Contributions to Zoology 88 (2019) 236-256
CTOZ
brill.com/ctoz
The invasive alien freshwater flatworm Girardia tigrina
(Girard, 1850) (Platyhelminthes, Tricladida) in Western
Europe: new insights into its morphology, karyology and
reproductive biology
Giacinta Angela Stocchino
Dipartimento di Medicina Veterinaria, Università di Sassari, Via Vienna 2, 07100,
Sassari, Italy
stocchin@uniss.it
Ronald Sluys
Naturalis Biodiversity Center, P.O. Box 9517, 2300, RA Leiden, The Netherlands
Abdel Halim Harrath
Department of Zoology, College of Science, King Saud University, P.O. Box 2455,
Riyadh 11451, Saudi Arabia
Lamjed Mansour
Department of Zoology, College of Science, King Saud University, P.O. Box 2455,
Riyadh 11451, Saudi Arabia
Renata Manconi
Dipartimento di Medicina Veterinaria, Università di Sassari, Via Vienna 2, 07100,
Sassari, Italy
Abstract
Invasions of alien species form one of the major threats to global biodiversity. Among planarian flatworms
many species are known to be invasive, in several cases strongly affecting local ecosystems. Therefore, a
detailed knowledge on the biology of an invasive species is of utmost importance for understanding the
process of invasion, the cause of its success, and the subsequent ecological impact on native species. This
paper provides new information on the biology of introduced populations of the freshwater flatworm
Girardia tigrina (Girard, 1850) from Europe. This species is a native of the Nearctic Region that was accidentally introduced into Europe in the 1920s. Since then, numerous records across the European continent bear witness of the invasiveness of this species, although only a few studies focused on the biology
© stocchino et al., 2019 | doi:10.1163/18759866-20191406
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EX-FISSIPAROUS ITALIAN FLATWORM GIRARDIA TIGRINA
237
of the introduced populations. We report on the morphology of sexualized individuals from a fissiparous
Italian population, representing the second record of spontaneous sexualization of fissiparous individuals in this species. A detailed morphological account of the reproductive apparatus of these ex-fissiparous
animals is presented. Our results increased the number of morphological groups previously recognized
for European populations of G. tigrina, thus corroborating the hypothesis on multiple independent introductions to this continent. Karyological results obtained from our fissiparous Italian individuals revealed a constant diploid chromosome complement of sixteen chromosomes. Further, we document the
marked intraspecific variation in several morphological features of this species.
Keywords
ectopic hyperplasic ovaries – ex-fissiparous Dugesiidae – Italian non-native species – morphotypes –
supernumerary copulatory apparatus
1
Introduction
Invasions of alien species are considered to
be one of the major threats to global biodiversity and to form the second major cause
of animal extinctions (Gherardi et al., 2007
and references therein). Among planarian
flatworms or triclads many species are known
to be invasive, in several cases strongly affecting local ecosystems (cf. Sluys, 2016). Therefore, detailed knowledge on the biology of
an invasive species is of utmost importance
for understanding the process of invasion,
the cause of its success, and the subsequent
ecological impact on native species (Ducey
et al., 2005; Boll et al., 2015). To this end, we
provide in this paper further insights into
the biology of introduced populations of the
freshwater flatworm Girardia tigrina (Girard,
1850).
Girardia tigrina is a widespread, native species of the Nearctic Region that has been accidentally introduced into Europe since the
1920s, very likely through the international
trade in aquatic plants and the activity of
aquarists. Subsequently, it has been recorded
also for South America, Israel, Japan, and Australia (Marcus, 1946; Gourbault, 1969; Benazzi
et al., 1970; Dahm & Gourbault, 1978; Ball &
Reynoldson, 1981; Kawakatsu et al., 1985, 1993;
Ribas et al., 1989; Benazzi, 1993; Sluys et al.,
1995, 2005; Vila et al., 2004).
