Tesi di Laurea di Aurora Sotgiu

Risposte agli stress termici e ricerca di marcatori molecolari nell’ottocorallo zooxantellato Maasella edwardsi (De Lacaze-Duthiers, 1888)

Nome Cognome

Aurora Sotgiu

Contatto

scienze.biologiamarina-info@unibo.it

Relatore/Correlatore

Ente e Laboratorio di Ricerca

C.I.R.S.A. Laboratorio di ecologia

Asbtract

The increase of thermal anomalies frequency, as result of global climate change, highlights the importance of studies on the biological responses and on the geographical distribution of the potentially sensitive species.

In this study we improved the knowledge on the biology and ecology of Maasella edwardsi (De Lacaze-Duthiers, 1888), one of the few zooxanthellate octocorals endemic of the Mediterranean Sea. Its dispersal ability and its sensibility to increasing temperature were investigated. In addition, the experimental protocols for the species genetic characterization, through the use of molecular markers at different resolving power, have been developed.

The response of two populations of Maasella edwardsi to an increased sea water temperature of 24°C, a value recorded in the field during a summer heat wave, has been investigated after 3 and 6 days. The natural habitats of the two populations are very different: one population live at 5 m in depth along the rocky coast of Porto San Paolo (OT), while the other at 20 m in depth on a coralligenous outcrop off of Chioggia (VE). The responses to thermal stress were evaluated in terms of variation in the zooxanthellae and photosynthetic pigments (chlorophyll a and c2) concentrations and of the expulsion rate of zooxanthellae by the colonies. The concentration of zooxanthellae didn’t vary in relation to the thermal stress applied. The effect of thermal stress was observed only in the Porto San Paolo population, where the concentration of chlorophyll a significantly decreased with the increasing temperature. The Chioggia population didn’t show any effect due to the thermal treatment. None of the two populations displayed differences in the chlorophyll c2 concentration under thermal stress, even after 6 days. The population of Chioggia had a higher concentration of pigment than the population of Porto San Paolo. The daily expulsion rate didn’t vary with thermal stress in any of the two populations; an inverse linear relationship between the daily expulsion rate and the concentration of zooxanthellae in the tissues of the organism has been detected.The dispersal ability of the species M. edwardsi was assessed through a colonization field experiment performed in Chioggia (2007-2009). Some artificial panels have been positioned with increasing distance from the rocky outcrop colonised by the octocoral. The colonization of travertine panels has been periodically monitored by photographic sampling. M. edwardsi recruited only on the panels located within the rocky outcrop, in the middle of the natural population, only after 2 years. This suggested a very low dispersal ability of the species.Genetic variation in both populations of Porto San Paolo and TM1 was analysed by testing the sequence polymorphism of the internal region of transcription of ribosomal DNA (ITS1) and polymorphism of a portion of the mitochondrial gene mtMSH. Efficacy of the markers has been identified by evaluating the degree of polymorphism and their ability to highlight genetic differentiation between the two populations. The two regions, albeit with a different level of interpopulation variability, have revealed a significant genetic structuring between the population of Porto San Paolo and Chioggia. Especially, the marker mtMSH in M. edwardsi resulted highly polymorphic (7 haplotypes of 14 sequences analysed).The high thermal stress resistance of Maasella edwardsi, compared to many other tropical and temperate zooxanthellate species, together with its fragmented geographical distribution and its low dispersal ability made it an interesting model species. The molecular markers developed will allow to study dispersion and differentiation in the Mediterranean Sea, in relation to its tolerance and / or evolutionary adaptation to the global change.