30
Michelle Waycott, Gabriele Procaccini, Donald H. Les and Thorsten B. H. Reusch
among six separate families (Table 1, last column)
in preference over classification systems that recognize fewer families. Moreover, it is now evident that
Ruppiaceae are closely allied with Posidoniaceae
and Cymodoceaceae rather than representing a subdivision of the freshwater family Potamogetonaceae.
Molecular analyses also confirm the distinctness of
the Zosteraceae and remote relationship of marine
Hydrocharitaceae to all other seagrasses. Finally,
molecular studies have provided compelling evidence that marine Hydrocharitaceae occur within
a single, monophyletic clade, thereby invalidating
classifications of the family that divide the three marine genera otherwise among the freshwater representatives.
At this time, higher-level seagrass relationships
appear to be clarified quite adequately, at least well
enough to provide a meaningful, defensible classification. Additional sequence data could corroborate
the integrity of seagrass clades further, but would
unlikely result in any major alterations of phylogenetic relationships already disclosed at higher levels.
Instead, more emphasis should be made to continue
refining phylogenetic relationships at lower, i.e. interspecific levels within seagrass genera (see below).
B. Interspecific Relationships
Cladistic analyses of molecular and non-molecular
data have been applied to questions of seagrass
species relationships, but this approach only has
been taken relatively recently and many species remain unstudied. Overall, seagrasses are not diverse
with most genera being quite species poor.
Enhalus (Hydrocharitaceae) is regarded as monotypic, comprising the sole species E. acoroides.
Ara˜ no et al. (2003) reported a complete lack of detectable RFLP variation among E. acoroides populations from different geographical regions, which
would support the existence of a single Enhalus
species. However, a recent comparison of ITS sequences from Chinese and Australian material of
Enhalus show that they are quite distinct (Les
and Moody, unpublished). Further investigation of
Enhalus is warranted and may disclose the existence
of additional species.
Four marine genera (Amphibolis, Syringodium,
Thalassia, Thalassodendron) each contain only two
species, thus showing unequivocal intrageneric phylogenetic relationships if the genera are truly monophyletic. Les et al. (1997) included both species
of Syringodium and Thalassia, which confirmed
the monophyly of Thalassia, but not Syringodium
(paraphyletic with Cymodocea). The monophyly of
Thalassia and its subdivision into two species (Old
and New Worlds) has been demonstrated further by
molecular sequence data from nuclear (18S, ITS)
and other cpDNA (trnL) loci. The monophyly of
Amphibolis and Thalassodendron (sister genera in
rbcL analyses) could be tested similarly by obtaining rbcL sequences for Amphibolis griffithii and
Thalassodendron ciliatum to include within the
data in Les et al. (1997). Only two of the four
Cymodocea species were included in the rbcL
analysis and it would be desirable to obtain
data for the unsampled species (C. rotundata, C.
angustata) as well. Further details of relationships in the Amphibolis/Cymodocea/Syringodium/
Thalassodendron clade should be investigated by
sampling all 10 species using rbcL and additional
molecular loci as well as incorporating a complete morphological data set. Precise delimitation
of generic boundaries in this group of seagrasses
remains unsettled.
Halodule (three species, plus) has been well
investigated using nuclear (ITS) and cpDNA (rbcL,
trnL) markers (Les et al., 1997; Waycott, unpublished), yet relationships remain complicated. All
markers (ITS, rbcL, trnL) examined thus far show
the genus to be monophyletic. Within the genus,
nuclear (ITS) sequence data analysis yielded one
monophyletic clade comprising the New World H.
wrightii and another containing the Old World H.
pinifolia and H. uninervis (Fig. 2A). Populations of
the latter two species overlap and are not clearly differentiated by ITS data. However, cpDNA (trnL) data
show considerable overlap among populations of all
three species, thus complicating the interpretation of
the molecular analyses (Fig. 2B). Halodule would
benefit from a genus wide intensive study of population level relationships.
Posidonia (five species) has been studied cladistically using morphological data and molecular sequence data from nuclear (ITS) and cpDNA (rbcL;
trnL) loci. Phylogenetic analyses of these different
data sets are consistent and indicate a similar overall
pattern of relationships in the genus. Combination
of these congruent data sets (Waycott and Les, unpublished data) suggests that the Mediterranean P.
oceanica (the outlier geographically) is basal, with
the Australian taxa derived (Fig. 3). Posidonia sinuosa, P. australis and P. angustifolia are relatively
Michelle Waycott, Gabriele Procaccini, Donald H. Les and Thorsten B. H. Reusch
among six separate families (Table 1, last column)
in preference over classification systems that recognize fewer families. Moreover, it is now evident that
Ruppiaceae are closely allied with Posidoniaceae
and Cymodoceaceae rather than representing a subdivision of the freshwater family Potamogetonaceae.
