26
Michelle Waycott, Gabriele Procaccini, Donald H. Les and Thorsten B. H. Reusch
environment. The evolutionary trends that have
lead to these, and other convergent characteristics,
are difficult to interpret without adequate insight
into their origins in both an ecological and evolutionary context. Significantly, the suite of seagrass adaptations (see “Foreword”) represents dramatic evidence of the adaptive capacity of flowering
plants to evolve and survive in extreme environments and as such warrants careful study.
Our understanding of seagrass ecology, physiology and adaptation has been constrained by our ability to develop a clear understanding of many biological features of these plants such as their ability
to pollinate in water (Cox, 1993; Ackerman, 1995;
Verduin, 1996; Ackerman, Chapter 4). In contrast
the details of seagrass photosynthetic mechanisms
(as inferred through the application of PAM, e.g.
Larkum et al., Chapter 14) during different phases
of their daily, weekly or monthly growth are rapidly
being unravelled due to the now widespread use of
new technologies. However, some of the most basic questions of biology such as evolutionary relatedness, plant (genet) age and growth, selection for
adaptive traits, breeding system, and disturbance response strategies, are poorly studied. It is now possible to investigate these questions using a range
of readily available DNA based markers (for review see Reusch, 2001b). With these techniques we
can now study almost any level in the hierarchy
of seagrass relationships and population processes
(Les et al., 1997; Waycott, 2000a; Waycott, 2000b;
Reusch, 2001b).
The use of genetic markers to study questions of
seagrass evolution and ecology began during the
1980–1990s (Les, 1988; McMillan, 1991; Triest,
1991a). These earlier studies largely concluded that
seagrasses were remarkable in their genetic uniformity indicating that the application of allozyme data
to the study of population processes would not be
fruitful. However, later studies (Laushman, 1993;
Ruckelshaus, 1995; Waycott, 1995; Williams and
Davis, 1996; Waycott et al., 1997), demonstrated
that greater sampling efforts could better detect genetic polymorphism thus improving the utility of allozyme markers. The introduction of readily applied
DNA markers provided significantly greater sensitivity (Alberte et al., 1994; Procaccini et al., 1996;
Reusch et al., 1999c).
In this chapter, we discuss the outcomes of genetic
approaches to the study of seagrass biology. Our aim
is to provide insight into the outcomes gained from
the use of powerful molecular based methodologies.
We do not attempt to explain these methods, as many
other reviews are available (e.g. Bachmann, 1994;
Jarne and Lagoda, 1996; Mueller and Wolfenbarger,
1999; Ouborg et al., 1999; Bachmann, 2001; Reusch,
2001b; Gibson, 2002; Judd et al., 2002) and we direct
the reader to these sources for further details and additional literature. This chapter includes two major
sections. The first discusses molecular systematics
and insights obtained from its application, as an approach independent of earlier exclusively morphological based methods. This section deals first with
the systematic placement of families and insights
gained from recent large-scale molecular phylogenetic analyses of the entire monocotyledon subclass
(Alismatidae) that contains the seagrasses. We then
discuss the current state of molecular based intrageneric studies and the insights they provide into
seagrass evolution. The second section is concerned
with genetic approaches to the investigation of ecological and population processes including seagrass
growth, reproduction and adaptation.
II. The Evolution of Seagrass Diversity
A. Classification and Higher
Level Relationships
Early impressions of seagrass relationships reflected superficial morphological comparisons to
other plant groups. These crude concepts are illustrated by the 1792 generic name Phucagrostis
Cavolini (literally ‘seaweed grass’), now a synonym
of Cymodocea K. D. Koenig. Agardh (1821) actually included Amphibolis antarctica among the algae (Sauvageau, 1891) and the marine angiosperms
remained less studied than most algae throughout
the mid 19th Century (Ascherson, 1867). Eventually, their angiospermous affinity was elucidated, but
misconceptions regarding their precise relationships
persisted. Delpino (1870) regarded Zostera as an
aquatic aroid (Araceae) and Posidonia as an aquatic
grass (Poaceae) (Schenck, 1886/2003). However, by
the early 20th Century, authors consistently accepted
the placement of all seagrasses within the monocotyledonous Helobiae (subclass Alismatidae).
