Genetic structure in the Mediterranean seagrass Posidonia oceanica: disentangling past vicariance events from contemporary patterns of gene flow

The Mediterranean Sea is a two‐basin system, with the boundary zone restricted to the Strait of Sicily and the narrow Strait of Messina. Two main population groups are recognized in the Mediterranean endemic seagrass Posidonia oceanica, corresponding to the Western and the Eastern basins. To address the nature of the East–West cleavage in P. oceanica, the main aims of this study were: (i) to define the genetic structure within the potential contact zone (i.e. the Strait of Sicily) and clarify the extent of gene flow between the two population groups, and (ii) to investigate the role of present water circulation patterns vs. past evolutionary events on the observed genetic pattern. To achieve these goals, we utilized SSR markers and we simulated, with respect to current regime, the possible present‐day dispersal pattern of Posidonia floating fruits using 28‐day numerical Lagrangian trajectories. The results obtained confirm the presence of the two main population groups, without any indices of reproductive isolation, with the break zone located at the level of the Southern tip of Calabria. The populations in the Strait of Sicily showed higher affinity with Western than with Eastern populations. This pattern of genetic structure probably reflects historical avenues of recolonization from relict glacial areas and past vicariance events, but seems to persist as a result of the low connectivity among populations via marine currents, as suggested by our dispersal simulation analysis.     

Persistence of Modifications Induced by Osmo-saline Shocks on Chloride Transport and Some Related Metabolic Processes in Roots of Maize Plants

Excised roots obtained from maize plants, previously treatedfor 24 h with different solutes (K₂SO₄, Na₂SO₄, or mannitol)at decreasing osmotic potential values, showed clearly the persistenceof osmo-saline shock in their chloride transport and in somemetabolic processes. In particular, vacuolar accumulation ofchloride was much reduced when the osmotic potential in pretreatmenthad been low; this effect was greater with solutions of electrolytes.Protein synthesis and [¹⁴C]leucine uptake were also reduced;thus there appeared to be a correlation between chloride accumulationin the vacuole and protein synthesis. By contrast, accumulationof chloride in the cytoplasm was only slightly modified or evenstimulated. Chloride influx was affected less by osmotic pretreatmentthan accumulation, except in the case of very low osmotic potentialswhere influx was depressed. Chloride efflux was slightly enhancedby osmo-saline shocks and the fluxes at the tonoplast were lessaffected in comparison with those at the plasmalemma. Malateaccumulation was stimulated by shock, but this effect was notpersistent, so that a correlation with the effects on accumulationand fluxes was not possible. Oxygen uptake was affected onlyslightly except that at osmotic potential values of –750kPa, perhaps because of a lowered viability of the roots subjectedto very low osmotic potential for 24 h.     

Phosphate Absorption, Fluxes, and Symplasmic Transport in Osmotically-Shocked Zea mays Roots

Osmotic shock with sequential 30 min treatments in ice-coldsaline solutions and distilled water inhibited both the subsequentuptake of orthophosphate (Pi) and its transport into the xylemof excised corn (Zea mays L.) roots. Measurements of Pi fluxeswith ³²P indicated that the decrease in net Pi uptake over a24 h period caused by osmotic shock was due primarily to delayedrecovery of Pi influx rather than to increasing efflux. Despitecomplete recovery of Pi absorption within 2–6 h aftershocking with 150–200 mM NaCl, transport to the xylemduring the subsequent 24 h only partially recovered. Leucineuptake and incorporation into protein was also markedly inhibitedby osmotic shock but both almost completely resumed controlrates within 24 h after shocking with up to 150 mM NaCl. Tetracyclineinhibited recovery of Pi uptake after NaCl treatment whereaspuromycin did not. These results with corn roots are consistentwith the hypothesis that recovery of Pi uptake activity aftermoderate osmotic shock requires de novo synthesis of membraneproteins. Incomplete recovery of Pi transport to the xylem suggeststhat osmotic shock may damage plasmodesmata. Key words: Corn, Ion uptake, Leucine uptake, NaCl, Puromycin, Tetracycline