Chapter 2 Seagrass Genetics and Evolution
41
lack of spatially explicit information is unfortunate
as it can be utilized to infer clone sizes age, as well as
to characterize the genetic neighborhood of genets
within populations (see below).
The large differences in sampling effort, physical distances among samples, and in the resolution
of genetic markers employed make comparisons
among studies assessing clonal structure difficult.
Nevertheless, the use of high-resolution genetic
markers has repudiated earlier notions of clonal uniformity in several species, notably Thalassia testudinum (Kirsten et al., 1998), Posidonia australis
(Waycott, 1998) and Zostera marina (Reusch et al.,
1999d). Previously all three species were thought
to comprise only a few clones based on allozyme
markers, while DNA based markers revealed several
distinct genotypes at the m-scale.
A second major finding was that a widely varying
degree of clonal diversity occurs among locations
within species. For example, in Posidonia oceanica, there are sites in the Adriatic Sea with only a
single detectable genotype (Ruggiero et al., 2002);
whereas, other populations in more central areas
of the Mediterranean are multi-clonal (Procaccini
et al., 2001). A similar range in clonal diversity has
been observed in the Australian species Posidonia
australis (Waycott, 1998). The northern-temperate
species Zostera marina shows an even wider range of
clonal diversity across locations, from monoclonal
stands to populations where each genotype is unique
(Reusch et al., 2000).
The causes of widely varying clonal diversities
across locations and species are largely unknown.
The proximate reason for the variation observed is
probably a shift in the relative success of sexual vs.
vegetative reproduction. Although almost nothing is
known regarding how variation in reproductive output in seagrass populations translates into rates of
successful recruitment, some evidence points to extrinsic forcing favouring sexual recruitment. There is
limited evidence from eelgrass (Zostera marina) that
clonal diversity is higher at sites with greater physical disturbance (H¨ ammerli and Reusch, 2003b).
This is a plausible scenario because the chances of
establishment in closed seagrass canopies are low,
while gaps may facilitate the emergence of seedlings
and hence, new genotypes to the local population
(Eriksson and Fr¨ oborg, 1996). A recent study by
H¨ ammerli and Reusch (2003b) also found that genotypes with a higher individual heterozygosity, measured at nine polymorphic DNA microsatellite loci,
were larger on average than more inbred clones.
Over time, clonal competition seems to favour those
genets, which have a higher genetic diversity at the
level of individuals. Thus, the local diversity and
size distribution of clones also has a deterministic
component.
Populations at the distribution limit may also reveal low clonal diversity because sexual reproduction at marginal sites may be reduced or even completely prevented (‘geographic parthenogenesis’ of
Bierzychudek (1985). Such a scenario may apply to
seagrasses. For three different seagrass species findings from DNA based markers indicate that populations at the distributional margin may consist of very
large clones [Cymodocea nodosa: (Alberto et al.,
2001); Posidonia oceanica: (Ruggiero et al., 2002);
Zostera marina: (Reusch et al., 1999a); or possess
significantly reduced genetic diversity as in Posidonia australis: (Waycott et al., 1997)]. As a corollary,
successful sexual reproduction at entirely clonal sites
may be practically undetectable, although it must
have occurred at one point in time if we are not to invoke re-establishing of rafting vegetative fragments
as an explanation. Clearly, much more experimental
work at the within-population scale, coupled with
censuses at the geographic scale are needed to explain extent and variation in clonal diversity among
seagrasses, both at the species, and the population
level.
The clonal life-history strategy has important
ramifications for processes within populations (see
Duarte et al., Chapter 11), in particular for the mating system. In the centre of large clones, focal ramets
may be surrounded by members of the same clone
(Handel, 1985). In monoecious (Zostera spp.) and
hermaproditic (Posidonia spp.) seagrass genera, the
opportunity for outcrossing is thus constrained by
the spatial architecture of the genets in a meadow.
Not surprisingly, seeds in focal flowering shoots in
monoecious Zostera marina show selfing rates up
to 65% due to between-ramet geitonogamy when
growing in a selfing neighborhood (Reusch, 2001a).
Selection against selfed progeny effectively restores
Hardy-Weinberg-equilibrium in the adult population. Since fitness costs associated with inadvertent
between ramet selfing are substantial (Ruckelshaus,
1995; Reusch, 2001a), these findings may provide
one possible explanation for the evolution of dioecy
in seagrasses. Evidently, within-clone selfing is impossible in dioecious seagrasses, as there are separate male and female plants. However, the high
41
lack of spatially explicit information is unfortunate
as it can be utilized to infer clone sizes age, as well as
to characterize the genetic neighborhood of genets
within populations (see below).
