Eterogeneità ambientale e variabilità genetica in Aphanius fasciatus (Teleostea)
Author(s)
Pontremolesi, Alessandra
Date Issued
February 10, 2011
Type
Doctoral Thesis
Abstract
The study of genetic diversity is fundamental in understanding basic process in
evolutionary ecology. The relationships between the level of genetic diversity and the
ecological features of the habitat has been widely explored in recent years, but some doubt
still exists about the reciprocal relevance of the evolutionary mechanisms involved.
In this thesis the ciprinodont fish Aphanius fasciatus, typical of brackish waters, has
been studied to elucidate the influence of environmental fluctuations of the gene pool of
natural populations. The reason to make this choice has been, among others, that there are
previous studies carried on the population living at the Saline di Tarquinia (Tarquinia
Salterns) so that a mean term study, encompassing many generations, could be performed.
Also, the population inhabiting Tarquinia Salterns has undergone a progressive stiffening of
the environmental conditions due to the end of the salt production then followed by the
ecological restoration of the site (EU Life Natura Project). This implies that the site of study
allowed investigating the evolutionary response of local population not only to the seasonal
strong fluctuation of the environment but also to the progressive habitat degradation (and
subsequent recovery). The study has been therefore structured according to a time table
reflecting the levels of environmental stress recorded: basic situation, immediately after the
end of salt extraction (1998 – pre-LIFE); stressed situation, immediately before and during
the works for the ecological restoration (2003-2005 – LIFE stage); after the recovery of the
salterns (2007-2010 – after LIFE).
The genetic structure of A. fasciatus was investigated using allozymes (Mendelian
markers possibly under selection) and microsatellites (hypervariable and neutral Mendelian
markers) in order to evaluate the relevance of the main evolutionary forces in moulding the
gene pool of the species under stressing conditions. In particular, genetic drift and selection
were under score since gene flow is notably restricted in this species, due to the natural
fragmentation and isolation of its habitat, the coastal lagoons.
Both markers (allozymes and microsatellites) agree in evidencing that the stiffening
of the environmental conditions (increasing of water temperature and salinity and reduction
of dissolved oxygen) is associated with genetic erosion. This loss of genetic variability is
widespread over a relevant number of loci (mainly allozymic) and is associated to a decrease
of the effective size of the population (Ne), thus suggesting that the genetic drift is the
mechanism underlying the random loss of alleles recorded. Also, the reduced water
circulation among the salt-pans before the environmental recovery has determined a Nord-
South cline in the salinity (increasing) and oxygen concentration (decreasing). Coincident to
this cline there is a correspondent continuum variation at locus ADA, with the allele ADA100
negatively correlated to the oxygen concentration. This finding suggests that selective
processes are also in action and such hypothesis is supported by the finding of a significant
heterozygote deficit at ADA and by statistics showing ADA as an “outlier” locus candidate
for selection.
The comparison of allozyme and microsatellite data reveals some discrepancies,
mainly a low, unexpected variability at microsatellites in the Tarquinia sample. This
observation could be explained considering that in heterogeneous habitats, such as salterns,
the maintenance of genetic variability at coding loci may be due to natural selection,
favouring different genotypic arrays in different micro-habitats. In other words, the allozyme
pattern would be due to the combined action of both genetic drift and selection, acting
simultaneously during environmental stress, but in different directions: the genetic drift
would erode the genetic variability affecting all loci while selection would maintain different
genotypes (hence polymorphisms) at some coding loci. As a consequence, the stiffening of
the environmental conditions would increase the strength of genetic drift alone on
microsatellite (neutral) loci, causing a stronger decrease in the level of their polymorphism.
Another piece of data comes from the analysis of the genetic structure of some
populations of A. fasciatus representative of the whole species range. The results obtained
have shown that geographical and hydrographical features of the coastal habitat have a
notable influence on the genetic structure of A. fasciatus. In homogeneous habitats
characterized by high human impact and anoxic crises, such as Orbetello lagoon, both
allozymes and microsatellites show a significant decrease of the effective population size and
a low genetic variability. On the contrary, habitat characterized by an environmental
heterogeneity, as Tarquinia salterns are, show a balancing action of genetic drift and
selection so that a certain extent of genetic variability is maintained, linked to the adaptive
response to the environmental changes. Finally, there are “mix” environments, as in the
lagoon Stagnone di Marsala, where a large, stable lagoon is in close proximity to a still
operating saltern. Here high levels of genetic variability are observed with both markers.
This is likely due to the fact that the lagoon represents a steady habitat, hosting a
demographically stable population able to keep its own genetic variability, periodically
increased by the immigration of specimens from the nearby saltern, where the selective
processes and the habitat choice, associated to the fluctuating environmental conditions,
maintain a number of genotypic arrays.
The case of the Stagnone di Marsala provides some final considerations concerning
the conservation of coastal habitats, with particular regard to the Tarquinia salterns. If the
final aim of the management is the maintenance of high demographic numbers and genetic
variability of local populations, a good solution could be the to have a wider steady lagoon
neighbouring a smaller heterogeneous (in space and time) zone, this latter able to provide
different genotypes to the lagoon, warranting in turn a demographic stability.
Additional information
Dottorato di ricerca in Ecologia e gestione delle risorse biologiche
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