Endlicher (1836–1840) included Thalassia
among the genera of ‘Fluviales’ such as Althenia,
Cymodocea, Halodule, Posidonia, Ruppia, and
Zostera. However, other authors placed Enhalus,
Michelle Waycott, Gabriele Procaccini, Donald H. Les and Thorsten B. H. Reusch
environment. The evolutionary trends that have
lead to these, and other convergent characteristics,
are difficult to interpret without adequate insight
into their origins in both an ecological and evolutionary context. Significantly, the suite of seagrass adaptations (see “Foreword”) represents dramatic evidence of the adaptive capacity of flowering
plants to evolve and survive in extreme environments and as such warrants careful study.
Our understanding of seagrass ecology, physiology and adaptation has been constrained by our ability to develop a clear understanding of many biological features of these plants such as their ability
to pollinate in water (Cox, 1993; Ackerman, 1995;
Verduin, 1996; Ackerman, Chapter 4). In contrast
the details of seagrass photosynthetic mechanisms
(as inferred through the application of PAM, e.g.
Larkum et al., Chapter 14) during different phases
of their daily, weekly or monthly growth are rapidly
being unravelled due to the now widespread use of
new technologies. However, some of the most basic questions of biology such as evolutionary relatedness, plant (genet) age and growth, selection for
adaptive traits, breeding system, and disturbance response strategies, are poorly studied. It is now possible to investigate these questions using a range
of readily available DNA based markers (for review see Reusch, 2001b). With these techniques we
can now study almost any level in the hierarchy
of seagrass relationships and population processes
(Les et al., 1997; Waycott, 2000a; Waycott, 2000b;
Reusch, 2001b).
The use of genetic markers to study questions of
seagrass evolution and ecology began during the
1980–1990s (Les, 1988; McMillan, 1991; Triest,
1991a). These earlier studies largely concluded that
seagrasses were remarkable in their genetic uniformity indicating that the application of allozyme data
to the study of population processes would not be
fruitful. However, later studies (Laushman, 1993;
Ruckelshaus, 1995; Waycott, 1995; Williams and
Davis, 1996; Waycott et al., 1997), demonstrated
that greater sampling efforts could better detect genetic polymorphism thus improving the utility of allozyme markers. The introduction of readily applied
DNA markers provided significantly greater sensitivity (Alberte et al., 1994; Procaccini et al., 1996;
Reusch et al., 1999c).
In this chapter, we discuss the outcomes of genetic
approaches to the study of seagrass biology. Our aim
is to provide insight into the outcomes gained from
the use of powerful molecular based methodologies.
We do not attempt to explain these methods, as many
other reviews are available (e.g. Bachmann, 1994;
Jarne and Lagoda, 1996; Mueller and Wolfenbarger,
1999; Ouborg et al., 1999; Bachmann, 2001; Reusch,
2001b; Gibson, 2002; Judd et al., 2002) and we direct
the reader to these sources for further details and additional literature. This chapter includes two major
sections. The first discusses molecular systematics
and insights obtained from its application, as an approach independent of earlier exclusively morphological based methods. This section deals first with
the systematic placement of families and insights
gained from recent large-scale molecular phylogenetic analyses of the entire monocotyledon subclass
(Alismatidae) that contains the seagrasses. We then
discuss the current state of molecular based intrageneric studies and the insights they provide into
seagrass evolution. The second section is concerned
with genetic approaches to the investigation of ecological and population processes including seagrass
growth, reproduction and adaptation.
II. The Evolution of Seagrass Diversity
A. Classification and Higher
Level Relationships
Early impressions of seagrass relationships reflected superficial morphological comparisons to
other plant groups. These crude concepts are illustrated by the 1792 generic name Phucagrostis
Cavolini (literally ‘seaweed grass’), now a synonym
of Cymodocea K. D. Koenig. Agardh (1821) actually included Amphibolis antarctica among the algae (Sauvageau, 1891) and the marine angiosperms
remained less studied than most algae throughout
the mid 19th Century (Ascherson, 1867). Eventually, their angiospermous affinity was elucidated, but
misconceptions regarding their precise relationships
persisted. Delpino (1870) regarded Zostera as an
aquatic aroid (Araceae) and Posidonia as an aquatic
grass (Poaceae) (Schenck, 1886/2003). However, by
the early 20th Century, authors consistently accepted
the placement of all seagrasses within the monocotyledonous Helobiae (subclass Alismatidae).
Endlicher (1836–1840) included Thalassia
among the genera of ‘Fluviales’ such as Althenia,
Cymodocea, Halodule, Posidonia, Ruppia, and
Zostera. However, other authors placed Enhalus,