The first European record of this alien
species was from Germany (Meinken, 1925),
after which it was reported from France
(De Beauchamp, 1946; Gourbault, 1969; Vila
et al., 2004), Switzerland (Dahm, 1955), GreatBritain (Reynoldson, 1956), The Netherlands
(Den Hartog, 1959), Luxembourg (Hoffmann,
1964), Iberian Peninsula and the Balearic
Islands, Azores (Baguña et al., 1980; Ribas
et al., 1989; Malhão et al., 2007; Vila-Farré et al.,
2011 and references therein), and also from
the Balkan peninsula from countries such as
Romania (An der Lan, 1962; Babalean, 2018)
and Herzegovina (Knezović et al., 2015). In
the Italian peninsula, since the 1960s populations of G. tigrina were recorded from many
localities (Stagni & Grasso, 1965; Stella & Margaritora, 1966; Benazzi, 1981, 1993; Stocchino
et al., 2013a). A fissiparous population of G.
tigrina was very recently discovered in a river
on the island of Sardinia, Italy (Stocchino,
2018).
In its native Nearctic Region G. tigrina is
represented by sexual, asexual (fissiparous)
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238
and seasonally alternating sexual/asexual
populations (Kenk, 1937). In contrast, the
great majority of the allochthonous populations in other parts of the world is fissiparous
(cf. Sluys et al., 2005).
Only a few (n = 5) immigrant sexual populations were discovered until now, all occurring
in Western Europe, viz. two populations from
Great-Britain (Reynoldson, 1985; Gee et al.,
1998), and single populations from Menorca Island (Spain) (Ribas et al., 1989), Italy
(Benazzi, 1993), and France (Vila et al., 2004).
The Italian sexual population was recorded
from a lake in the southern part of the peninsula. Successive studies revealed a high degree
of sterility of these Italian sexual specimens,
despite the great number of cocoons that was
laid (Benazzi & Giannini Forli, 1996).
Among fissiparous populations of G. tigri
na, the occurrence of so-called “ex-fissiparous”
individuals, i.e. planarians that stop multiplying by fission and acquire the sexual state, has
been scarcely reported (cf. Grasso, 1974 and
references therein; Ribas et al., 1989). Moreover, these very few records did not report details on the morphology of the reproductive
apparatus of these “sexualized” animals.
Here we report (1) the occurrence of sexualized individuals of G. tigrina from a fissiparous Italian population, representing the
second record for this species of spontaneous sexualization of fissiparous individuals;
(2) a detailed morphological description of
the reproductive apparatus of ex-fissiparous
specimens of G. tigrina; (3) the chromosome
complement of the Italian population; (4)
the marked intraspecific variation in several
morphological features of this species; (5)
some peculiarities of the hyperplasic ovaries
as well as morphological aberrations of the
reproductive apparatus in the Italian population. Possible means of dispersal and subsequent establishment of populations are
discussed.
Stocchino et al.
2
Materials and methods
2.1
Collection and culturing
Planarians (n = 6) were first collected in December 2009 from a rounded tank (~3 m in
diameter) localised outside of the greenhouse
of tropical terrestrial plants at the Botanical
Garden of the University of Genoa (Liguria,
Northwestern Italy), harbouring fishes (redwhite carps) and aquatic plants, such as Nim
phaea alba (L.), Nuphar lutea (L.) Sm., and
Lemna sp. (fig. 1A–B). The Botanical Garden
is located within the city’s historical centre.
More recently (May 2018), in two other tanks
planarians were found on the underside of
semi-submerged leaves of N. alba: a) a small
square tank (~60 × 60 cm, water depth 2 cm),
in which the planarians were associated with
gastropods and larvae of insects; and b) a
larger rounded tank (~3 m in diameter, water
depth 10 cm) in which the associated fauna
were leeches and gastropods.
All individuals were exclusively fissiparous at collection, with signs of fission being
visible. The collected specimens were transferred to the laboratory and were reared in
glass bowls under semi-dark conditions at 18
+/- 2°C; the worms were fed weekly with fresh
beef liver, while the bowls were cleaned within 8-12 h after feeding.
