44
Michelle Waycott, Gabriele Procaccini, Donald H. Les and Thorsten B. H. Reusch
(Waycott and Sampson, 1997) although the exact
structure of the population sampled for mating system analysis was not documented. It should be noted
however that a large proportion of seagrass populations studied to date have mixed population structure
(see proceeding sections this chapter) and in such
populations there will be a high probability that seed
production will result from outcrossing in these populations. However, it is important to document fine
scale population structure alongside the mating system in seagrasses to fully appreciate the implications
of any measurement of outcrossing (or by implication inbreeding). In addition, the hydrodynamics of
pollen movement needs to be understood better to account for pollen movement and the interaction of the
biotic and abiotic factors affecting successful sexual reproduction (Cox et al., 1992; Verduin, 1996;
Ackerman, 1997).
Mating system estimates provide a more detailed
understanding of population processes such as small
scale gene flow, the frequency of inbreeding and estimates of inbreeding depression (e.g. Ruckelshaus,
1995; H¨ ammerli and Reusch, 2003b). The consequences of inbreeding can significantly impact
a species primarily due to reduction in progeny
fitness (Charlesworth and Charlesworth, 1987;
Ellstrand and Elam, 1993). For this reason alone,
estimates of inbreeding are valuable and should be
obtained for a greater range of seagrass species.
IV. Concluding Remarks
A. Inferences for Ecology, Conservation
and Rehabilitation
Outcomes from population genetic analysis can
substantially influence the way we perceive how seagrass ecosystems interact and can identify where
important reservoirs of genetic diversity may exist.
Different scales of gene flow can be very important
for assessing the real significance of geographic isolation and habitat fragmentation in the context of
protecting and managing seagrass meadows. Many
recent results suggest that seagrasses are capable of
at least some long distance dispersal (e.g. Reusch
et al., 2000; Waycott and Barnes, 2001; Les et al.,
2002; Reusch, 2002; Waycott et al., 2002). These results based on broader scale population genetic and
phylogeographic studies imply that the perception
of seagrasses as isolated disconnected populations
requires reassessment. Considerable insight will be
gained from studies that explore these processes using both an ecological and genetic approach. In addition to the outcomes for an improved understanding
of population ecology such studies enhance our ability to reconstruct historical processes that have lead
to isolation of populations.
Determining population genetic isolation is particularly important for defining correct strategies for
collecting transplants to re-establish lost seagrass
meadows, a practice now widespread (Fonseca et al.,
1998). Broad scale studies identify areas with lower
genetic diversity and lower exchange with other localities. The definition of genetic distinction among
populations facilitates selection of appropriate donor
beds for transplantation programs, according to their
relative levels of polymorphism and genetic distinctiveness. In such studies, genetic diversity needs to
be carefully defined, as it has two levels in seagrasses, genotypic diversity and genetic diversity.
Previous studies were often unable to distinguish
between both levels since markers were not polymorphic enough. However, this may be critical when
selecting donor beds. For example, a few large heterozygous clones may be ideal starting material for
transplantations, yet they could be assessed as genetically uniform when clonal diversity cannot be
separated from genetic diversity. A transplant study
on Posidonia oceanica has suggested, in fact, that
transplants coming from genetically polymorphic
populations have a greater probability of survival
and perform better in common garden experiments
(Procaccini and Piazzi, 2001). In addition, understanding the genetic consequences of transplantation
may allow us to better infer the ability of those beds
to survive in longer time frames (e.g. see Williams,
2001).
B. Interactions of Scale and the Appropriate
Use of Molecular Markers
The preceding sections of this chapter present a
broad range of scale in genetic analyses of seagrasses from phylogenetic relationships at the subclass level to centimetre scale measures of gene flow.
