827
Qualitative shift to indirect development in the parasitic sea
anemone Edwardsiella lineata
Adam M. Reitzel,1 James C. Sullivan, and John R. Finnerty
Department of Biology, Boston University, 5 Cummington Street, Boston, MA 02215, USA
Synopsis Direct development lies at 1 end of a continuum that encompasses various degrees of indirect development.
Indirect development exists where a larval stage is interposed between the embryo and the adult and undergoes metamorphosis, though the ecological and morphological distinctiveness of the larval stage relative to the adult stage can vary
tremendously. There are numerous empirical examples where direct development has evolved from indirect development,
but little empirical evidence describing a recent transition from direct to indirect development. Here, we suggest 4 criteria for
defining indirect, and therefore metamorphic, life histories. We then apply these criteria to address the planula–polyp
transition in cnidarians, focusing on 2 species in the anthozoan family Edwardsiidae. The lined sea anemone,
Edwardsiella lineata, has made a qualitative shift towards indirect development that coincides with, and was potentially
facilitated by, the evolution of endoparasitism. We compare E. lineata’s development with that of a closely related sea
anemone, Nematostella vectensis, where the nonfeeding planula gradually develops the morphology of the adult polyp. In
E. lineata, a novel parasitic life history stage is interposed between the planula and the polyp. We discuss how the evolution of
endoparasitism could facilitate the evolution metamorphic life histories.
Introduction: defining metamorphic
criteria
Animal life histories can be broadly categorized into 2
classes: direct development and indirect development.
Indirect development encompasses a larval stage that
undergoes a metamorphic transition into a juvenile.
Direct development by definition lacks a larval stage.
Despite such an apparently clear dichotomy, defining
sharp limits for what is and is not a larva remains
difficult due to a continuum of developmental strategies (reviewed by Hickman 1999). Emphasis for
making life history classifications has been placed on
both morphological (McEdward and Janies 1993) and
ecological plus morphological criteria (Chia 1974).
Metamorphic transitions between larval and adult
stages are frequently accompanied by dramatic shifts in
(1) morphology, (2) feeding ecology, and (3) habitat.
The designations of metamorphic life histories in animals is therefore dependent on delineating what are
and are not distinct, intermediate stages (Heyland and
others 2005). The recognition of distinct intermediate
stages is difficult in many cases because larvae may
differ in the degree to which they are ecologically
and/or morphologically distinct from the adult, and
indirect life histories may differ substantially with
respect to the duration of the larval stage. For these
reasons, a graded classification scheme that recognizes
degrees of indirect development may be more representative of the diversity of organismal life histories
than a dichotomous classification scheme (for example, Hanken 1999; Nagy and Grbić 1999; Webb 1999).
In general, we suggest that 4 criteria will prove useful
in delineating metamorphic events: (1) unique morphological structures and/or developmental regulatory
genes involved in body plan patterning in a preadult
stage, (2) tissue remodeling between stages involving
apoptosis, (3) specific exogenous and endogenous
signaling that elicits and/or coordinates a transition
between stages, and (4) qualitative niche shift characterized by changes in ecologically relevant characters
such as feeding method or habitat.
Evolutionary shifts between direct
and indirect development
Phylogenetic analyses suggest that many, if not all
clades with direct and indirect developing species,
direct development is derived, in some cases occurring
independently multiple times (Jägersten 1972;
Strathmann 1978, 1985). Indirect life histories are
considered primitive in most phyla (Nielsen 1998),
From the symposium “Metamorphosis: A Multikingdom Approach” presented at the annual meeting of the Society for Integrative and
Comparative Biology, January 4–8, 2006, at Orlando, Florida.
1 E-mail: [email protected]
Integrative and Comparative Biology, volume 46, number 6, pp. 827–837
doi:10.1093/icb/icl032
Advance Access publication August 30, 2006
Ó The Author 2006. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology. All rights reserved.
For permissions please email: [email protected].
