Chapter 4 Seagrass Sexual Reproduction
105
relevant information from these studies is that sexual
reproduction may or may not be of local importance.
A recent study, however, revealed that developmental and ecological interaction (e.g. abortion and seed
predation) can have significant effects on fruit and
seed production, which could be interpreted as failures in pollination success in the absence of careful
longitudinal study (Balestri and Cinelli, 2003). This
is clearly a topic that should be examined in more
detail.
There is considerable evidence from enzyme and
genetic analysis that sexual reproduction is important for local gene flow (Waycott et al., Chapter 2).
For example, this appears to be the case in Thalassia for local gene flow but not for exchanges over
km scales (Schlueter and Guttman, 1998). Molecular
analyses reveal high outcrossing rates in Zostera and
Posidonia (Ruckleshaus, 1995; Waycott and Sampson, 1997; Reusch, 2000), but not for example in
Amphibolis (Waycott et al., 1996). It is interesting
to note that previous reports of genetic uniformity
in some seagrasses have proven false with the application of different or new techniques (see review in
Reusch, 2001). From the perspective of this contribution, it is satisfying to note that outcrossing rates in
Posidonia australis were higher in energetic environments where pollen transfer would be enhanced, and
that these rates were more similar to those of entomophilous plants than anemophilous ones (Waycott
and Sampson, 1997). Clearly, additional research in
this area is needed, especially studies that integrate
the examination of pollen transport phenomena with
genetic analysis.
X. Conclusions
The reproductive biology of the 12–13 genera of seagrasses is reviewed from a morphological, ecological, and functional perspective. Seagrasses are an
artificial grouping of unrelated monocotyledonous
plants that have successfully invaded the marine
coastal environment. Whereas much of their reproductive biology resembles that of their freshwater
relatives, there are a number of characteristics that
are unique and which have evolved independently
within the various lineages. Generally, seagrasses
have drab, highly reduced flowers, and exhibit dioecy
(9 genera) and dichogamy (4 genera), all characteristics of abiotically and submarine pollinated plants.
The floral structures are either axillary near the
seafloor, or borne on reproductive shoots within or
near the top of the canopy; one genus has a pedunculate female inflorescence and free-floating male
flowers. Carpellate reproductive structures are generally elongated bifid stigmas, although unbranched,
thrice-branched, and disc-shaped stigmas are also
present. Staminate structures are generally simple
anther sacs in which filiform or functionally filiform pollen, which have evolved convergently and
functionally, are housed. Anthesis generally occurs
following the elongation of a simple filament and
water currents are required to disperse pollen, which
is released under water from detached anthers or in
some cases on the water surface from free-floating
male flowers or exposed plants. Pollination appears
to be mediated by fluid–dynamic interactions between the filiform pollen and the velocity gradients around carpellate flowers, which lead to rotational movements and repositioning in the flow as
pollen is transported around flowers. This biomechanical response of filiform pollen increases the
opportunity for pollination and thus represents a
selective advantage for these submarine pollinated
plants.
It is evident that the patterns of sexual reproduction in seagrasses are becoming better understood,
especially with respect to morphology and phenology, although much of the data exist for a few model
species. For example, it is possible to map the particular pollen morphology onto the general phylogeny
of seagrasses and come to some reasonable conclusions about the convergent evolution of this group.
Much information, however, is lacking; for example,
morphological measurements of pollen length remain to be reported in most species and indeed some
genera (see Table 2). Notwithstanding this progress,
there is an almost complete lack of understanding of
the pollination process in most seagrass genera (see
above) and much of the descriptive information is
from the 19
th century! Again, we have what amounts
to a reasonable understanding of the physical ecology of pollination in Zostera marina and a glimpse
of what occurs in Amphibolis antarctica (Labille)
Sonder et Ascherson, several Australian Posidonia
species, and Syringodium filiforme. Whereas this
does not establish a paradigm in the true sense, it
does provide a biomechanical model that can be examined rigorously in the laboratory and the field.
More importantly, this realization provides considerable opportunities for exciting research in coastal
ecosystems throughout the globe.