The large differences in sampling effort, physical distances among samples, and in the resolution
of genetic markers employed make comparisons
among studies assessing clonal structure difficult.
Nevertheless, the use of high-resolution genetic
markers has repudiated earlier notions of clonal uniformity in several species, notably Thalassia testudinum (Kirsten et al., 1998), Posidonia australis
(Waycott, 1998) and Zostera marina (Reusch et al.,
1999d). Previously all three species were thought
to comprise only a few clones based on allozyme
markers, while DNA based markers revealed several
distinct genotypes at the m-scale.
A second major finding was that a widely varying
degree of clonal diversity occurs among locations
within species. For example, in Posidonia oceanica, there are sites in the Adriatic Sea with only a
single detectable genotype (Ruggiero et al., 2002);
whereas, other populations in more central areas
of the Mediterranean are multi-clonal (Procaccini
et al., 2001). A similar range in clonal diversity has
been observed in the Australian species Posidonia
australis (Waycott, 1998). The northern-temperate
species Zostera marina shows an even wider range of
clonal diversity across locations, from monoclonal
stands to populations where each genotype is unique
(Reusch et al., 2000).
The causes of widely varying clonal diversities
across locations and species are largely unknown.
The proximate reason for the variation observed is
probably a shift in the relative success of sexual vs.
vegetative reproduction. Although almost nothing is
known regarding how variation in reproductive output in seagrass populations translates into rates of
successful recruitment, some evidence points to extrinsic forcing favouring sexual recruitment. There is
limited evidence from eelgrass (Zostera marina) that
clonal diversity is higher at sites with greater physical disturbance (H¨ ammerli and Reusch, 2003b).
This is a plausible scenario because the chances of
establishment in closed seagrass canopies are low,
while gaps may facilitate the emergence of seedlings
and hence, new genotypes to the local population
(Eriksson and Fr¨ oborg, 1996). A recent study by
H¨ ammerli and Reusch (2003b) also found that genotypes with a higher individual heterozygosity, measured at nine polymorphic DNA microsatellite loci,
were larger on average than more inbred clones.
Over time, clonal competition seems to favour those
genets, which have a higher genetic diversity at the
level of individuals. Thus, the local diversity and
size distribution of clones also has a deterministic
component.
Populations at the distribution limit may also reveal low clonal diversity because sexual reproduction at marginal sites may be reduced or even completely prevented (‘geographic parthenogenesis’ of
Bierzychudek (1985). Such a scenario may apply to
seagrasses. For three different seagrass species findings from DNA based markers indicate that populations at the distributional margin may consist of very
large clones [Cymodocea nodosa: (Alberto et al.,
2001); Posidonia oceanica: (Ruggiero et al., 2002);
Zostera marina: (Reusch et al., 1999a); or possess
significantly reduced genetic diversity as in Posidonia australis: (Waycott et al., 1997)]. As a corollary,
successful sexual reproduction at entirely clonal sites
may be practically undetectable, although it must
have occurred at one point in time if we are not to invoke re-establishing of rafting vegetative fragments
as an explanation. Clearly, much more experimental
work at the within-population scale, coupled with
censuses at the geographic scale are needed to explain extent and variation in clonal diversity among
seagrasses, both at the species, and the population
level.
The clonal life-history strategy has important
ramifications for processes within populations (see
Duarte et al., Chapter 11), in particular for the mating system. In the centre of large clones, focal ramets
may be surrounded by members of the same clone
(Handel, 1985). In monoecious (Zostera spp.) and
hermaproditic (Posidonia spp.) seagrass genera, the
opportunity for outcrossing is thus constrained by
the spatial architecture of the genets in a meadow.
Not surprisingly, seeds in focal flowering shoots in
monoecious Zostera marina show selfing rates up
to 65% due to between-ramet geitonogamy when
growing in a selfing neighborhood (Reusch, 2001a).
Selection against selfed progeny effectively restores
Hardy-Weinberg-equilibrium in the adult population. Since fitness costs associated with inadvertent
between ramet selfing are substantial (Ruckelshaus,
1995; Reusch, 2001a), these findings may provide
one possible explanation for the evolution of dioecy
in seagrasses. Evidently, within-clone selfing is impossible in dioecious seagrasses, as there are separate male and female plants. However, the high