It was observed that in this culture, as
well as in a strain of a Sardinian population
(M. Pala & G.A. Stocchino, pers. obs.), animals
were extremely sticky, and that, for example,
when they were attached firmly to the surface
of the containers or to the paint brushes used,
it was very difficult to remove the animals
from these substrates.
2.2
Morphology and karyology
For morphological study, sexualized specimens were fixed for 24 h in Bouin’s fluid,
dehydrated in a graded ethanol series, then
transferred to clove oil, and, subsequently,
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EX-FISSIPAROUS ITALIAN FLATWORM GIRARDIA TIGRINA
embedded in synthetic wax or paraffin. Serial
sections were made at intervals of 8 μm and
stained with Mallory-Cason. Reconstructions
of the copulatory complex were obtained
by using a camera lucida attached to a compound microscope.
Chromosome metaphasic plates were obtained by the squashing method on single
caudal regenerative blastemas of 5 fissiparous
specimens. Blastemas were first treated with
a solution of colchicine (0.3%) for 4 h, then
transferred onto glass slides and treated with
a solution of acetic acid (5%) for 5 min. Subsequently, they were stained with acetic orcein
for 2 h and squashed using a small coverslip.
The chromosome complement was characterised on the basis of 5-6 metaphasic plates per
specimen. Chromosomal nomenclature follows Levan et al. (1964).
2.3
Material examined
ZMA V.Pl. 7283.1, freshwater tank at the Botanical Garden of the University of Genoa
(44°25’35” N, 8°54’54” E) Genoa, Italy, December 2009, coll. Paola Ramoino and Renata
Manconi, sagittal sections on 4 slides; CGAS
Pla 18.1, ibid., sagittal sections on 3 slides;
CGAS Pla 18.2, ibid., sagittal sections on 3
slides.
The material is deposited in the collections
of Naturalis Biodiversity Center, Leiden, The
Netherlands (ZMA collection code), and in
the Giacinta A. Stocchino collection (CGAS),
University of Sassari.
2.4
Abbreviations used in the figures
a, atrium; b, brain; ba, blind atrium; bc, bursal
canal; bro, branch of right oviduct; ca, common atrium; cb, copulatory bursa; cg, cement
glands; cm, circular muscles; csv, common
seminal vesicle; di, diverticulum; dp, diplotene oocytes; e, eye; ec, ectopic hyperplasic
ovaries; ed, ejaculatory duct; g, gonopore; h,
head; ho, hyperplasic ovaries; lm, longitudinal
239
muscle; lvd, left vas deferens; m, mouth; ma,
male atrium; o, oviduct; og, oogonia; pb, penis
bulb; ph, pharynx; php, pharyngeal pocket;
pp, penis papilla; rbc, right bursal canal; ro,
right oviduct; s, sperm; sba, supplementary
blind atrium; sd, spermatids; sg, shell glands;
sp, supplementary penis; spv, spermiducal
vesicles; st, spermatogonia; sv, seminal vesicles; vd, vas deferens.
3
Results
3.1
Sexualization
From the animals obtained during the first
collection three specimens sexualized, after
having been kept in the laboratory for about
five years, during which also fissioning occurred. These sexualized animals displayed
the characteristic features of ex-fissiparous
individuals: large body size, development of
the copulatory apparatus, hyperplasic ovaries.
Four cocoons were also laid, one of which was
found opened, but no juveniles were observed.
3.2
Morphological description
Body size of preserved, sexualized specimens
ranged from 4 to 6 mm in length and 1.2-2 mm
in width. Head triangular, with bluntly pointed anterior tip. Two eyes are present in the
middle of the head at the level of auricles, positioned close together and located in broad
pigment-free patches. Unpigmented auricular
grooves are marginally placed just posteriorly
to the eyes (fig. 1C).
The dorsal pigmentation pattern is of the
“striped type” (sensu Hyman, 1939): on the
yellowish-brown background colour run two
longitudinal stripes, made up of black spots,
separated by a lighter yellowish-brown middorsal band; further there are also black
specks haphazardly distributed on the dorsal
surface, while a clear zone runs along the entire body margin (fig. 1C).
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240
Figure 1
Stocchino et al.
