Chapter 2 Seagrass Genetics and Evolution
37
Fig. 6. Distribution of the genetic variability of Posidonia oceanica in the Mediterranean basin. Analysis is based on data from six
microsatellite loci. Tree is based on (δµ)
2 distance values (modified from Procaccini et al., 2002).
presence of seasonal circulation gyres in this area
(Astraldi and Gasparini, 1994). The Mediterranean
Sea has experienced dramatic changes of environmental parameters related to glaciations and hypersalinity crises (Maldonado, 1985; Thiede, 1978).
The evolution of the basin was controlled by the
interplay between geological processes and environmental parameters determining species loss and recolonisation in particular areas. Dramatic fluctuations of more than 100 m of sea water level during
last glaciation surely have caused the disappearance
of Posidonia from some areas. Thus, the actual distribution could have originated by recolonisation from
populations persisting in relict zones and the low genetic variability detected in some areas could have
originated from a founder effect. The latter possibility seems to be the case for the uniclonal patches of
the North-Adriatic, where locally adapted genotypes
are now present in the area (Ruggiero et al., 2002).
New polymorphic di-nucleotide microsatellite
loci recently selected (Alberto et al., 2003a) are
showing higher polymorphism levels. Preliminary
analysis at basin level is confirming at broad scale
the separation between eastern and western Mediterranean populations. Intra-population analyses instead are showing higher heterozygosity and clonal
diversity values (Alberto et al., 2003a; ArnaudHaond personal communication).
4. Zostera marina
This species has had the greatest number of publications in the genetic analysis literature for seagrasses
(Moore et al., Chapter 16). Nonetheless, only recently a first study has been produced that documents the genetic diversity and genetic structure of
the species throughout almost its entire geographical range (Olsen et al., 2004). This shortcoming
may be due in part to the particularly widespread
distribution of this species which occurs across the
majority of available habitats in the northern hemisphere. Studies along the Pacific coasts of North
America date back to 1992–1994 and have been carried out using RFLP (Fain et al., 1992), allozymes
(Laushman, 1993) and DNA fingerprinting analyses
(Alberte et al., 1994). Of these studies, DNA fingerprinting showed higher polymorphism within meadows, as expected for this marker type. More recently,
other studies have utilized allozymes to compare distinct Z. marina meadows in California and Chesapeake Bay, showing Fst values ranging from 0.06 to
0.335, depending on the geographic distance among
meadows (Williams and Davis, 1996; Williams and
Orth, 1998). In the last few years, genetic diversity in Z. marina has been studied extensively along
the northern coasts of Europe using polymorphic
microsatellite loci (Reusch, 1999c among the others). The use of the same markers shows high allelic richness in the Pacific populations in respect
to the Atlantic ones and clear genetic distinction
between southern and northern east-Pacific populations (Olsen et al., 2004). Unfortunately, the values of polymorphism observed using microsatellites
could not be compared directly with results obtained
from markers utilized previously.
The aforementioned study on the distribution of
microsatellite genetic diversity along the whole geographic range of the species (Olsen et al., 2004)
37
Fig. 6. Distribution of the genetic variability of Posidonia oceanica in the Mediterranean basin. Analysis is based on data from six
microsatellite loci. Tree is based on (δµ)
2 distance values (modified from Procaccini et al., 2002).
presence of seasonal circulation gyres in this area
(Astraldi and Gasparini, 1994). The Mediterranean
Sea has experienced dramatic changes of environmental parameters related to glaciations and hypersalinity crises (Maldonado, 1985; Thiede, 1978).
The evolution of the basin was controlled by the
interplay between geological processes and environmental parameters determining species loss and recolonisation in particular areas. Dramatic fluctuations of more than 100 m of sea water level during
last glaciation surely have caused the disappearance
of Posidonia from some areas. Thus, the actual distribution could have originated by recolonisation from
populations persisting in relict zones and the low genetic variability detected in some areas could have
originated from a founder effect. The latter possibility seems to be the case for the uniclonal patches of
the North-Adriatic, where locally adapted genotypes
are now present in the area (Ruggiero et al., 2002).
New polymorphic di-nucleotide microsatellite
loci recently selected (Alberto et al., 2003a) are
showing higher polymorphism levels. Preliminary
analysis at basin level is confirming at broad scale
the separation between eastern and western Mediterranean populations. Intra-population analyses instead are showing higher heterozygosity and clonal
diversity values (Alberto et al., 2003a; ArnaudHaond personal communication).
4. Zostera marina
This species has had the greatest number of publications in the genetic analysis literature for seagrasses
(Moore et al., Chapter 16). Nonetheless, only recently a first study has been produced that documents the genetic diversity and genetic structure of
the species throughout almost its entire geographical range (Olsen et al., 2004). This shortcoming
may be due in part to the particularly widespread
distribution of this species which occurs across the
majority of available habitats in the northern hemisphere. Studies along the Pacific coasts of North
America date back to 1992–1994 and have been carried out using RFLP (Fain et al., 1992), allozymes
(Laushman, 1993) and DNA fingerprinting analyses
(Alberte et al., 1994). Of these studies, DNA fingerprinting showed higher polymorphism within meadows, as expected for this marker type. More recently,
other studies have utilized allozymes to compare distinct Z. marina meadows in California and Chesapeake Bay, showing Fst values ranging from 0.06 to
0.335, depending on the geographic distance among
meadows (Williams and Davis, 1996; Williams and
Orth, 1998). In the last few years, genetic diversity in Z. marina has been studied extensively along
the northern coasts of Europe using polymorphic
microsatellite loci (Reusch, 1999c among the others). The use of the same markers shows high allelic richness in the Pacific populations in respect
to the Atlantic ones and clear genetic distinction
between southern and northern east-Pacific populations (Olsen et al., 2004). Unfortunately, the values of polymorphism observed using microsatellites
could not be compared directly with results obtained
from markers utilized previously.
The aforementioned study on the distribution of
microsatellite genetic diversity along the whole geographic range of the species (Olsen et al., 2004)