828
even where the basal group within the phylum is composed of direct developing species (for example,
Nemertea, Maslakova and others 2004) The common
ancestor of crown group metazoans is widely thought
to have been an indirect developer although the exact
route by which this ancestral life history evolved is
disputed (Jägersten 1972; Nielsen 1998, 2000; Collins
and Valentine 2001).
The developmental innovations underlying evolutionary shifts between direct and indirect life histories
have been studied extensively. Cornerstone examples
include the evolution of direct development in echinoderms (Wray 1995; Raff 1999; McEdward and Miner
2001; Sly and others 2003), particularly within the
echinoid genus Heliocidaris (Wray and Raff 1989;
Raff 1992; Emlet 1995), ascidians (Huber and others
2000) and amphibians. Direct developing amphibians
have evolved from indirect developing ancestors both
through the loss of the larval stage (Callery and others
2001) the loss of the adult stage (Voss and Shaffer 1997,
2000). In each of these systems, closely related species
represent divergent life histories with the direct
developing species being derived.
Due to the ancient origins of indirect development
in extant phyla, and the relative rarity with which
indirect development has evolved from direct development in recent times, very few studies have investigated
the causes and consequences of such a life history shift.
One group of organisms where the evolution of indirect life history from direct developing ancestors has
been studied is the insects (Truman and Riddiford
2002). However, holometabolous insects are a monophyletic group that diverged from hemimetabolous
ancestors 300 Mya (Kristensen 1999), so this transition from direct to indirect development happened
only once, and it happened long ago.
Metamorphosis in the Cnidaria
Applying the term metamorphosis to species’ life
histories within the Cnidaria is a challenge because
cnidarians display remarkable diversity in life histories,
representing a spectrum of sexual and asexual reproductive patterns (Fautin 2002). Some life history
transitions involving asexual reproduction could be
regarded as metamorphosis, especially where 2 stages
differ dramatically in morphology and ecology (for
example, strobilation in scyphozoans, cubomedusae
formation, Straehler-Pohl and Jarms 2005). However,
much of the discussion over cnidarian metamorphosis has focused on the transition from pelagic
larva (planula) to benthic polyp (juvenile/adult form;
Müller and Leitz 2002).
A. M. Reitzel et al.
The transition from planula to polyp has been
intensively studied in certain colonial hydrozoans,
especially Hydractinia (Frank and others 2001;
Müller and Leitz 2002) and Pennaria (Martin and
Archer 1986, 1997). In these taxa, 3 of the 4 criteria
that we have suggested for designation of metamorphosis are clearly met. First, there is some evidence
that these hydrozoans are characterized by unique
morphological structures and developmental regulatory genes (criterion 1). The transition from the
planula to the polyp is characterized by the loss of
some larval-specific cell types (Martin and Archer
1997)—for example, RFamide-positive neural network (Martin 2000). The patterning of the primary
oral–aboral axis appears to be established in the planula
stage and maintained in the primary polyp. Very
little data are available to assess whether there are distinctive larval versus polyp gene expression patterns.
Only a few developmental regulatory genes have been
studied in multiple stages (ems: Mokady and others
1998; Cnox-2: Cartwright and others 1999; MasudaNakagawa and others 2000; Cnox-1: Kamm and others
2006). The expression of Cnox-2 is consistent between
planula and polyp in Podocoryne carnea (MasudaNakagawa and others 2000), but the expression of
Cnox-1 undergoes a qualitative shift during
this transition in another hydrozoan, Eleutheria
dichotoma (Kamm and others 2006).
Despite the modest morphological and changes
during the planula to polyp transition, these hydrozoans undergo extensive tissue remodeling and apoptosis
at settlement (criterion 2). There are cell migrations
along the oral–aboral axis, particularly in the aboral
region where the base or stolon forms. Apoptosis is
also widespread and particularly prominent in the
aboral portion of the larva, beginning shortly after
metamorphosis is induced and continuing until
adult patterning commences with tentacle formation
(Seipp and others 2001). Indeed, apoptosis is essential
for this shift because inhibition of caspase-mediated
apoptosis results in a failure to develop the polyp
(Seipp and others 2006).
