Processi microevolutivi e genesi degli hotspot di biodiversità intraspecifica
Author(s)
Sacco, Florinda
Date Issued
February 10, 2011
Type
Doctoral Thesis
Abstract
The southern Mediterranean peninsulas are well-known hotspots of biodiversity at both species
and intraspecific level. This characteristic makes them excellent natural laboratories for studies to shed
light on microevolutionary dynamics that determine the geographic distribution of genetic diversity and
the genesis of intraspecific biodiversity hotspots.
The richness in intraspecific genetic diversity has been attributed to the role of refuge played by
these areas for temperate species during the severe climatic oscillations of the Pleistocene. One
proposed scenario describes these areas as historical repositories of genetic variability for many
temperate species, given that they offered the possibility for the long-term persistence of large and
demographically stable populations. According to another scenario, known as “refugia within refugia”,
high variability in areas of putative refuge would instead be attributed to microevolutionary processes
that would lead to an increase in genetic diversity, such as allopatric fragmentation, possibly followed
by secondary contacts.
In this study we investigated the geographic variation of three species of amphibians, Salamandra
salamandra, Lissotriton italicus and Rana dalmatina, with the aim of investigating whether and to what
extent the two evolutionary scenarios described above may have influenced the history of populations
of these species. The results of the individual studies were then compared, to individuate lines of
concordance and/or divergence, in order to detect aspects of general validity for the microevolutionary
processes most involved in determining the formation of a hotspot of intraspecific biodiversity.
Genetic variation in the Italian populations of S. salamandra was investigated using both nuclear
(23 allozyme loci) and mitochondrial markers. Although the two classes of markers both showed a
geographically structured pattern of genetic diversity, they disagree strongly regarding the location of
the main phylogeographic discontinuity observed. This discrepancy indicates a secondary contact
between the two subspecies S.s. salamandra and S. s. giglioli, owing to an event of differential
introgresseve hybridization at different loci. Seven allozyme loci suggest the Ligurian Apennines as the
area of secondary contact. There subsequently took place a selectively favoured introgression of the S.
s. salamandra’ mitochondrial form into the S. s. giglioli. This introgression replaced the S. s. giglioli’
mitochondrial form completely as far as the Volturno basin, the phylogeographical break for the
southern and northern groups of S. salamandra gigliolii. Overall, the hotspot of genetic diversity of Italian
S. salamandra is located in the Ligurian and Northern Apennines, and its origin can be attributed to a
process of allopatric fragmentation and introgressive hybridization.
Genetic variation in populations of Lissotriton italicus was investigated by analysis of sequences of
two mitochondrial gene fragments (ND2 and ND4; overall 1897bp) of individuals from 23 localities
spanning the entire species range. The analysis revealed a phylogeographical pattern of unexpected
complexity, according to a previous allozyme survey of variation. Indeed, our data provided evidence of
two strongly divergent lineages (6,8%), which had a parapatric distribution, one restricted to part of the
Calabrian peninsula, the other widespread throughout the rest of the species range. Both lineages are
further subdivided, giving a total of eight terminal haplogroups, five of which are found in Calabria.
This pattern suggests that the species underwent repeated cycles of allopatric fragmentation throughout
the Plio-Pleistocene; in particular, during the Late Pleistocene glacial cycle the Italian newt was
fragmented into eight refugia. The discrepancy between our data and a previous allozyme survey of
variation is due to the former study’s incomplete sampling in Calabria, in the exact area that was
subsequently identified as multiple refugia for the Italian newt.
Genetic variation in populations of Rana dalmatina was first investigated by analysis of
sequences of one mitochondrial gene fragment (16S; 421 bp) of individuals sampled in 22 localities
from Italy, Slovenia and Croatia, along with individual from Spain, Germany and Moldavia taken from
a GenBank. Our results provided evidence that the highest level of genetic diversity of R. dalmatina is
located in Italy. Genetic variation in Italian, Croatian and Slovenian populations of the agile frog was
further analysed using two other mitochondrial gene fragments (Cyt-b and COI; overall 1404 bp) of
individuals from forty localities. Two divergent haplogroups were identified (4,2%), one found in the
population located above and the other below the northern edge of the Volturno river’s plain. Both
haplogroups are further subdivided into other haplogroups. Overall, our data showed a geographically
structured pattern of genetic diversity. The Calabrian peninsula proved to be the hotspot of
intraspecific diversity for R. dalmatina, as one principal haplogroup and 59% of haplotypes were found
there. Both phylogeographical and historical demographical analysis suggested that the pattern of
genetic diversity was principally shaped by allopatric fragmentation in multiple refugia, followed by
expansion and then by secondary contact.
Overall, despite the peculiarities of their evolutionary histories, patterns of genetic diversity of
the three species studied were similar to each other, since all showed a highly structured geographical
pattern. Comparing these patterns, important spatial coincidences in the main locations of phylogenetic
discontinuity were also identified. Although the locations of their hotspots were different, the origin of
each hotspot was determined by allopatric fragmentation, possibly followed by expansion and
introgressive hybridization and not by demographic stability of large populations. Similar patterns,
determined by the same processes as described above, have been documented in other species not only
in the south of the Italian peninsula, but also in species from other regions of the world with different
paleogeographic and/or paleoclimatic history. Therefore, these processes are probably the common
feature in the history of the formation of a hotspot of intraspecific diversity.
Finally, those areas where we found phylogeographic discontinuity and/or contact zones should
for many species not be regarded as simple hotspots of intraspecific diversity, but as evolutionary
hotspots. Identifying such areas will help us realize the fundamental goal of conservation biology:
protecting biodiversity and the processes that generate and sustain it.
Additional information
Dottorato di ricerca in Ecologia e gestione delle risorse biologiche
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