Girardia tigrina. A. Geographic range of allochthonous sexual populations (filled circles) and populations with sexualized animals (triangles and asterisk) in the Western Palaearctic; asterisk: population
from Liguria investigated in the present study. B. Aquatic plants as preferential shaded microhabitat in
a tank at the Botanical Garden of the University of Genoa, Liguria. C. Habitus of a living ex-fissiparous
specimen of the Ligurian population. Scale bar not available.
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EX-FISSIPAROUS ITALIAN FLATWORM GIRARDIA TIGRINA
The pharynx is unpigmented, with the inner
and outer pharyngeal musculature bilayered,
i.e. without an extra, third, outer longitudinal
muscle layer. The position of the mouth opening is different in the three specimens examined. In specimen ZMA V.Pl. 7283.1 the mouth
is located at the hind end of the pharyngeal
pocket (fig. 2A), whereas in specimens CGAS
Pla 18.1 and CGAS Pla 18.2 the mouth opening is shifted anteriad, in that it is located at
about 1/6 and 1/4 , respectively, of the distance
between the posterior end of the pharyngeal
pouch and the root of the pharynx (fig. 2B, C).
The ovaries are hyperplasic in all three
specimens examined, with several scattered
masses of oocytes and oogonia in the body region directly posterior to the brain, occupying
almost the entire dorso-ventral space of the
body (fig. 3A). All oocytes show a regular meiotic maturation process, from prophase up to
the diplotene phase, when meiosis ends. Diplotene oocytes scattered in the ovarian masses
do not show degenerative nuclear or cytoplasmic stages (fig. 3C). Moreover, in specimen
ZMA V.Pl. 7283.1 some ovarian masses are located ventrally on the right-hand side of the
copulatory apparatus, that is, far into the posterior part of the body (fig. 3B). Also in these
ovarian masses no anomalies in the oocytes
were observed.
The anterior portion of the oviducts is expanded to form a seminal receptacle or ampulla, which communicates with the ovarian
masses at a variable position, depending on
the hyperplasic condition of the ovaries.
The infranucleated oviducts run ventrally
in caudal direction to the level of the copulatory apparatus and then curve dorsad towards
the vaginal area, subsequently opening symmetrically into the angled hind part of the
bursal canal (figs 4A, 5).
The testes are situated ventrally and extend from the level of the most caudal postcephalic ovarian masses into the posterior end
of the body. The follicles are well developed in
241
ZMA V.Pl. 7283.1 and CGAS Pla 18.1, whereas
they are under-developed in specimen CGAS
Pla 18.2. In the mature testes spermatogenesis
appears to proceed in a regular fashion, in
that no anomalies, such as irregularly shaped
spermatids or spermatozoa, were observed
(fig. 3D). Vitellaria are located, as usual, between the intestinal branches.
In ZMA V.Pl. 7283.1 two copulatory apparatuses are present in the post-pharyngeal
region: the fully developed system as well as
another, second one, the latter located at a
short distance behind the pharyngeal pocket.
This second copulatory apparatus is not completely developed and consists of only a dorso-ventrally oriented penis.
A single, left vas deferens can be observed
penetrating the muscular penis bulb of the
incompletely developed, second copulatory
apparatus. The penis bulb consists of loosely
interwoven layers of circular and longitudinal
muscle fibres. In this not fully developed system, the sperm duct enlarges to form a seminal vesicle, which continues as a very short
ejaculatory duct that opens at the tip of the
papilla. The ovoid penis papilla is covered by
a partially infranucleated epithelium, which
is underlain by a subepithelial layer of circular muscle, followed by a layer of longitudinal
muscle fibres. The penis papilla is housed in
a genital atrium, which is lined by a cuboidal, partially infranucleated epithelium, surrounded by a subepithelial layer of circular
muscles, followed by a layer of longitudinal
muscle fibres. The genital atrium is not divided into a male and common atrium and does
not communicate ventrally with a proper
gonopore (figs 4A, 6A).
Posterior to this incompletely developed
copulatory apparatus lies the main, welldeveloped apparatus (figs 4A, 6A). The following description is based on the fully
developed copulatory apparatus of ZMA V.Pl.