Work with Hydractinia has shown that both specific
exogenous and endogenous signaling elicits and
coordinates a transition between planula and polyp
(criterion 3). Hydractinia planulae are induced for
settlement through (1) a number of positively charged
ions (Spindler and Müller 1972; Müller 1973) or (2)
other direct activators of protein kinase C (Leitz
and Müller 1987; Leitz 1993; Schneider and Leitz
1994) that stimulate excitable neurosensory cells in
the aboral portion of the embryo. Following settlement,
experimental work has shown that a particular class
of neuropeptides orchestrates internal signaling for
829
Metamorphosis in E. lineata
metamorphosis (Leitz 1998). These compounds are
synthesized and released from the aboral end of the
larva and are thought to travel along axonal fibers to
induce oral cells. If settlement cues are eliminated,
larvae will swim indefinitely and eventually die.
Similarly, settlement inhibitors such as taurine and
the betaines that are present in the larvae have also
been identified (Müller and Leitz 2002). These inhibitors decrease in quantity when larvae begin settlement.
Finally, Hydractinia, Pennaria, and other hydrozoans with similar life histories in general show a
qualitative niche shift characterized by changes in
both habitat and feeding (criterion 4). Pelagic larvae
undergo settlement to benthic, sessile polyps. Because
these planulae are effectively nonfeeding, there is a
transition in feeding mode to feeding in the adult
stage, although this shift is of limited utility in defining
metamorphic life histories.
Family Edwardsiidae: direct to
indirect development?
The family Edwardsiidae is a monophyletic group of
largely burrowing sea anemones in the order Actinaria
(class Anthozoa, phylum Cnidaria). A systematic
study of relationships of genera in the Edwardsiidae
by Daly (2002b) supported 2 major clades in the
family: the Edwardsiinae (Edwardsia, Scolanthus) and
the Milneedwardsiinae (Paraedwardsia, Drillactis,
Edwardsiella, and Nematostella). Although few studies
have reported on the development and life history
of species of Edwardsiid anemones, it appears likely
that the life history exhibited by Edwardsia and
Nematostella, and by most Anthozoa, is the ancestral
life history: external fertilization is followed by the
development of a ciliated planula, followed by a
benthic juvenile and an adult polyp (Widersten 1972).
Nematostella vectensis: direct development?
Nematostella vectensis is a representative Actinarian
species that is an emerging experimental system for
studying evolution, development, and evolutionary
genomics (Darling and others 2005). N. vectensis
undergoes both sexual and asexual reproduction.
During sexual reproduction, fertilized eggs undergo
asynchronous development through a free-living planula into the polyp (Reitzel and others in review, and
references therein). The gradual transition between
planula and polyp stages is accompanied by the loss
of the planula’s sensory organ (the apical tuft) and the
development of the adult’s feeding structures (tentacles, pharynx, and mouth), digestive (mesentery), and
gamete producing tissues (mesentery) (Fig. 1).
By nearly any measure, N. vectensis’s development
from embryo to polyp is gradual and rather direct.
From a morphological and developmental perspective,
Fig. 1 Life history comparison for 2 species in the family Edwardsiidae, E. lineata and N. vectensis. Early developmental
stages prior to the parasitic stage have not been observed in E. lineata.
830
the transition from planula to polyp does not appear
abrupt. Adult morphological characters (pharynx,
mesenteries, and tentacles) begin to develop during
the planula stage, well prior to settlement. The apical
tuft, a bundle of long cilia at the aboral end of the planula,
is the only planula structure lost prior to settlement, but
its loss is not synchronous with settlement. It is not yet
known whether certain larval cell types might be lost
prior to or during the transition to the polyp stage, as
in Hydractinia and Pennaria. For example, we might
expect that neurons associated with the cilia of the apical
organ would degenerate. As the only example of a qualitative structural distinction between planula and polyp,
the apical tuft alone does not appear sufficient to designate N. vectensis as an indirect developer.
