38
Michelle Waycott, Gabriele Procaccini, Donald H. Les and Thorsten B. H. Reusch
provides a much comprehensive, comparable picture of the overall phylogeographic pattern of this
species. Among the previous broader scale studies on Zostera marina to date, pronounced genetic
sub-structuring was observed among eight European populations, with a strong linear relationship of
genetic differentiation along geographic distances
of 12–4,500 km (Reusch et al., 2000). However, on
a larger geographic scale the observed pattern was
counterintuitive, with two North American populations clustering with those from the Baltic Sea
and North Sea. New data support recent genetic
exchange in Z. marina, between the east Pacific,
west and east Atlantic, suggesting a still-active transArctic connections (Olsen et al., 2004). Instead, a
weak correlation of genetic and geographic distance
was found in populations sampled in the northern
Wadden Sea and south-western Baltic Sea, indicating enhanced metapopulation dynamics in the area
(Reusch, 2002). Recolonisation in the two areas may
have occurred recently, masking any signal resulting from recent gene flow among populations. In
general, populations showed high connectivity (θ =
0.018), despite the fact that populations are annual
in the Wadden Sea and perennial in the Baltic. Theoretically annual Wadden Sea populations may show
higher connectivity due to greater propagule production and stronger tidal currents. The vectors of such
high gene flow are still unclear. Assignment tests
conducted on rafting reproductive shoots shows that
they can have an important role in dispersing genotypes up to 50 km (Reusch, 2002).
5. Thalassia testudinum
This species occurs throughout the Caribbean and
Gulf of Mexico (Tussenbroek et al., Chapter 18),
where periodic die-off of above ground biomass has
been observed (Robblee et al., 1991). Studies on genetic diversity among different localities have been
conducted using allozymes (Schlueter and Guttman,
1998), RAPD (Kirsten et al., 1998) and AFLP markers (Waycott and Barnes, 2001); microsatellite markers are currently under development (Waycott, unpublished; Kor-Jent unpublished). As for the other
species discussed above, a comparison among the
results obtained is difficult to make because of differences in the level of genotypic polymorphism detected by the different markers utilized. Eighteen
meadows have been sampled in the lower Florida
Keys and allozyme analyses show, as expected, low
genetic variation within and between sites (Gst =
0.050). Although these low values of genetic variability could be due to low inherent polymorphism
of the markers, the general trend indicated seems
to be real. RAPD and AFLP markers, in fact, also
confirm high population connectivity with higher,
but still low, within-site genetic variation. In both
RAPD and AFLP analyses the partitioning of genetic
diversity was distributed similarly, with a higher percent of variation (81.2–96.6%) within meadows. No
AFLP differentiation was found between the two regions analysed (overall mean Fst = 0.03). In general, T. testudinum appears to be a species with weak
genetic structure and high homogeneity within its
distributional range. The nature of that uniformity
is still uncertain, although possibly the result of a
recent colonization of the region, following changes
in habitat occurring since the Eocene (Waycott and
Barnes, unpublished data). Studies on the effects of
heavy human impact in the area, the importance of
Thalassia for the coastal dynamics and more detailed genetic analyses using co-dominant molecular
markers (microsatellites) are underway.
