José Luis Silva Hernández de Santaolalla, University of Seville. Supervisor: J.A. Mejías. Sept. 2014.
Summary:
The section Pustulati of the genus Sonchus
(Asteraceae) is only constituted by three species of cliff habitat: S.
pustulatus Willk., S. fragilis Ball and S. masguindalii Pau & Font Quer,
whose populations are restricted to very reduced non-overlapping areas
within the Baetic-Rifan region (Western Mediterranean). The three
species are in the Rifan range (northwestern Africa) and only S.
pustulatus is in the Baetic range (southeastern Iberian Peninsula). The
two main goals of this PhD thesis have been (i) to unravel the causes
that have led to these little known species to such stenoicity and
disjunction on both sides of the Alboran Sea, and (ii) to diagnose the
state of conservation of their populations and to analyze the factors
that determine their persistence. To address these objectives, a
multidisciplinary approach has been used, based on a detailed study of
the demographic, ecological, phylogenetic and reproductive
characteristics of these species, covering virtually all known
populations. From the application of molecular dating it has been
estimated that the origin and diversification of these species happened
during the Messinian Salinity Crisis and the subsequent Zanclean
reflooding of the Mediterranean basin prior to the establishment of the
Mediterranean climate, when the environment was mild and wetter in the
region. The Zanclean reflooding, the subsequence establishment of the
Mediterranean climate and the Quaternary glaciations should lead to
strong constrictions of the geographical distribution of the three
species, which would be responsible for their current rarity and the
condition of relict. The biological features detected in these species
suggest that the current geographical disjunction of S. pustulatus is
due to a vicariance process mediated by the reflooding of the
Mediterranean Sea. Nonetheless, these species developed a strong
resilience in certain cliffs which have served them as essentially
microclimatic refuge. The northerly orientation, rocky substratum,
proximity to the sea and low altitude of these cliffs maintain enough
levels of air humidity, which buffer the extreme high and low
temperatures characteristic of the Mediterranean climate and the
Quaternary glaciations, respectively. It has been found out that, while
the persistence of the taxa in the Rifan range seems to basically depend
on the conservation of their habitats, S. pustulatus in the Baetic range
is seriously threatened, and it therefore requires greater conservation
efforts. In the latter range, the species seems to be on the limit of
its ecological amplitude as is suggested, among other features, by the
demographic decline, the reduced area of occupancy, the small size of
its populations and the low neutral and S allele (responsible for the
self-incompatibility) genetic diversities. On the other hand, the
presence of a sporophytic genetic control of the self-incompatibility
systems has been demonstrated, which is not always totally effective at
individual level. The presence of a potentially mixed mating system in a
population of S. fragilis has also been highlighted, that is an
outstanding material for future studies about the evolution of plant
reproductive systems in a species with demographically stable
populations and a relatively high lifespan.