Consideration of additional developmental events
from embryo to juvenile are consistent with the
designation of N. vectensis as an essentially direct
developing species. The principal body regions exhibited by the polyp are established early in development:
pharyngeal regions, tentacular zone, body column, and
A. M. Reitzel et al.
physa. Concordantly, the expression territories of
many axial patterning genes, including representatives
from the Hox family of transcription factors (Finnerty
and others 2004) and Wnt signaling peptides
(Kusserow and others 2005), remain largely unchanged
through the planula to polyp transition (Fig. 2).
Consistent with the gradual developmental transition
from embryo to polyp, there does not appear to be extensive tissue remodeling between stages as exhibited in the
aforementioned hydrozoans. N. vectensis does not appear
to undergo the tissue dynamics characterized by settling
Hydractinia or Pennaria, showing only elongation from
planula to polyp. For example, TUNEL-staining (as per
Seipp and others 2001) with N. vectensis specimens during
various stages of development has indicated little, if any
apoptosis in the developing planula or polyp. In addition,
we did not observe TUNEL-positive cells during larval
developmental stages that coincide with stages where
the apical tuft is lost, suggesting that the widespread apoptosisobservedinhydrozoansduringtransitiontothepolyp
does not occur in N. vectensis.
Fig. 2 In situ hybridizations for 4 developmental regulatory genes during the development of N. vectensis. For each of
these genes, expression is continuous from embryo to juvenile, supporting direct development for this species. Forkhead:
(A) early planula, (B) later planula, (C) juvenile. Anthox6: (D) early planula, (E) later planula, (F) juvenile. Snail: (G) early
planula, (H) later planula, (I) juvenile Gsx: ( J) planula, (K) juvenile. (From Finnerty and others 2003; Finnerty and others
2004; Martindale and others 2004). Reproduced from Finnerty JR, Paulson D, Burton P, Pang K, Martindale MQ. 2003.
Early evolution of a homeobox gene: the parahox gene Gsx in the Cnidaria and the Bilateria. Evol Dev 5:331–45 with
permission from Blackwell Publishing.
831
Metamorphosis in E. lineata
Specific exogenous signaling is not required for the
transition from planula to polyp. Solitary planulae will
readily undergo differentiation to the benthic polyps in
bowls of sterile artificial seawater. It is unclear if or how
endogenous signals may operate during the planula to
polyp transition. Neuropeptides play a prominent role
in the metamorphosis of Hydractinia as well as in
morphogenesis of Hydra (Steele 2002). In addition,
endocrine-like signaling in addition to other bioregulatory molecules have been identified in a number of
cnidarians (Tarrant 2005), but we know little about
their involvement in life history processes.
Finally, it is uncertain if the planula to polyp
transition for N. vectensis could be considered a qualitative shift in their niche. The ciliated planulae generally remain motionless on the bottom of culture
dishes in the laboratory (Williams 1975; Hand and
Uhlinger 1992). In the only potential field collection
of planulae, Williams (1975) reported larvae residing in
the same sediment occupied by the sessile adults. We
have observed similar larval behavior in laboratory
observations where larvae introduced to a dish containing sediment will passively settle and burrow below
the surface. As such, there is little evidence to indicate
that the planula would typically occupy the pelagic
environment. Laboratory observations can only
approximate the likely field behavior of these larvae
and observations of natural larval behavior would be
necessary to validate laboratory observations and studies. Recent population genetic studies of N. vectensis
have supported our view that larvae are unlikely to be
pelagic stages. Significant population genetic structure
is evident between closely spaced subpopulations
within a single salt marsh (Darling and others 2004;
Reitzel and others in review).
There is currently little descriptive evidence of
Edwardsiid development and until further studies
with additional species are reported, we cannot definitively qualify N. vectensis’ development as ancestral
for the family Edwardsiidae. Although there are
distinct similarities with reported stages from other
Edwardsiids and anthozoans in general, the underlying
tissue dynamics, signaling, and niche shifts may vary.
There is some reason to suggest that N. vectensis may
have followed a novel trajectory, potentially in response
to specialization for a novel habitat (estuaries).