7283.1 and on those of CGAS Pla 18.1 and CGAS
Pla 18.2.
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242
Figure 2
Stocchino et al.
Girardia tigrina from Liguria. Photomicrographs of the pharynx; sagittal sections (anterior to the left).
A. ZMA V.Pl. 7283.1, mouth opening located at the hind end of the pharyngeal pocket; B. CGAS Pla 18.1,
mouth opening located at about 1/6 of the distance between the posterior end of the pharyngeal pouch
and the root of the pharynx; C. CGAS Pla 18.2, mouth opening located about at 1/4 of the distance
between the posterior end of the pharyngeal pouch and the root of the pharynx.
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EX-FISSIPAROUS ITALIAN FLATWORM GIRARDIA TIGRINA
Figure 3
243
Girardia tigrina from Liguria. Photomicrographs of hyperplasic ovaries and testes. A. ZMA V.Pl. 7283.1,
hyperplasic ovaries located behind the brain; B. ZMA V.Pl. 7283.1, ectopic hyperplasic ovarian masses
located at the level of the copulatory apparatus; C. ZMA V.Pl. 7283.1, magnification of hyperplasic
ovaries, with oocytes at different stages of maturation; D. ZMA V.Pl. 7283.1, mature testes with sperm.
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244
Figure 4
Stocchino et al.
Girardia tigrina from Liguria. ZMA V.Pl. 7283.1 A. sagittal reconstruction of the two copulatory apparatuses (anterior to the left); B. sagittal reconstruction of the main copulatory apparatus at the level of
the right branch of the bursal canal and the blind cavity (anterior to the left).
The small sac-shaped copulatory bursa is
lined by a columnar, glandular epithelium
bearing basal nuclei and it is surrounded by a
network of muscle fibres. The copulatory bursae of the three specimens examined contain
no spermatophores.
As usual, the bursal canal runs in a caudal direction to the left of the copulatory
apparatus. The posterior portion of the bursal canal curves sharply towards the ventral
body surface (thus constituting the so-called
“angled” bursal canal), and at this point receives the separate, symmetrical openings of
the oviducts, while, subsequently, it communicates with the common atrium. The bursal
canal is lined by cuboidal, nucleated, and ciliated cells and is surrounded by a subepithelial
layer of circular muscles, followed by a layer
of longitudinal muscle (figs 4–6).
Shell glands, producing a fine-grained,
erythrophil secretion, which is very abundant
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EX-FISSIPAROUS ITALIAN FLATWORM GIRARDIA TIGRINA
Figure 5
245
Girardia tigrina from Liguria. CGAS Pla 18.1, sagittal reconstruction of the copulatory apparatus (anterior to the left).
18.1, open into the bursal canal just ectally,
i.e. ventrally to the separate openings of the
oviducts.
From the bursal canal of specimen ZMA
V.Pl. 7283.1 originates a left branch at a short
distance from the postero-dorsal wall of its
bursa that, subsequently, curves downward to
communicate with a blind sac or cavity located on the left side of the body; this cavity may
be considered to represent a small, additional
genital atrium.
This blind cavity is lined by an infranucleated epithelium that is underlain by a subepithelial layer of longitudinal muscle fibres and
receives the openings of several cement glands
(fig. 4B). At the level of the posteriorly located
ectopic ovaries, from the right oviduct originates a dorsal branch, which ascends to the
supernumerary left branch of the bursal canal
and then opens into it just at the point where
the canal opens into the blind additional atrium. Shell glands open into this oviduct just before the duct opens into the bursal canal.
The penis bulb is large and globose in ZMA
V.Pl. 7283.1 and CGAS Pla 18.1, whereas it is only
moderately developed in CGAS Pla 18.2, consisting of loosely interwoven layers of circular
and longitudinal muscle fibres (figs 4–6).
The sperm ducts form well-developed
spermiducal vesicles, packed with sperm, in
specimens ZMA V.Pl. 7283.1 and CGAS Pla 18.1.