6. Other Studies
A modicum of additional information exists on the
distribution of genetic diversity in other seagrass
species. The introduced species Halophila stipulacea has high RAPD diversity in meadows sampled along the Sicilian coasts and high population
connectivity between distinct localities (Procaccini
et al., 1999a). Absence of clear differentiation in
the ITS regions between Mediterranean and Red
Sea populations supports the hypothesis of a recent Lessepsian introduction in the Mediterranean
(Ruggiero and Procaccini, 2004). The congeneric
H. johnsonii shows very high clonality and genetic homogeneity in south-eastern Florida, with the
same RAPD phenotype present in more than 50%
of the samples (Freshwater et al., 2003). In a recent analysis of morphological and genetic variation
of Halophila hawaiiana (McDermid et al., 2003),
no genetic structure was revealed, although the authors utilized a chloroplast DNA homopolymer that
may not yield more than species level variability
(Waycott and Freshwater, unpublished). Cymodocea
nodosa seems to occur only as one or few genotypes in Ria Formosa (southern Portugal) (Alberto
et al., 2001); whereas, higher genetic variability was found in meadows in the central/southern
Michelle Waycott, Gabriele Procaccini, Donald H. Les and Thorsten B. H. Reusch
provides a much comprehensive, comparable picture of the overall phylogeographic pattern of this
species. Among the previous broader scale studies on Zostera marina to date, pronounced genetic
sub-structuring was observed among eight European populations, with a strong linear relationship of
genetic differentiation along geographic distances
of 12–4,500 km (Reusch et al., 2000). However, on
a larger geographic scale the observed pattern was
counterintuitive, with two North American populations clustering with those from the Baltic Sea
and North Sea. New data support recent genetic
exchange in Z. marina, between the east Pacific,
west and east Atlantic, suggesting a still-active transArctic connections (Olsen et al., 2004). Instead, a
weak correlation of genetic and geographic distance
was found in populations sampled in the northern
Wadden Sea and south-western Baltic Sea, indicating enhanced metapopulation dynamics in the area
(Reusch, 2002). Recolonisation in the two areas may
have occurred recently, masking any signal resulting from recent gene flow among populations. In
general, populations showed high connectivity (θ =
0.018), despite the fact that populations are annual
in the Wadden Sea and perennial in the Baltic. Theoretically annual Wadden Sea populations may show
higher connectivity due to greater propagule production and stronger tidal currents. The vectors of such
high gene flow are still unclear. Assignment tests
conducted on rafting reproductive shoots shows that
they can have an important role in dispersing genotypes up to 50 km (Reusch, 2002).
5. Thalassia testudinum
This species occurs throughout the Caribbean and
Gulf of Mexico (Tussenbroek et al., Chapter 18),
where periodic die-off of above ground biomass has
been observed (Robblee et al., 1991). Studies on genetic diversity among different localities have been
conducted using allozymes (Schlueter and Guttman,
1998), RAPD (Kirsten et al., 1998) and AFLP markers (Waycott and Barnes, 2001); microsatellite markers are currently under development (Waycott, unpublished; Kor-Jent unpublished). As for the other
species discussed above, a comparison among the
results obtained is difficult to make because of differences in the level of genotypic polymorphism detected by the different markers utilized. Eighteen
meadows have been sampled in the lower Florida
Keys and allozyme analyses show, as expected, low
genetic variation within and between sites (Gst =
0.050). Although these low values of genetic variability could be due to low inherent polymorphism
of the markers, the general trend indicated seems
to be real. RAPD and AFLP markers, in fact, also
confirm high population connectivity with higher,
but still low, within-site genetic variation. In both
RAPD and AFLP analyses the partitioning of genetic
diversity was distributed similarly, with a higher percent of variation (81.2–96.6%) within meadows. No
AFLP differentiation was found between the two regions analysed (overall mean Fst = 0.03). In general, T. testudinum appears to be a species with weak
genetic structure and high homogeneity within its
distributional range. The nature of that uniformity
is still uncertain, although possibly the result of a
recent colonization of the region, following changes
in habitat occurring since the Eocene (Waycott and
Barnes, unpublished data). Studies on the effects of
heavy human impact in the area, the importance of
Thalassia for the coastal dynamics and more detailed genetic analyses using co-dominant molecular
markers (microsatellites) are underway.
6. Other Studies
A modicum of additional information exists on the
distribution of genetic diversity in other seagrass
species. The introduced species Halophila stipulacea has high RAPD diversity in meadows sampled along the Sicilian coasts and high population
connectivity between distinct localities (Procaccini
et al., 1999a). Absence of clear differentiation in
the ITS regions between Mediterranean and Red
Sea populations supports the hypothesis of a recent Lessepsian introduction in the Mediterranean
(Ruggiero and Procaccini, 2004). The congeneric
H. johnsonii shows very high clonality and genetic homogeneity in south-eastern Florida, with the
same RAPD phenotype present in more than 50%
of the samples (Freshwater et al., 2003). In a recent analysis of morphological and genetic variation
of Halophila hawaiiana (McDermid et al., 2003),
no genetic structure was revealed, although the authors utilized a chloroplast DNA homopolymer that
may not yield more than species level variability
(Waycott and Freshwater, unpublished). Cymodocea
nodosa seems to occur only as one or few genotypes in Ria Formosa (southern Portugal) (Alberto
et al., 2001); whereas, higher genetic variability was found in meadows in the central/southern