atlantic Oceans.     

Symbiosis and host morphological variation: Symbiodiniaceae photosynthesis in the octocoral Briareum asbestinum at ambient and elevated temperatures

Intra-species morphological variation may occur in sessile organisms, such as corals, living in different habitats. Conversely, the octocoral Briareum asbestinum exhibits both encrusting and upright branching morphologies at the same shallow water habitat, enabling studying physiological differences uncoupled from habitat variation due to depth or reef location. We investigated the mutualism between endosymbiotic dinoflagellate algae, Breviolum spp. (previously clade B Symbiodinium), and these B. asbestinum morphologies at ambient and elevated temperatures. Based on msh1 gene sequences, the host morphologies were genetically similar although they differed in protein content, polyp expansion behavior, and associated Breviolum (B19 for encrusting and B21 for branching B. asbestinum). Due to colony orientation, polyps in encrusting B. asbestinum experienced irradiance levels nearly three times higher than polyps in the branching morph, which probably contributed to the lower photochemical and light absorption efficiencies of the Breviolum in encrusting fragments. The light-limited portion of photosynthesis–irradiance curves and the intracellular chlorophyll concentrations, however, indicated that Breviolum in both morphologies were acclimated to similar internal irradiances. Encrusting B. asbestinum exhibited higher Breviolum density, areal chlorophyll a, and greater photosynthetic rates cm⁻² compared to branching B. asbestinum. Notably, elevated temperature did not cause bleaching in either morphology, as Breviolum and chlorophyll densities did not differ significantly from ambient temperature, although the two morphologies adjusted some of the measured parameters, indicating coping with the stressor. In the face of continued ocean warming, the high thermal tolerance of octocorals may reinforce the shift of Caribbean reefs from scleractinian coral to octocoral dominance.

     

Sperm surface galactosyltransferase activities during in vitro capacitation

Studies using genetic and biochemical probes have suggested that mouse sperm surface galactosyltransferases may participate during fertilization by binding N- acetylglucosamine (GlcNAc) residues in the egg zona pellucida. In light of these results, we examined sperm surface galactosyltransferase activity during in vitro capacitation to determine whether changes in enzymatic activity correlated with fertilizing ability. Results show that surface galactosyltransferases on uncapacitated sperm was preferentially loaded with poly N-acetyllactosamine substrates. As a consequence of capacitation in Ca(++)-containing medium, these polylactosaminyl substrates are spontaneously released from the sperm surface, thereby exposing the sperm galactosyltransferase for binding to the zona pellucida. Sperm capacitation can be mimicked, in the absence of Ca(++), either by washing sperm in Ca(++)-free medium, or by pretreating sperm with antiserum that reacts with the galactosyltransferase substrate. In both instances, sperm galgactosylation of endogenous polylactosaminyl substrates is reduced, coincident with increased galactosylation of exogenous GlcNAc, and increased binding to the zona pellucida. Binding of capacitated sperm to the egg can be inhibited by pronase-digested high molecular weight polyactosaminyl glycoside extracted from epidymal fluids or from undifferentiated F9 embryonal carninoma cells. Thus, these glycosides function as “decapacitation factors” when added back to in vitro fertilization assays. These glycoside “decapacitation factors” inhibit sperm-egg binding by competeing for the sperm surface galactosyltransferase, since (a) they are galactosylated by sperm in the presence of UDP[(3)H]galactose, and (b) enzymatic removal of terminal GlcNAc residues reduces “decapacitation factio” competition. On the other hand “conventional” low molecular weight glycosides, isolated from either epididymal fluid or differentiated F9 cells, fail to inhibit capacitated sperm binding to the zona pellucida. These results define a molecular mechanism for one aspect of sperm capacitation, and help explain why removal of “decapacitation factos” is a necessary prerequisite for sperm binding to the zona pellucida.