In ZMA V.Pl. 7283.1 and CGAS Pla 18.2, the vasa
deferentia at first curve dorsad before symmetrically penetrating the anterior wall of the
penis bulb. In its ascending course, the left vas
deferens of ZMA V.Pl. 7283.1 forms a closed
loop (fig. 4A). In CGAS Pla 18.1 the vasa deferentia recurve considerably in caudal direction
before separately and symmetrically penetrating the antero-dorsal wall of the penis
bulb (fig. 5). Once inside the penis bulb, each
vas deferens enlarges to form a wide vesicle
in ZMA V.Pl. 7283.1, whereas the ducts fuse to
form a single intrabulbar vesicle in CGAS Pla
18.1 and CGAS Pla 18.2.
In all specimens the seminal vesicle(s) continue as a single ejaculatory duct that opens at
the tip of the penis papilla. In ZMA V.Pl. 7283.1
the ejaculatory duct follows a basically ventral
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Figure 6
Stocchino et al.
Girardia tigrina from Liguria. Photomicrographs of the copulatory apparatus; sagittal sections. A. ZMA
V.Pl. 7283.1, supernumerary penis and the main, fully developed copulatory apparatus; B. CGAS Pla 18.1,
copulatory bursa with the bursal canal, penis, male atrium, and common atrium with diverticulum; C.
CGAS Pla 18.2, copulatory bursa with the bursal canal, penis, and male atrium.
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EX-FISSIPAROUS ITALIAN FLATWORM GIRARDIA TIGRINA
centrally located in CGAS Pla 18.1 and CGAS
Pla 18.2 (figs 4–6). In CGAS Pla 18.2 the ejaculatory duct is very short.
In all specimens examined the intrabulbar seminal vesicles, including that of the
supernumerary penis in ZMA V.Pl. 7283.1, are
filled with a longitudinally oriented web-like
fibrillate secretion. In CGAS Pla 18.2 many
erythrophil granules are also present in the
seminal vesicle (figs 4–6).
In ZMA V.Pl. 7283.1 the penis papilla has the
shape of an elongated cone; in CGAS Pla 18.1
it is a stubby cone, while it is barrel-shaped
and more dorso-ventrally oriented in CGAS
Pla 18.2 (figs 4–6). The penis papilla is covered
by an infranucleated epithelium in ZMA V.Pl.
7283.1 and CGAS Pla 18.1, but in CGAS Pla 18.2
it is provided with a nucleated epithelium.
The epithelium of the papilla is underlain
with a subepithelial layer of circular muscle
fibres, followed by a layer of longitudinal
muscles.
In CGAS Pla 18.1 and CGAS Pla 18. 2 the genital atrium is clearly divided into a common
atrium and a male atrium, which communicate via a pronounced narrowing. In ZMA V.Pl.
7283.1 this division into two atria is less obvious, very likely due to the elongation of the
penis papilla, which occupies the entire common atrium, with its tip extending into the
gonopore (figs 4–6). The atria are lined by an
infranucleated epithelium, which is underlain
by a subepithelial layer of circular muscle, followed by a layer of longitudinal muscle fibres.
The common atrium opens ventrally through
the gonopore and it receives the coarsely granular, xanthophil secretion of very abundant
cement glands in ZMA V.Pl. 7283.1 and CGAS
Pla 18.1, while in CGAS Pla 18.2 these glands
are only moderately developed (figs 4–6).
In CGAS Pla 18.1 a well-developed posterior
diverticulum is present in the hind wall of its
common atrium, whereas in ZMA V.Pl. 7283.1
and CGAS Pla 18.2 this diverticulum is much
less pronounced (figs 4–6).
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3.3
Karyology
Metaphasic plates revealed that the fissiparous animals are characterized by a constant
diploid set of 16 chromosomes with n = 8 as
haploid number (fig. 7). The karyotype consists of 8 pairs of metacentric chromosomes
in descending order, with the first four chromosomes being metacentric isobrachial,
while the other chromosomes are metacentric
heterobrachial, with the exception of chromosome 8, which is at the border between
metacentric and submetacentric.
