Chapter 2 Seagrass Genetics and Evolution
39
Tyrrhenian Sea (Mediterranean Sea) and in the Cadiz
Bay, Spain (Atlantic Sea) by means of microsatellite analyses (Alberto et al., 2003b; Ruggiero et al.,
in press; Ruggiero and Procaccini, unpublished).
Zostera noltii seems to posses a moderate degree of
variability, but very low population connectivity in
populations sampled in central/southern Tyrrhenian
Sea (Procaccini and Ruggiero, unpublished). Ongoing studies on the complete distribution range of
the species are showing significant clustering among
distinct geographical regions and high degree of
population differentiation at smaller regional scale
(Coyer et al., submitted). More data on both Cymodocea nodosa and Zostera noltii will soon be available
using the same markers from several distinct localities inside and outside Mediterranean (Coyer et al.,
in press). Halodule wrightii appears to be distributed
in genetically distinct meadows, with high variability, along Texas and Florida coasts (Angel, 2002). A.
antarctica seems to be represented by a single clone
along the whole southern Australian coast (Waycott
et al., 1996).
To date, studies generally confirm that patterns
of genetic diversity in seagrass species are determined by a series of interconnected factors that are
not always easy to distinguish. There is no readily
identifiable trend common among species belonging to the same genus, or those theoretically having
similar or identical theoretical dispersal capabilities.
To more complete understanding of factors causing
genetic structure over wide geographical ranges requires the knowledge of local environmental factors,
current regimes, human impact and historical colonization events of the area. Difficulties in understanding and predicting population structure often
arise from an inadequate knowledge of the factors
mentioned above.
B. Population Structure
and Reproductive Strategies
As many of the other chapters in this book will
describe, vegetative growth through rhizome extension is a dominant feature of seagrasses. This
growth habit allows great flexibility for a single genetic individual to survive in both space and time
through clonality resulting from vegetative fragmentation. Most seagrass populations exhibit a mixture
of clonal growth along with sexual reproduction and
understanding the structure of these populations has
proved challenging using standard ecological approaches. The clonal growth form of many seagrass
species results in a hierarchy of different organisational levels. The most elementary level of organisation is the ramet (sensu Harper, 1977), the potentially
independent individual. In seagrasses, this individual typically is the leaf bundle, a piece of rhizome,
and a root bundle (Tomlinson, 1974). Depending on
the longevity of rhizome connections between ramets, several ramets can form physiologically integrated clusters, the second level of organization. The
size of these clusters varies greatly between species,
and may comprise several hundreds of leaf shoots
in the genus Posidonia. The sexual individual, or
genet, are all ramets or ramet clusters which originated from the same zygote. From an evolutionary point of view, it is only the genet that matters
and eventually transmits genes to the next generation. Since genets may be exceedingly large, the
determination of genet fitness, reproductive output,
and longevity poses formidable practical problems.
Most ecological studies have ignored the potential
genetic identity of leaf shoots or ramets when selecting experimental plants, or plots. Whether or not
this has resulted in biased ecological conclusions
remains unclear because quantification of heritable
differences in ecological performance of clones is
lacking in almost all species (but see H¨ ammerli and
Reusch, 2002).
Since physical connections between ramets persist only for a limited time interval, the identification
of the ramet/genet organisation in field populations
is possible only using genetic markers. While traditional allozyme markers often provide too little polymorphism, modern DNA based molecular markers such as RAPD (random amplified polymorphic
DNA), AFLP (amplified fragment length polymorphism) or DNA microsatellites allow precise assignment of leaf shoot samples to ramets and genets
based on multi-locus genotypes. As an additional
advantage, the Mendelian inheritance of DNA microsatellites allows the calculation of error probabilities for an incorrect assignment of ramets to
genets (Reusch et al., 1999d). To date, information on the clonal structure of nine seagrass species
is available (Cymodocea nodosa, Halophila stipulacea, Halophila johnsonii, Posidonia oceanica,
Posidonia australis, Thalassia testudinum, Thalassodendron ciliatum, Zostera marina). In only a few
studies were genotypes mapped in space (cf. Waycott, 1995; Kirsten et al., 1998) (e.g. Fig. 7). This
39
Tyrrhenian Sea (Mediterranean Sea) and in the Cadiz
Bay, Spain (Atlantic Sea) by means of microsatellite analyses (Alberto et al., 2003b; Ruggiero et al.,
in press; Ruggiero and Procaccini, unpublished).