105
relevant information from these studies is that sexual
reproduction may or may not be of local importance.
A recent study, however, revealed that developmental and ecological interaction (e.g. abortion and seed
predation) can have significant effects on fruit and
seed production, which could be interpreted as failures in pollination success in the absence of careful
longitudinal study (Balestri and Cinelli, 2003). This
is clearly a topic that should be examined in more
detail.
There is considerable evidence from enzyme and
genetic analysis that sexual reproduction is important for local gene flow (Waycott et al., Chapter 2).
For example, this appears to be the case in Thalassia for local gene flow but not for exchanges over
km scales (Schlueter and Guttman, 1998). Molecular
analyses reveal high outcrossing rates in Zostera and
Posidonia (Ruckleshaus, 1995; Waycott and Sampson, 1997; Reusch, 2000), but not for example in
Amphibolis (Waycott et al., 1996). It is interesting
to note that previous reports of genetic uniformity
in some seagrasses have proven false with the application of different or new techniques (see review in
Reusch, 2001). From the perspective of this contribution, it is satisfying to note that outcrossing rates in
Posidonia australis were higher in energetic environments where pollen transfer would be enhanced, and
that these rates were more similar to those of entomophilous plants than anemophilous ones (Waycott
and Sampson, 1997). Clearly, additional research in
this area is needed, especially studies that integrate
the examination of pollen transport phenomena with
genetic analysis.
X. Conclusions
The reproductive biology of the 12–13 genera of seagrasses is reviewed from a morphological, ecological, and functional perspective. Seagrasses are an
artificial grouping of unrelated monocotyledonous
plants that have successfully invaded the marine
coastal environment. Whereas much of their reproductive biology resembles that of their freshwater
relatives, there are a number of characteristics that
are unique and which have evolved independently
within the various lineages. Generally, seagrasses
have drab, highly reduced flowers, and exhibit dioecy
(9 genera) and dichogamy (4 genera), all characteristics of abiotically and submarine pollinated plants.
The floral structures are either axillary near the
seafloor, or borne on reproductive shoots within or
near the top of the canopy; one genus has a pedunculate female inflorescence and free-floating male
flowers. Carpellate reproductive structures are generally elongated bifid stigmas, although unbranched,
thrice-branched, and disc-shaped stigmas are also
present. Staminate structures are generally simple
anther sacs in which filiform or functionally filiform pollen, which have evolved convergently and
functionally, are housed. Anthesis generally occurs
following the elongation of a simple filament and
water currents are required to disperse pollen, which
is released under water from detached anthers or in
some cases on the water surface from free-floating
male flowers or exposed plants. Pollination appears
to be mediated by fluid–dynamic interactions between the filiform pollen and the velocity gradients around carpellate flowers, which lead to rotational movements and repositioning in the flow as
pollen is transported around flowers. This biomechanical response of filiform pollen increases the
opportunity for pollination and thus represents a
selective advantage for these submarine pollinated
plants.
It is evident that the patterns of sexual reproduction in seagrasses are becoming better understood,
especially with respect to morphology and phenology, although much of the data exist for a few model
species. For example, it is possible to map the particular pollen morphology onto the general phylogeny
of seagrasses and come to some reasonable conclusions about the convergent evolution of this group.
Much information, however, is lacking; for example,
morphological measurements of pollen length remain to be reported in most species and indeed some
genera (see Table 2). Notwithstanding this progress,
there is an almost complete lack of understanding of
the pollination process in most seagrass genera (see
above) and much of the descriptive information is
from the 19
th century! Again, we have what amounts
to a reasonable understanding of the physical ecology of pollination in Zostera marina and a glimpse
of what occurs in Amphibolis antarctica (Labille)
Sonder et Ascherson, several Australian Posidonia
species, and Syringodium filiforme. Whereas this
does not establish a paradigm in the true sense, it
does provide a biomechanical model that can be examined rigorously in the laboratory and the field.
More importantly, this realization provides considerable opportunities for exciting research in coastal
ecosystems throughout the globe.