4
Discussion
4.1
Reproductive modes and
sexualization process
Sexualization of individuals from fissiparous
strains of freshwater planarians has been
highlighted since the early 1970s for species
of the genus Dugesia (cf. Benazzi, 1974). These
planarians were called “ex-fissiparous” and
are characterized by (a) an increase in body
dimensions, (b) development of a complete
copulatory apparatus, (c) hyperplasic ovaries,
(d) underdeveloped testes, and (e) sterility
or, at least, low fertility (cf. Stocchino et al.,
2012, 2014; Harrath et al., 2013, 2014).
Only a few cases of sexualized individuals from fissiparous strains of G. tigrina have
been documented (Grasso, 1974 and references therein; Ribas et al., 1989). Grasso (1974)
obtained sexualization of many fissiparous
individuals from a north-Italian population
(Lake Maggiore), but only after the worms
had been fed for many weeks with crushed
tissues of sexually mature specimens of Poly
celis nigra (Müller, 1774). Typical hyperplasic
ovaries were reported for these ex-fissiparous
animals. It is noteworthy that this worker did
not succeed in obtaining sexualized animals
from this population when the flatworms
were fed for more than ten years with the
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Figure 7
Stocchino et al.
Girardia tigrina from Liguria. A. Metaphasic plate; B. Karyogram.
Many years later, Ribas et al. (1989) reported
on some rare, large ex-fissiparous individuals
from Spanish mainland populations, which
never laid cocoons. However, these Spanish
specimens were of the spotted morphotype
(see below, under “Spotted vs. striped morphotypes” section) with strongly and coarsely
pigmented pharynx, in contrast to the Italian
animals that we examined, which belong to
the striped morphotype with unpigmented
pharynx. Unfortunately, both Grasso (1974)
and Ribas et al. (1989) did not provide any
details on the reproductive apparatus of their
sexualized planarians. The present paper
provides the first detailed description of exfissiparous individuals of G. tigrina.
It is noteworthy that in one of the our animals examined an extra, albeit incomplete,
copulatory apparatus is present, as well as a
doubling of some structures in the main, completely developed apparatus, together with
an unusual course of the left vas deferens. It
is interesting to note that with respect to the
genus Girardia occurrence of supernumerary copulatory apparatuses was reported for
G. dorotocephala (Woodworth, 1897) by Kenk
(1935). This worker described that 17 out of 71
fissiparous specimens from a particular locality, which were kept at a constant low temperature and were fed with beef liver, developed
2-4 genital pores. These animals were on average larger than those with one genital pore,
while they also had a comparatively larger
post-pharyngeal region as compared with the
single-gonopore specimens. Three specimens
(with 4, 3, and 2 gonopores, respectively) that
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EX-FISSIPAROUS ITALIAN FLATWORM GIRARDIA TIGRINA
Kenk (1935) examined histologically showed
normal testes, hyperplasic ovaries, and
multiple copulatory apparatuses, developed
to greater or lesser extent and sometimes
connected to each other, being arranged one
behind the other along the anterior-posterior
axis of the body. After sexualization also cocoons were laid.
Supernumerary sexual structures were also
induced by low temperature in a laboratory
strain of Dugesia gonocephala (presently D. ja
ponica) from Japan (Ogukawa, 1955). However, differently from the Girardia species these
aberrations occurred only in sexual animals
with normal ovaries and testes.
A peculiar condition encountered in the
present study is that besides hyperplasic ovaries, located as usual just behind the brain, in
the aberrant specimen ectopic ovarian masses are localized ventrally at the level of the
copulatory apparatus, thus very far from the
usual anterior position. Such a very peculiar
phenomenon has never been reported before
for freshwater triclads.
It is surprising that in this case neoblasts not
destined to become germ cells were instead
induced toward this kind of differentiation.
In point of fact, it has been demonstrated that
the nanos gene is required for postembryonic
development, regeneration, and maintenance
of planarian germ cells (cf. Newmark et al.,
2008 and references therein). However, also in
fissiparous animals nanos-positive cells were
detected at positions in which germ cells are
first observed post-embryonically in sexual
planarians and, thus, they were considered to
be “presumptive germ cells” that are unable to
complete their differentiation (cf. Newmark
et al., 2008 and references therein).
