Site Variation in Spatial Aggregation and Phorophyte Preference in Psychilis monensis (Orchidaceae)

Aggregation patterns of plants vary according to spatial scale and developmental stage, and are dependent on vegetation dynamics and species composition. We describe the aggregation patterns of the epiphytic orchid Psychilis monensis from two sites with different vegetation structure and composition on Mona Island, Puerto Rico. We analyzed spatial variation for seedlings, juveniles, and adults using the density-independent, standardized Morisita index (I_MS). We censused a total of 879 plants. Strong preferences for some phorophyte species were evident, including dead trees, but association with bark roughness was equivocal. The highest densities occurred in the site with the lowest fruit and seed production, suggesting that the best sites for pollination and seedling establishment were not the same. Seedlings at one site were significantly aggregated, but all other stages were indistinguishable from a random distribution. Nevertheless, adults at both sites had the lowest I_MS values indicating that they tend to be the least aggregated of the three life history stages. The abundance and age structure of P. monensis were clearly affected by the frequency of their preferred hosts, but site-specific factors affecting seedling survival probably play a major role in site differences.     

A Model of the Quaternary Structure of Enolases, Based on Structural and Evolutionary Analysis of the Octameric Enolase from Bacillus subtilis

Purified enolase from Bacillus subtilis has a native mass of approximately 370 kDa. Since B. subtilis enolase was found to have a subunit mass of 46.58 kDa, the quaternary structure of B. subtilis is octameric. The pl for B. subtilis enolase is 6.1, the pH optimum (pH_o) for activity is 8.1–8.2, and the K _m for 2-PGA is approximately 0.67 mM. Using the dimeric Cα structure of yeast dimeric enolase as a guide, these dimers were arranged as a tetramer of dimers to simulate the electron microscopy image processing obtained for the octameric enolase purified from Thermotoga maritima. This arrangement allowed identification of helix J of one dimer (residues 86–96) and the loop between helix L and strand 1 (HL–S1 loop) of another dimer as possible subunit interaction regions. Alignment of available enolase amino acid sequences revealed that in 16 there are two tandem glycines at the C-terminal end of helix L and the HL–S1 loop is truncated by 4–6 residues relative to the yeast polypeptide, two structural features absent in enolases known to be dimers. From these arrangements and alignments it is proposed that the GG tandem at the C-terminal end of helix L and truncation of the HL–S1 loop may play a critical role in octamer formation of enolases. Interestingly, the sequence features associated with dimeric quaternary structure are found in three phylogenetically disparate groups, suggesting that the ancestral enolase was an octamer and that the dimeric structure has arisen independently multiple times through evolutionary history.     

Germ cell transplantation in goats

Transplantation of spermatogonial stem cells provides a unique approach for the study of spermatogenesis and manipulation of the male germ line. This technique may also offer an alternative to the currently inefficient methods of producing transgenic domestic animals. We have recently established the technique of spermatogonial transplantation, originally developed in laboratory rodents, in pigs, and this study was aimed to extend the technique to the goat. Isolated donor testis cells were infused into the seminiferous tubules of anesthetized recipient goats through an ultrasonographically-guided catheter inserted into the rete testis. Donor cells were obtained by enzymatic digestion of freshly collected testes from immature goats (either from the recipients' contralateral testis or from unrelated donors). Prior to transplantation, testis cells were labeled with a fluorescent marker to allow identification after transplantation. Recipient testes were examined for the presence and localization of labeled donor cells at 3-week intervals up to 12 weeks after transplantation. Labeled donor cells were found in the seminiferous tubules of all testes, comprising 10-35% of the examined tubules. Histological examination of the recipient testes did not reveal evident tissue damage, except for limited fibrotic changes at the site of needle insertion. Likewise there were no detectable local or systemic signs of immunologic reactions to the transplantations. These results indicate that germ cell transplantation is technically feasible in immature male goats and that donor-derived cells are retained in the recipient testis for at least three months and through puberty. This study represents the first report of germ cell transplantation in goats.     

