MOLECULAR EFFECTS ON SPERMATOZOA OF MYTILUS GALLOPROVINCIALIS EXPOSED TO SEVERE HYPOSALINE CONDITIONS: A CASE OF FERTILITY PRESERVATION STRATEGY DUE TO GAMETE PLASTICITY.

Climate changes are affecting species physiology, pushing environmental tolerance limits and shifting biogeographic distribution ranges of marine organisms. In addition to temperature and ocean acidification, global climate changes can also occur through changes in seawater salinity. Salinity levels may change also by anthropogenic factors such as mining activity and agricultural and industrial processes. Salinity is considered one of the most significant environmental stressors for marine bivalves. Mussels, in close proximity to coasts and in estuaries, in fact, periodically experience hypo-saline stress, particularly during intense precipitations. Many bivalve species, when experiencing water with high temperatures and low salinity, have the skill to acclimatize, or migrate to deeper water which is cooler and more saline. Mytilus galloprovincialis, is distributed in the north Atlantic Ocean, Mediterranean Sea and Black Sea and in recent years has invaded new places, including South Africa, Japan and California. This mussel is a common member of the intertidal estuarine and coastal areas where it can meet different salinities. Mytilus galloprovincialis can survive at elevated heat exposure, but it is susceptible to hyposalinity, while the native Pacific coast M. trossulus is more tolerant to hypo-saline conditions but vulnerable to heat stress. Since salinity represents a critical factor in reproduction of marine species, we analyzed the responses of Mytilus galloprovincialis spermatozoa to hyposaline stress. We exposed mussels, in laboratory tanks, for 24 hours at 18°C to control (35.9 psu) and three hyposaline (17.1; 22.6 and 26.2 psu) conditions, and evaluated the expression of sperm hsp70 and protamine-like proteins genes. Further we analyzed the electrophoretic pattern, the DNA binding and the release from sperm nuclei of protamine-like proteins. For all experimental approaches used, the results obtained at 17.1 psu condition were very similar to those obtained in control condition, while alterations were always recorded at 22.6 and 26.2 psu conditions. Particularly, at 22.6 and 26.2 psu, was observed: 42.5 and 17.1-fold increase in hsp70 expression, respectively and hypo-expression of PL-II/PLIV protamine-like proteins genes. Further, electrophoretic mobility shift assays and salt-induced release of nuclear proteins from sperm nuclei, revealed alterations in the PL proteins/DNA binding, in these two hyposaline conditions. The similarity between the results obtained in control and in the more severe hyposaline condition (17.1 psu) could indicate a phenomenon of fertility preservation strategy due to gamete plasticity.