Molecular analyses also confirm the distinctness of
the Zosteraceae and remote relationship of marine
Hydrocharitaceae to all other seagrasses. Finally,
molecular studies have provided compelling evidence that marine Hydrocharitaceae occur within
a single, monophyletic clade, thereby invalidating
classifications of the family that divide the three marine genera otherwise among the freshwater representatives.
At this time, higher-level seagrass relationships
appear to be clarified quite adequately, at least well
enough to provide a meaningful, defensible classification. Additional sequence data could corroborate
the integrity of seagrass clades further, but would
unlikely result in any major alterations of phylogenetic relationships already disclosed at higher levels.
Instead, more emphasis should be made to continue
refining phylogenetic relationships at lower, i.e. interspecific levels within seagrass genera (see below).
B. Interspecific Relationships
Cladistic analyses of molecular and non-molecular
data have been applied to questions of seagrass
species relationships, but this approach only has
been taken relatively recently and many species remain unstudied. Overall, seagrasses are not diverse
with most genera being quite species poor.
Enhalus (Hydrocharitaceae) is regarded as monotypic, comprising the sole species E. acoroides.
Ara˜ no et al. (2003) reported a complete lack of detectable RFLP variation among E. acoroides populations from different geographical regions, which
would support the existence of a single Enhalus
species. However, a recent comparison of ITS sequences from Chinese and Australian material of
Enhalus show that they are quite distinct (Les
and Moody, unpublished). Further investigation of
Enhalus is warranted and may disclose the existence
of additional species.
Four marine genera (Amphibolis, Syringodium,
Thalassia, Thalassodendron) each contain only two
species, thus showing unequivocal intrageneric phylogenetic relationships if the genera are truly monophyletic. Les et al. (1997) included both species
of Syringodium and Thalassia, which confirmed
the monophyly of Thalassia, but not Syringodium
(paraphyletic with Cymodocea). The monophyly of
Thalassia and its subdivision into two species (Old
and New Worlds) has been demonstrated further by
molecular sequence data from nuclear (18S, ITS)
and other cpDNA (trnL) loci. The monophyly of
Amphibolis and Thalassodendron (sister genera in
rbcL analyses) could be tested similarly by obtaining rbcL sequences for Amphibolis griffithii and
Thalassodendron ciliatum to include within the
data in Les et al. (1997). Only two of the four
Cymodocea species were included in the rbcL
analysis and it would be desirable to obtain
data for the unsampled species (C. rotundata, C.
angustata) as well. Further details of relationships in the Amphibolis/Cymodocea/Syringodium/
Thalassodendron clade should be investigated by
sampling all 10 species using rbcL and additional
molecular loci as well as incorporating a complete morphological data set. Precise delimitation
of generic boundaries in this group of seagrasses
remains unsettled.
Halodule (three species, plus) has been well
investigated using nuclear (ITS) and cpDNA (rbcL,
trnL) markers (Les et al., 1997; Waycott, unpublished), yet relationships remain complicated. All
markers (ITS, rbcL, trnL) examined thus far show
the genus to be monophyletic. Within the genus,
nuclear (ITS) sequence data analysis yielded one
monophyletic clade comprising the New World H.
wrightii and another containing the Old World H.
pinifolia and H. uninervis (Fig. 2A). Populations of
the latter two species overlap and are not clearly differentiated by ITS data. However, cpDNA (trnL) data
show considerable overlap among populations of all
three species, thus complicating the interpretation of
the molecular analyses (Fig. 2B). Halodule would
benefit from a genus wide intensive study of population level relationships.
Posidonia (five species) has been studied cladistically using morphological data and molecular sequence data from nuclear (ITS) and cpDNA (rbcL;
trnL) loci. Phylogenetic analyses of these different
data sets are consistent and indicate a similar overall
pattern of relationships in the genus. Combination
of these congruent data sets (Waycott and Les, unpublished data) suggests that the Mediterranean P.
oceanica (the outlier geographically) is basal, with
the Australian taxa derived (Fig. 3). Posidonia sinuosa, P. australis and P. angustifolia are relatively