N. vectensis is the only estuarine member of the
Edwardsiidae. Its preferred habitat is very patchy in
distribution. For this reason, N. vectensis may have
evolved strategies to limit dispersal in order to prevent
the offspring from being flushed out of suitable estuarine pools. Such strategies would include the evolution
of benthic egg masses and possibly demersal development. Accelerating the development of adult characters
so that they appear in the planula (adultation) would
likewise serve to minimize dispersal and would result
in the loss of a larval stage. Whether these developmental events are unique to N. vectensis and indeed
represent heterochronic shifts is unclear and determination awaits further description of development
from other Edwardsiid anemones.
Edwardsiella lineata: indirect development and
endoparasitism
In stark contrast to N. vectensis, the lined sea anemone,
Edwardsiella lineata, undergoes indirect development,
with a biphasic life cycle that includes a parasitic stage
(Crowell 1976). Recent phylogenetic analyses suggest
that Nematostella and Edwardsiella are closely related
genera with the family Edwardsiidae, perhaps even
sister taxa (Daly 2002a; Daly 2002b). The lined sea
anemone, E. lineata, lives in shallow ocean water in
the northwestern Atlantic, from Cape Cod to Cape
Hatteras (Daly 2002a). Adults reproduce both sexually
and asexually and have been found in mats of hundreds
of individuals; presumably these colonies are the result
of local asexual reproduction (Crowell and Oates
1980).
Crowell (1976) and later Crowell and Oates (1980)
described portions of the life history of E. lineata. We
have added further observations for this species
(Fig. 1). Free-living ciliated planulae selectively infect
ctenophores of the species Mnemiopsis leidyi either
through the epidermis (Crowell 1976) or through the
gastrovascular cavity (Reitzel, Sullivan, and Finnerty
unpublished data). The anemones position themselves
along the pharynx or in the ciliated area near the esophagus with their oral end just inside the digestive
cavity (Crowell 1976; Reitzel, Sullivan, and Finnerty
unpublished data). The parasites elongate and assume
a vermiform shape with a differentiated pharynx. The
parasites occasionally possess thin mesenteries, sometimes numbering up to 4. Some time after infecting
the host, the parasite assumes a more spherical shape
and exits the host as a planula-like stage. At this
point in the life history, E. lineata can either reinfect
another ctenophore or settle as the benthic juvenile
polyp. At this settling stage, tentacles emerge and all
8 mesenteries are present.
E. lineata’s life history reflects a qualitative shift
towards indirect life history—the parasite is clearly a
novel life history stage, and it represents a detour
from the ancestral planula-to-polyp developmental trajectory. At the morphological level, while the parasitic
stage does not exhibit any unique structure, it does
exhibit a novel combination of features. For example,
like the polyp, it lacks the well-developed external
832
A. M. Reitzel et al.
Fig. 3 TUNEL-positive cells indicating regional expression of apoptosis in the life history of E. lineata. (A) Aboral end of
recently excised parasite from the ctenophore host. TUNEL-positive cells were found exclusively in the ectoderm tissue
layer. The oral end showed little to no apoptosis. (B) Magnified view of A. (C) Aboral end of settling postparasitic
planula showing apoptosis in the ectoderm, similar to the excised parasite. (D) Developing tentacles showed extensive
apoptosis during the initial stages after settlement. After 24 h, tentacles no longer show TUNEL-positive cells.
TUNEL-positive cells were not observed in any developmental stage of N. vectensis. Scale bar, A–C, 300 mm; D,
500 mm.
ciliation characteristic of planula. However, like the
planula, the parasite lacks the well-developed retractor
muscles characteristic of the polyp. It lacks tentacles
entirely, but it does possess a well-developed pharynx,
and larger individuals may exhibit 1 or 2 pairs of thin
mesenteries.
At 2 life stage transitions, E. lineata does undergo
extensive tissue remodeling involving apoptosis (1)
when the parasite re-assumes the planula form
when exiting the host and (2) when the postparasite
planula settles to form a polyp. In both instances,
TUNEL-positive cells are found in the aboral ectoderm,
reminiscent of Hydractinia larvae induced to settle
(Fig. 3). In addition, TUNEL-positive cells are found
in newly differentiated tentacles. Apoptosis in the parasite to planula transition correlates with the compaction of the vermiform-shaped parasitic body into the
more rounded planula.
