Chapter 4 Seagrass Sexual Reproduction
93
ascribed to them (Grace, 1993). Apparently this
may be the case in dicotyledons where most of
the 30 rhizomatous families (i.e. ∼10% of total
families) are semi-aquatic or aquatic, but not in
the monocotyledons where a high proportion (i.e.
70%) of all monocotyledon families are rhizomatous (Grace, 1993). Seagrasses, as monocotyledons,
possess many characteristics of the Alismatidae including clonal growth. Regardless of the reason for
a rhizomatous existence, the necessity of sexual reproduction remains a valid question given the advantages of asexual reproduction (Jackson et al., 1985):
i.e. (1) it maintains and propagates “good” genotypes through growth and dispersal via fragmentation (e.g. Campbell, 2003); (2) it removes the needs
for the clones of the opposite sex in the dioecious
populations; (3) it provides multiple versions of the
same genotype in the event of mortality (i.e. escape
in numbers); and (4) it removes the real costs of
reproduction (Obeso, 2002). Similarly, there are a
number of advantages for sexual reproduction (e.g.
Williams, 1975) that include: (1) the maintenance
of genetic variation through meiosis and fertilization, which is essential for natural selection; (2) the
masking of deleterious genes, and perhaps the generation of Sisyphean genotypes that can colonize new
habitats or niches; (3) the deposition of a dormant
stage (e.g. seeds) that provides an escape in time;
and (4) the facilitation of dispersal, which is a fundamental process that follows sexual reproduction.
In other words, sexual reproduction is important for
the long-term stability of populations under dynamic
change (Silander, 1985; cf. Rasheed, 1999), and also
in the short-term continued presence of populations
that are annual due to excessive heat or ice scouring
(e.g. Keddy and Patriquin, 1978; Meling-Lopez and
Ibarra-Obando, 1999). Moreover, genetic analyses
have revealed high outcrossing rates in a number of
seagrasses (Ruckleshaus, 1995; Waycott and Sampson, 1997; Reusch, 2000; Waycott et al., Chapter
2), which speaks to the importance of pollination in
natural populations. Given these findings, there can
be little doubt of the role of sexual reproduction in
seagrasses.
The evolutionary ecology of sexual reproduction
in seagrasses is not unlike that of other plants with
abiotic pollination (Ackerman, 2000). In this context, a number of characteristics are present, presumably to facilitate outcrossing and thus limit the
probability of self pollination (Charlesworth, 1993).
Specifically, there is a separation of carpellate and
staminate reproductive structures (1) in space, in
the case of the nine genera with dioecious species
and/or (2) in time via dichogamous pollination (i.e.
phenological separation between pollen reception
and pollen release; usually protogyny), in the case
of the four monoecious genera (Halophila, Heterozostera, Nanozostera, and Zostera; Posidonia is bisexual with perfect flowers; see Table 2; Tomlinson,
1982; Pettitt, 1984; Les et al., 1997). It is also important to note that considerable biochemical evolution occurred in the transition to hydrophily given
that the pollen and stigmatic surfaces are exposed to
wet conditions (Pettitt, 1984; McConchie and Knox,
1989a). In addition to problems associated with osmotic balance, water may disrupt pollen germination
cues, interfere with chemical cues on stigmatic surface, and transport material that may foul surfaces.
Consequently, most aquatic plants reproduce above
the water surface using entomophily (insect pollination; ∼242/380 genera) and anemophily (wind
pollination; ∼119 genera) similar to their terrestrial relatives (Ackerman 1995, 2000; Cook, 1996a;
Philbrick and Les, 1996). Indeed, despite the relatively high proportion of wind-pollinated aquatics, only four genera have evolved anemophily from
an entomophilous ancestry (Cook, 1988, 1996b). A
very small fraction of the aquatics are hydrophilous
(∼19 genera) and pollinate without animal vectors
(abiotic pollination) either on (ephydrophily) or in
(hyphydrophily) the water (Ackerman, 1995, 2000).
Ephydrophily is common in freshwater plants and
in shallow coastal habitats in the case of Enhalus
acoroides (L.f.) Royle and sometimes when the reproductive organs of intertidal seagrasses may lie
on the water surface (Tomlinson, 1982; Ackerman,
2000). Submarine pollination is, however, the dominant pollination mode in the other 12 genera of seagrasses and is limited to a few freshwater genera
(Cook, 1996a; Ackerman, 2000). Ackerman (2000)
summarized the ecological factors in a surface pollination syndrome to include largely clonal plants
that are dioecious or diclinous, in which spheroidal
pollen is released under moderate flows in shallow
habitats leading to reasonably high outcrossing rates.
The ecological factors in a submarine pollination
syndrome include mostly dioecious clonal plants
with dicliny (protogyny) in which filiform pollen is
released under flowing conditions in coastal marine
habitats leading to high outcrossing rates.