Microtubule Depolymerization Inhibits Ethanol-Induced Enhancement of GABAA Responses in Stably Transfected Cells

Abstract: We studied whether microtubule organization is important for actions of ethanol on GABA_A ergic responses by testing the effects of microtubule depolymerization on ethanol enhancement of GABA action in mouse L(tk⁻) cells stably transfected with GABA_A receptor α₁β₁γ_2L subunits. The microtubule-disrupting agents colchicine, taxol, and vinblastine completely blocked ethanol-induced enhancement of muscimol-stimulated chloride uptake. β-Lumicolchicine, a colchicine analogue that does not disrupt microtubules, had no effect on ethanol action. Colchicine did not alter the potentiating actions of flunitrazepam or pentobarbital on muscimol-stimulated chloride uptake. Thus, colchicine specifically inhibited the potentiating action of ethanol. From these findings, we conclude that intact microtubules are required for ethanol-induced enhancement of GABA_A responses and suggest that a mechanism involving microtubules produces posttranslational modifications that are necessary for ethanol sensitivity in this cell system.     

Gap Dynamics in a Seagrass Landscape

We investigated gap dynamics within a shallow subtidal landscape characterized by seagrass vegetation and examined the relationship between gap formation and selected physical factors. The study was conducted over 2 y by using a biannual mapping of seagrass and water depth across an 48,800-m² area in Tampa Bay, Florida. In addition, monthly sediment deposition or erosion was recorded at 96 locations within the landscape. Gaps represented from 2.4% to 5.7% of the seagrass landscape, and all were within monospecific stands of Halodule wrightii. Gaps ranged in size from 10 to 305 m² and most frequently decreased in size over time. Most gaps were small and short lived (less than 6-mo duration), but the second age group most frequently recorded was at least 1.5 y old. No new species of seagrass invaded the gaps with Halodule replacing itself 100% of the time. Gaps were recorded over the entire range of water depths within the landscape. Neither gap area nor persistence of gaps was related to water depth. However gap area was associated positively with the number of extreme sedimentation events. Gaps originated not only from removal of interior vegetation (similar to classic gaps) but also from differential growth of the seagrass margin (similar to edaphic gaps). Distinct seasonal components to the mode of formation were detected with interior-produced gaps originating primarily in the winter and margin gaps most commonly during summer. These results combine to illustrate the importance of large-scale studies with fine-scale resolution for deciphering unique features of seagrass landscape dynamics. Our historical information suggests that a static enumeration of gaps may not provide an accurate assessment of disturbance intensity in this system, and the seagrass mosaic probably is explained best by a combination of disturbance regimes and edaphic factors, such as sediment stability. Moreover, we suggest that even in areas characterized by monospecific stands of vegetation and over short or moderate time periods, gaps indirectly may influence community structure and ecosystem function via modification of habitat arrangement. Received 17 September 1998; accepted 26 April 1999.     

Positive interactions between herbivores and plant diversity shape forest regeneration

The effects of herbivores and diversity on plant communities have been studied separately but rarely in combination. We conducted two concurrent experiments over 3 years to examine how tree seedling diversity, density and herbivory affected forest regeneration. One experiment factorially manipulated plant diversity (one versus 15 species) and the presence/absence of deer (Odocoileus virginianus). We found that mixtures outperformed monocultures only in the presence of deer. Selective browsing on competitive dominants and associational protection from less palatable species appear responsible for this herbivore-driven diversity effect. The other experiment manipulated monospecific plant density and found little evidence for negative density dependence. Combined, these experiments suggest that the higher performance in mixture was owing to the acquisition of positive interspecific interactions rather than the loss of negative intraspecific interactions. Overall, we emphasize that realistic predictions about the consequences of changing biodiversity will require a deeper understanding of the interaction between plant diversity and higher trophic levels. If we had manipulated only plant diversity, we would have missed an important positive interaction across trophic levels: diverse seedling communities better resist herbivores, and herbivores help to maintain seedling diversity.