While specific exogenous chemical cues have not
been described for the infection of M. leidyi, the specificity of the host–parasite relationship suggests that
E. lineata planula may rely on host cues for infection.
Over 2 sampling seasons, we collected 871 E. lineata
parasites from 4015 sampled M. leidyi at Woods Hole,
MA, USA. Over the same period, we sampled 600 ctenophores of the genus Pleurobrachia and found no
infections in any of these ctenophores. While M. leidyi
tends to be larger than Pleurobrachia species, we have
found many infected M. leidyi within the size range of
Pleurobrachia. In addition, various cnidarian medusae
are present in E. lineata’s range and none have been
reported to harbor E. lineata parasites.
As with N. vectensis, endogenous signals regulating
the transitions between life history stages have not
been identified in E. lineata, but any endogenous signal
transduction pathways are likely sensitive to chemical
signals from the host. In general, when parasites have
the opportunity to infect multiple hosts they benefit
from knowing the physiological status of the individual
they are infecting (Ueno 1994). Results from our
laboratory have shown that when hosts are exposed
to stressful conditions, parasites begin to undergo the
morphological modifications necessary to revert to a
free-living state to exit the host. Postparasitic planula
can follow 2 developmental trajectories: they may
reinfect another ctenophore and re-assume the parasite
body plan, or they may settle as a benthic polyp. Those
developmental regulatory pathways that re-establish
the parasite body plan must be downstream of some
signal derived from the host.
E. lineata displays a clear shift in ecological niche
between the parasite and polyp. The parasite feeds
through ciliary suspension feeding in the ctenophore
digestive system, typically the esophagus and more
833
Metamorphosis in E. lineata
Gene Expression
Ancestral
pattern
Morphology
Polyp
Ancestral
body plan
A
Hypothesis 1.
No change in the expression
of patterning genes.
Parasite
B
Hypothesis 2.
Shifted gene
expression territories.
C
Hypothesis 3.
Deleted gene
expression territories.
Derived
body plan
Fig. 4 Alternative hypotheses relating the expression of patterning genes to the development of the derived body plan
in the parasitic form. Each color represents the expression territory of a single patterning gene. (A) The expression
of patterning genes is not altered. The novel morphology must be caused by modifications to downstream
developmental events. (B) The relative boundaries and relative sizes of gene expression territories are altered,
resulting in an alteration of the body plan. (C) A subset of gene expression territories is deleted, resulting in an
alteration of the body plan.
rarely the pharynx. The ctenophore esophagus is lined
with dense bunches of cilia where ingested food is
disrupted for extracellular digestion (Bumann and
Puls 1997). In contrast, the polyp is a tentacular feeder,
likely feeding on pelagic zooplankton although field
dietary studies have not been completed. The pelagic
parasite represents a potential dispersal stage because
ctenophores travel great distances with the currents
while the polyps are sessile benthic organisms.
Molecular mechanisms underlying
changes in ontogeny
Analysis of expression of development regulatory genes
may provide fundamental insights regarding the
evolution of an indirect life cycle from a direct developing ancestor (Raff and Sly 2000). Morphological
differences between the parasite and other life history
stages suggest that the parasite may reasonably be
regarded as a simplified version of the polyp—a
simplified version of the polyp that does not develop
tentacles and seldom mesenteries. Concordant with the
lack of mesentery differentiation, the parasite does not
appear to possess retractor muscles. If the parasite is
missing pattern elements that are present in the adult
polyp, it seems likely that the genes responsible for
patterning the polyp body plan might underlie this
evolutionary transformation. If the spatiotemporal
expression of a particular patterning gene is compared
during the planula-to-polyp transition (ancestral)
and the planula-to-parasite transition (derived), the
following 3 outcomes emerge as logical possibilities
(Fig. 4). (1) The spatiotemporal expression of a particular patterning genes might be unaltered in the
derived developmental pathway. The loss of structures
in the derived ontogeny might then be attributable
to downstream events: for example, changes in how
these patterning genes interact with downstream
target genes, changes in the function of the downstream targets themselves, or quantitative change in
the number of transcripts produced at a particular
time in development (for example, novel leg repressing activity in the Ubx protein of insects; Galant
and Carroll 2002). (2) The expression territory of a
834
particular gene might be altered—either expanded or
contracted. Altered gene expression territories might
cause the loss of particular structures during parasite
development because co-expressed regulatory genes
can collaborate to affect downstream cell-fate decisions
in a kind of combinatorial code. Altering gene expression boundaries can perturb the ancestral code, resulting in a novel body plan (for example, shifting
boundaries of Ubx/abdA expression in crustaceans
and its effect on the form of the thoracic appendages;
Averof and Patel 1997). (3) The expression territory of
a particular gene might be deleted. The loss of particular structures might be caused by the deletion of
particular gene expression territories during parasite
development. If a key regulatory gene is switched off
in the planula-to-parasite transition then downstream
developmental events may not be initiated (for example, downregulation of the Manx gene in tailless ascidian embryos; Swalla and Jeffery 1996).