93
ascribed to them (Grace, 1993). Apparently this
may be the case in dicotyledons where most of
the 30 rhizomatous families (i.e. ∼10% of total
families) are semi-aquatic or aquatic, but not in
the monocotyledons where a high proportion (i.e.
70%) of all monocotyledon families are rhizomatous (Grace, 1993). Seagrasses, as monocotyledons,
possess many characteristics of the Alismatidae including clonal growth. Regardless of the reason for
a rhizomatous existence, the necessity of sexual reproduction remains a valid question given the advantages of asexual reproduction (Jackson et al., 1985):
i.e. (1) it maintains and propagates “good” genotypes through growth and dispersal via fragmentation (e.g. Campbell, 2003); (2) it removes the needs
for the clones of the opposite sex in the dioecious
populations; (3) it provides multiple versions of the
same genotype in the event of mortality (i.e. escape
in numbers); and (4) it removes the real costs of
reproduction (Obeso, 2002). Similarly, there are a
number of advantages for sexual reproduction (e.g.
Williams, 1975) that include: (1) the maintenance
of genetic variation through meiosis and fertilization, which is essential for natural selection; (2) the
masking of deleterious genes, and perhaps the generation of Sisyphean genotypes that can colonize new
habitats or niches; (3) the deposition of a dormant
stage (e.g. seeds) that provides an escape in time;
and (4) the facilitation of dispersal, which is a fundamental process that follows sexual reproduction.
In other words, sexual reproduction is important for
the long-term stability of populations under dynamic
change (Silander, 1985; cf. Rasheed, 1999), and also
in the short-term continued presence of populations
that are annual due to excessive heat or ice scouring
(e.g. Keddy and Patriquin, 1978; Meling-Lopez and
Ibarra-Obando, 1999). Moreover, genetic analyses
have revealed high outcrossing rates in a number of
seagrasses (Ruckleshaus, 1995; Waycott and Sampson, 1997; Reusch, 2000; Waycott et al., Chapter
2), which speaks to the importance of pollination in
natural populations. Given these findings, there can
be little doubt of the role of sexual reproduction in
seagrasses.
The evolutionary ecology of sexual reproduction
in seagrasses is not unlike that of other plants with
abiotic pollination (Ackerman, 2000). In this context, a number of characteristics are present, presumably to facilitate outcrossing and thus limit the
probability of self pollination (Charlesworth, 1993).
Specifically, there is a separation of carpellate and
staminate reproductive structures (1) in space, in
the case of the nine genera with dioecious species
and/or (2) in time via dichogamous pollination (i.e.
phenological separation between pollen reception
and pollen release; usually protogyny), in the case
of the four monoecious genera (Halophila, Heterozostera, Nanozostera, and Zostera; Posidonia is bisexual with perfect flowers; see Table 2; Tomlinson,
1982; Pettitt, 1984; Les et al., 1997). It is also important to note that considerable biochemical evolution occurred in the transition to hydrophily given
that the pollen and stigmatic surfaces are exposed to
wet conditions (Pettitt, 1984; McConchie and Knox,
1989a). In addition to problems associated with osmotic balance, water may disrupt pollen germination
cues, interfere with chemical cues on stigmatic surface, and transport material that may foul surfaces.
Consequently, most aquatic plants reproduce above
the water surface using entomophily (insect pollination; ∼242/380 genera) and anemophily (wind
pollination; ∼119 genera) similar to their terrestrial relatives (Ackerman 1995, 2000; Cook, 1996a;
Philbrick and Les, 1996). Indeed, despite the relatively high proportion of wind-pollinated aquatics, only four genera have evolved anemophily from
an entomophilous ancestry (Cook, 1988, 1996b). A
very small fraction of the aquatics are hydrophilous
(∼19 genera) and pollinate without animal vectors
(abiotic pollination) either on (ephydrophily) or in
(hyphydrophily) the water (Ackerman, 1995, 2000).
Ephydrophily is common in freshwater plants and
in shallow coastal habitats in the case of Enhalus
acoroides (L.f.) Royle and sometimes when the reproductive organs of intertidal seagrasses may lie
on the water surface (Tomlinson, 1982; Ackerman,
2000). Submarine pollination is, however, the dominant pollination mode in the other 12 genera of seagrasses and is limited to a few freshwater genera
(Cook, 1996a; Ackerman, 2000). Ackerman (2000)
summarized the ecological factors in a surface pollination syndrome to include largely clonal plants
that are dioecious or diclinous, in which spheroidal
pollen is released under moderate flows in shallow
habitats leading to reasonably high outcrossing rates.
The ecological factors in a submarine pollination
syndrome include mostly dioecious clonal plants
with dicliny (protogyny) in which filiform pollen is
released under flowing conditions in coastal marine
habitats leading to high outcrossing rates.