Chapter 2 Seagrass Genetics and Evolution
35
development and this will provide fertile ground for
future research activities.
III. Ecological and Reproductive Processes
A. Species-Wide Population Genetic Studies
1. Introduction
Seagrasses are distributed in coastal areas of all continents and belong to nine species assemblages (floras), six of which are exclusively temperate (Duarte,
2001). Distributional ranges of single species can
differ radically; ranging from species whose distribution is limited to a single flora (e.g. Posidonia
oceanica in the Mediterranean) to widely distributed
species such as Zostera marina. The present distribution of seagrass species is the result of processes
occurring at a number of different temporal scales
and may reflect events that occurred a very long
time ago (tens of thousands of years or more). However, the actual distribution of populations is also
the result of ongoing extant dynamic processes of
colonization/extinction. When population distributions are viewed using a genetic perspective, higher
genetic differentiation among populations reflects
lower genetic exchange (gene flow) and indicates
longer-term isolation. The application of molecular
markers to study levels of population connectivity
potentially can provide significant insights into the
factors influencing present-day distribution of seagrass species, particularly when applied across broad
spatial scales.
Almost half of the roughly 65 papers published on seagrass population genetic studies attempt
to quantify gene flow among distinct populations
(Table 2). To date studies addressing the broad
scale distribution genetic diversity has only been
documented for a few species (Table 2). However,
there are many studies in progress on different
species with new and potentially more polymorphic molecular markers. When viewed across all the
broader scale studies, levels of population connectivity are not always related to differences in dispersal
mode or reproductive characteristics among species,
and do not appear to be linked to their phylogenetic
relatedness.
Only few species have been studied in detail:
within the genus Posidonia, only the Australian endemic P. australis and the Mediterranean endemic
P. oceanica; within the genus Zostera, only Zostera
marina from Europe and North America and in the
genus Thalassia, only the Caribbean species, Thalassia testudinum (these species are the subject of
individual chapters in this volume, see Chapters 16,
17 and 18).
2. Posidonia australis
A broad scale study of 20 P. australis populations has
been performed with RAPD and allozyme markers
(Waycott et al., 1997; Waycott, 1998). Populations
have been sampled along their geographical range,
from the north-western populations of Shark Bay, in
warm-subtropical waters, to the eastern populations
of Lake Macquarie. RAPDs detected higher genotypic variability compared to allozymes, although in
a few cases the authors observed the opposite result. In general, populations showed high variability, with average RAPD Dg values of 0.91. Nevertheless, the analyses showed the existence of low
population connectivity (Gst = 0.623) at the scale
surveyed and identified regions with significant differences in genetic diversity. Meadows sampled in
the south-western region show highest allozyme diversity, whereas lower diversity was recorded at the
extremes of the range. Interestingly, regions with
higher genetic diversity corresponded to regions of
highest species diversity within the genus (Kuo and
Cambridge, 1984; Waycott et al., 1997). Explanations for this pattern of species diversity may reflect
the selective pressures experienced by the Australian
species of Posidonia, which reached their current
distribution from refugia occupied following Gondwanan break up millions of years ago. Selective
pressures may have facilitated speciation and high
genetic variability within some of the species. A
South–North oriented secondary expansion from
few genotypes would explain the low allozyme diversity of the north-western populations. Patterns of
phylogeographic relationships show a clear distinction between these localities and the southern and
eastern populations. Ongoing local reproduction and
recruitment could explain the existence of molecular
based genetic diversity in such localities (Waycott
et al., 1997; Waycott, 2000a). This study demonstrates the utility of sampling over a very wide range
to infer broader processes such as the observation of
low diversity in some populations and high diversity
in others (Waycott, 1998).
35
development and this will provide fertile ground for
future research activities.
III. Ecological and Reproductive Processes
A. Species-Wide Population Genetic Studies
1. Introduction
Seagrasses are distributed in coastal areas of all continents and belong to nine species assemblages (floras), six of which are exclusively temperate (Duarte,
2001). Distributional ranges of single species can
differ radically; ranging from species whose distribution is limited to a single flora (e.g. Posidonia
oceanica in the Mediterranean) to widely distributed
species such as Zostera marina. The present distribution of seagrass species is the result of processes
occurring at a number of different temporal scales
and may reflect events that occurred a very long
time ago (tens of thousands of years or more). However, the actual distribution of populations is also
the result of ongoing extant dynamic processes of
colonization/extinction. When population distributions are viewed using a genetic perspective, higher
genetic differentiation among populations reflects
lower genetic exchange (gene flow) and indicates
longer-term isolation. The application of molecular
markers to study levels of population connectivity
potentially can provide significant insights into the
factors influencing present-day distribution of seagrass species, particularly when applied across broad
spatial scales.
Almost half of the roughly 65 papers published on seagrass population genetic studies attempt
to quantify gene flow among distinct populations
(Table 2). To date studies addressing the broad
scale distribution genetic diversity has only been
documented for a few species (Table 2). However,
there are many studies in progress on different
species with new and potentially more polymorphic molecular markers. When viewed across all the
broader scale studies, levels of population connectivity are not always related to differences in dispersal
mode or reproductive characteristics among species,
and do not appear to be linked to their phylogenetic
relatedness.
Only few species have been studied in detail:
within the genus Posidonia, only the Australian endemic P. australis and the Mediterranean endemic
P. oceanica; within the genus Zostera, only Zostera
marina from Europe and North America and in the
genus Thalassia, only the Caribbean species, Thalassia testudinum (these species are the subject of
individual chapters in this volume, see Chapters 16,
17 and 18).
2. Posidonia australis
A broad scale study of 20 P. australis populations has
been performed with RAPD and allozyme markers
(Waycott et al., 1997; Waycott, 1998). Populations
have been sampled along their geographical range,
from the north-western populations of Shark Bay, in
warm-subtropical waters, to the eastern populations
of Lake Macquarie. RAPDs detected higher genotypic variability compared to allozymes, although in
a few cases the authors observed the opposite result. In general, populations showed high variability, with average RAPD Dg values of 0.91. Nevertheless, the analyses showed the existence of low
population connectivity (Gst = 0.623) at the scale
surveyed and identified regions with significant differences in genetic diversity. Meadows sampled in
the south-western region show highest allozyme diversity, whereas lower diversity was recorded at the
extremes of the range. Interestingly, regions with
higher genetic diversity corresponded to regions of
highest species diversity within the genus (Kuo and
Cambridge, 1984; Waycott et al., 1997). Explanations for this pattern of species diversity may reflect
the selective pressures experienced by the Australian
species of Posidonia, which reached their current
distribution from refugia occupied following Gondwanan break up millions of years ago. Selective
pressures may have facilitated speciation and high
genetic variability within some of the species. A
South–North oriented secondary expansion from
few genotypes would explain the low allozyme diversity of the north-western populations. Patterns of
phylogeographic relationships show a clear distinction between these localities and the southern and
eastern populations. Ongoing local reproduction and
recruitment could explain the existence of molecular
based genetic diversity in such localities (Waycott
et al., 1997; Waycott, 2000a). This study demonstrates the utility of sampling over a very wide range
to infer broader processes such as the observation of
low diversity in some populations and high diversity
in others (Waycott, 1998).