Given the plethora of gene expression work reported
from N. vectensis, there are a number of genes
expressed in specific structures and body regions for
both ectoderm and endoderm, thus providing a number of experimental tests for the hypotheses outlined
above. For example, mef2, soxD1, and m-LIM are 3
genes with spatially segregated expression in the tentacles of N. vectensis (Martindale and others 2004;
Magie and others 2005). Studying these genes in
E. lineata polyps and parasites might allow us to distinguish between hypotheses 2 and 3. If the genes were
not expressed in the tentacles during polyp development, but they are not expressed during parasite
development, this would suggest that these gene
expression territories have been deleted concordant
with the absence of tentacles (hypothesis 3). Alternatively, if the expression of these genes is altered relative to other patterning genes during parasite versus
polyp development, this would support hypothesis 2.
Studying gene expression between stages of E. lineata
would also address if the planula stage that forms
while exiting the host ctenophore could be best
regarded as a reversion to the original postembryogenesis planula stage as observed during reverse development in other cnidarians (Sammarco 1982; Piraino
and others 2004) or a unique intermediate stage.
Metamorphosis and endoparasitism
The evolution of parasitism from free-living ancestors
can result in dramatic changes in organismal life histories (for example, polyembryony in parasitoid wasps,
Grbić and Strand 1998; Grbić 2003). The evolution of
parasitism, particularly endoparasitism, results in the
necessity for the infecting species to (1) locate the
A. M. Reitzel et al.
appropriate host(s), (2) infect the host by entering
and establishing at appropriate location(s), and (3)
extract nutrition from the host, either through feeding
on the host’s tissues or the host’s ingested food. Thus,
many parasites fulfill a number of criteria for a metamorphic life history in that a stage parasitic on 1 host
usually differs from the free-living stage or parasitic
stage on another host in habitat, food sources, and
morphology.
Endoparasitic flatworms, particularly digenean
trematodes, exhibit remarkable evolutionary innovations resulting from the origin of obligate parasitism.
Their life histories typically involve a series of stages
(miracidium, sporocyst, redia, cercaria, metacircaria,
reproductive adult), with 2 or more hosts, and at
least 2 infective stages. Developmental transitions
between these stages generally involve events characteristic of metamorphosis (miracidium to sporocyst:
neodermis formation, shift to host habitat; cercaria
to metacircaria: tail, digestive tract, and suckers formation, shift to free-living environment (Ruppert and
others 2004). Cribb and others (2001) mapped various
digean life cycles on phylogenies and found changes in
life cycles and hosts is common and related. If each host
gain is accompanied by the addition of a new parasitic
stage adapted to living in the new host, then parasitism
is an engine for the evolution of new metamorphoses.
We suggest that the evolution of endoparasitism
was the precondition for the evolution of a novel metamorphosis in the life history of E. lineata. Prior to the
origin of its parasitic life history, predation by M. leidyi
may have selected for planula that could survive ingestion by lodging themselves in benign locations with
the ctenophore’s digestive tract until the ctenophore
died or escape digestion by burrowing through host
tissue. Once E. lineata planulae became able to survive
ingestion, selection could have favored those individuals that could utilize caloric intake from the host.