Zostera noltii seems to posses a moderate degree of
variability, but very low population connectivity in
populations sampled in central/southern Tyrrhenian
Sea (Procaccini and Ruggiero, unpublished). Ongoing studies on the complete distribution range of
the species are showing significant clustering among
distinct geographical regions and high degree of
population differentiation at smaller regional scale
(Coyer et al., submitted). More data on both Cymodocea nodosa and Zostera noltii will soon be available
using the same markers from several distinct localities inside and outside Mediterranean (Coyer et al.,
in press). Halodule wrightii appears to be distributed
in genetically distinct meadows, with high variability, along Texas and Florida coasts (Angel, 2002). A.
antarctica seems to be represented by a single clone
along the whole southern Australian coast (Waycott
et al., 1996).
To date, studies generally confirm that patterns
of genetic diversity in seagrass species are determined by a series of interconnected factors that are
not always easy to distinguish. There is no readily
identifiable trend common among species belonging to the same genus, or those theoretically having
similar or identical theoretical dispersal capabilities.
To more complete understanding of factors causing
genetic structure over wide geographical ranges requires the knowledge of local environmental factors,
current regimes, human impact and historical colonization events of the area. Difficulties in understanding and predicting population structure often
arise from an inadequate knowledge of the factors
mentioned above.
B. Population Structure
and Reproductive Strategies
As many of the other chapters in this book will
describe, vegetative growth through rhizome extension is a dominant feature of seagrasses. This
growth habit allows great flexibility for a single genetic individual to survive in both space and time
through clonality resulting from vegetative fragmentation. Most seagrass populations exhibit a mixture
of clonal growth along with sexual reproduction and
understanding the structure of these populations has
proved challenging using standard ecological approaches. The clonal growth form of many seagrass
species results in a hierarchy of different organisational levels. The most elementary level of organisation is the ramet (sensu Harper, 1977), the potentially
independent individual. In seagrasses, this individual typically is the leaf bundle, a piece of rhizome,
and a root bundle (Tomlinson, 1974). Depending on
the longevity of rhizome connections between ramets, several ramets can form physiologically integrated clusters, the second level of organization. The
size of these clusters varies greatly between species,
and may comprise several hundreds of leaf shoots
in the genus Posidonia. The sexual individual, or
genet, are all ramets or ramet clusters which originated from the same zygote. From an evolutionary point of view, it is only the genet that matters
and eventually transmits genes to the next generation. Since genets may be exceedingly large, the
determination of genet fitness, reproductive output,
and longevity poses formidable practical problems.
Most ecological studies have ignored the potential
genetic identity of leaf shoots or ramets when selecting experimental plants, or plots. Whether or not
this has resulted in biased ecological conclusions
remains unclear because quantification of heritable
differences in ecological performance of clones is
lacking in almost all species (but see H¨ ammerli and
Reusch, 2002).
Since physical connections between ramets persist only for a limited time interval, the identification
of the ramet/genet organisation in field populations
is possible only using genetic markers. While traditional allozyme markers often provide too little polymorphism, modern DNA based molecular markers such as RAPD (random amplified polymorphic
DNA), AFLP (amplified fragment length polymorphism) or DNA microsatellites allow precise assignment of leaf shoot samples to ramets and genets
based on multi-locus genotypes. As an additional
advantage, the Mendelian inheritance of DNA microsatellites allows the calculation of error probabilities for an incorrect assignment of ramets to
genets (Reusch et al., 1999d). To date, information on the clonal structure of nine seagrass species
is available (Cymodocea nodosa, Halophila stipulacea, Halophila johnsonii, Posidonia oceanica,
Posidonia australis, Thalassia testudinum, Thalassodendron ciliatum, Zostera marina). In only a few
studies were genotypes mapped in space (cf. Waycott, 1995; Kirsten et al., 1998) (e.g. Fig. 7). This