The interpretation of processes should be made with
these differences in scale in mind. For example, it is
clear that Zostera marina represents a distinct evolutionary lineage to the southern hemisphere Zostera
species based on molecular phylogenetic results (Les
et al., 2002). However, these analyses were done with
Michelle Waycott, Gabriele Procaccini, Donald H. Les and Thorsten B. H. Reusch
(Waycott and Sampson, 1997) although the exact
structure of the population sampled for mating system analysis was not documented. It should be noted
however that a large proportion of seagrass populations studied to date have mixed population structure
(see proceeding sections this chapter) and in such
populations there will be a high probability that seed
production will result from outcrossing in these populations. However, it is important to document fine
scale population structure alongside the mating system in seagrasses to fully appreciate the implications
of any measurement of outcrossing (or by implication inbreeding). In addition, the hydrodynamics of
pollen movement needs to be understood better to account for pollen movement and the interaction of the
biotic and abiotic factors affecting successful sexual reproduction (Cox et al., 1992; Verduin, 1996;
Ackerman, 1997).
Mating system estimates provide a more detailed
understanding of population processes such as small
scale gene flow, the frequency of inbreeding and estimates of inbreeding depression (e.g. Ruckelshaus,
1995; H¨ ammerli and Reusch, 2003b). The consequences of inbreeding can significantly impact
a species primarily due to reduction in progeny
fitness (Charlesworth and Charlesworth, 1987;
Ellstrand and Elam, 1993). For this reason alone,
estimates of inbreeding are valuable and should be
obtained for a greater range of seagrass species.
IV. Concluding Remarks
A. Inferences for Ecology, Conservation
and Rehabilitation
Outcomes from population genetic analysis can
substantially influence the way we perceive how seagrass ecosystems interact and can identify where
important reservoirs of genetic diversity may exist.
Different scales of gene flow can be very important
for assessing the real significance of geographic isolation and habitat fragmentation in the context of
protecting and managing seagrass meadows. Many
recent results suggest that seagrasses are capable of
at least some long distance dispersal (e.g. Reusch
et al., 2000; Waycott and Barnes, 2001; Les et al.,
2002; Reusch, 2002; Waycott et al., 2002). These results based on broader scale population genetic and
phylogeographic studies imply that the perception
of seagrasses as isolated disconnected populations
requires reassessment. Considerable insight will be
gained from studies that explore these processes using both an ecological and genetic approach. In addition to the outcomes for an improved understanding
of population ecology such studies enhance our ability to reconstruct historical processes that have lead
to isolation of populations.
Determining population genetic isolation is particularly important for defining correct strategies for
collecting transplants to re-establish lost seagrass
meadows, a practice now widespread (Fonseca et al.,
1998). Broad scale studies identify areas with lower
genetic diversity and lower exchange with other localities. The definition of genetic distinction among
populations facilitates selection of appropriate donor
beds for transplantation programs, according to their
relative levels of polymorphism and genetic distinctiveness. In such studies, genetic diversity needs to
be carefully defined, as it has two levels in seagrasses, genotypic diversity and genetic diversity.
Previous studies were often unable to distinguish
between both levels since markers were not polymorphic enough. However, this may be critical when
selecting donor beds. For example, a few large heterozygous clones may be ideal starting material for
transplantations, yet they could be assessed as genetically uniform when clonal diversity cannot be
separated from genetic diversity. A transplant study
on Posidonia oceanica has suggested, in fact, that
transplants coming from genetically polymorphic
populations have a greater probability of survival
and perform better in common garden experiments
(Procaccini and Piazzi, 2001). In addition, understanding the genetic consequences of transplantation
may allow us to better infer the ability of those beds
to survive in longer time frames (e.g. see Williams,
2001).
B. Interactions of Scale and the Appropriate
Use of Molecular Markers
The preceding sections of this chapter present a
broad range of scale in genetic analyses of seagrasses from phylogenetic relationships at the subclass level to centimetre scale measures of gene flow.
The interpretation of processes should be made with
these differences in scale in mind. For example, it is
clear that Zostera marina represents a distinct evolutionary lineage to the southern hemisphere Zostera
species based on molecular phylogenetic results (Les
et al., 2002). However, these analyses were done with