Regardless of the scenario that favored the evolution of
parasitism, the insertion of a successful parasitic stage
required coordinated spatio-temporal development in
a specific ctenophore species. Although the suite of
morphological characters exhibited by the parasite
does not differ drastically from other stages in the
life history, this may reflect the evolutionary recency
of this novel stage. If the parasitic stage is maintained in
E. lineata’s life history, it will continue to experience
different selection pressures than either the planula or
the polyp, and it is likely to evolve greater distinctiveness. Similar arguments have been used in describing
the evolution of the diversity in holometabolous insects
where more radical metamorphic life histories may
permit differential selection on different life history
stages (Yang 2001).
835
Metamorphosis in E. lineata
Larvae of sea anemones from the genus Peachia
are able to parasitize pelagic hydromedusae, and
such infections appear to be obligate in the case of
Peachia parasitica (¼ quinquecapitata, Spaulding
1972). The genus Peachia belongs to the family
Haloclavidae, and the parasitism exhibited by these
anemones almost certainly evolved independently
from the parasitism of E. lineata. Like E. lineata, the
parasitic Peachia lack tentacles, and they feed on
predigested material in the hydromedusa gastrovascular cavity via ciliary currents. However, Peachia differs
from E. lineata in that it also feeds on the gonads and
potentially other body parts of its host (Spaulding
1972). In addition, Peachia differs from E. lineata by
becoming ectoparasitic on the bell of its hydromedusa
host after developing tentacles. Future studies on the
development of Peachia species would provide an
informative comparison with E. lineata.
Conclusion
The evolution of indirect and direct life histories is a
central question in organismal life history evolution,
but studies on the transition from indirect to direct
development greatly outnumber studies on the transition from direct to indirect. E. lineata provides valuable
insights into the evolution of indirect development
from a more direct developing ancestor because this
parasitic anemone appears to have made an evolutionarily recent qualitative shift towards indirect development. In addition, E. lineata reveals parasitism to be an
evolutionarily plausible route from direct to indirect
development. A major challenge to understanding the
evolution of indirect development is the following.
How can exceedingly distinctive novel life history
stages, requiring the evolution of complex new developmental pathways, become interposed in the ontogeny of a direct developing ancestor? If an existing life
history stage (for example, a planula) can establish itself
as a parasite through fortuitous preadaptations, subsequent selection on the parasite can lead to the evolution of a novel stage that may require a metamorphic
transition. Ancient indirect developing lineages do not
allow us to track the evolutionary mechanisms underlying the origin of novel metamorphoses. However, in
the case of E. lineata, it is possible to make direct
comparisons to a closely related, free-living relative,
N. vectensis. By leveraging the molecular and genomic
tools developed for the closely related anemone
N. vectensis, we can begin to assess potential mechanisms for this change in life history. Future studies
addressing expression of developmental regulatory
genes in different life history stages as well as endogenous and exogenous signaling to coordinate the
changes between stages will aid in identifying mechanisms for how the novel stage develops. In addition, such
study may help elucidate steps in the evolution of
indirect life history; a critical innovation acquired in
many dominant animal taxa but something thought
difficult to study.
Acknowledgments
We would like to thank the symposium organizers for
organizing this symposium. We would like to thank all
audience-members from the platform and associatedsessions for constructive discussions. We are grateful to
the Society for Integrative and Comparative Biology
(SICB) for promoting and partially funding this
symposium. Furthermore, we would like to thank
the following organizations for their generous financial
support: the University of Florida, The Whitney
Laboratory for Marine Biosciences, the American
Microscopical Society (AMS), and the SICB Division
of Evolutionary Developmental Biology (DEDB). We
would also like to thank 2 anonymous reviewers for
insightful comments that improved this manuscript.
A.M.R. was funded by an American Microscopical
Society fellowship for the TUNEL-staining for
Nematostella and Edwardsiella.
Conflict of interest: None declared.
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