It is known that environmental factors,
especially temperature, influence sexual or
asexual reproduction in populations with alternating reproductive modes (Kenk, 1937).
Moreover, sexualization of our G. tigrina and
the G. dorotocephala studied by Kenk (1935)
249
was not induced by food items consisting of
pieces of sexual planarians, which may have
supplied sex-inducing substances, as has
occurred in certain experiments (see Grasso
& Benazzi, 1973; Sakurai, 1981). Thus, it may
be that under laboratory conditions (constant
temperature and lighting and regular food
supply) in the usual ex-fissiparous animals,
at least in the genus Girardia, sometimes an
extra sexual induction occurs with the effect
that besides development of supernumerary
sexual structures also neoblasts not belonging
to presumptive germ cells are able to differentiate into germ cells.
Hyperplasic ovaries in ex-fissiparous individuals have been reported for a number of
Dugesia species (De Vries, 1986; Pala et al.,
1995; Stocchino et al., 2002, 2009, 2012, 2013b,
2014, 2017; Harrath et al., 2013). In contrast, exfissiparous individuals of D. benazzii Lepori,
1951 and D. etrusca Benazzi, 1946 do not develop hyperplasic ovaries (Stocchino & Manconi,
2013).
Such hyperplasic ovaries are characterised
by degenerative processes, which lead to a
blockage of the maturation of the oocytes
(Gremigni & Banchetti, 1972a). It has been
demonstrated that a complex process of early
autophagy, followed by apoptotic processes,
occurs during the cell death of oocytes in the
hyperplasic ovaries of D. arabica and that cytokine-like molecules may contribute to this
pathology (Harrath et al., 2014, 2017).
As in species of the genus Dugesia occurrence of hyperplasic ovaries has been considered to be the result of a greater proliferation
of neoblasts into oogonia (Gremigni &
Banchetti, 1972b; Benazzi, 1974), we may surmise that the same process also produced the
hyperplasia in the ectopic ovaries.
Another peculiarity, evident in all three
G. tigrina specimens examined, concerns
the fact that, notwithstanding the hyperplasic condition, oogenesis appears regular
and the hyperplasic
ovaries lack the typical
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250
degenerative stages that are characteristic of
such sexualized specimens in several Dugesia
species (Gremigni & Banchetti, 1972a; Stocchino et al., 2009, 2012, 2013b; Harrath et al.,
2014, 2017).
Furthermore, in the aberrant individual,
and also in another specimen examined,
testes are also well-developed, exhibiting all
phases of sperm maturation. That spermatogenesis is regular is also highlighted by (1) the
presence of sperm inside the vasa deferentia,
of which the posterior tracts are enlarged as
spermiducal vesicles, and (2) the presence of
allosperm in the oviducts, indicating previous
mating(s) with another animal(s).
The regular female and male gametogenesis may be related to the eudiploid condition
of the Italian population (see below), in that it
allows a more regular meiosis and that, thus,
potential sexual reproduction cannot be ruled
out, although no juveniles were observed to
hatch from the opened cocoon of these sexualized animals.
With respect to sexualized freshwater
planarians, only one case of well-developed
testes has been reported, namely for D. bi
fida, albeit that in this species the ovaries are
only weakly hyperplasic. Furthermore, in this
species the chromosome complement is eudiploid, while sexualized animals laid fertile
cocoons (Stocchino et al., 2014). In contrast,
the majority of the fissiparous species of the
genus Dugesia are triploid or mixoploid, with
the sexualized specimens exhibiting gonadal
anomalies (Stocchino et al., 2014).
The presence of a fibrillate secretion in
the seminal vesicles has never been reported
for G. tigrina and may be compared with the
“web-like tissue” described by Hyman (1956)
for D. diabolis (presently G. dorotocephala
(Woodworth, 1897)) and later also reported by Ball (1971) and Chen et al. (2015) for
G. dorotocephala and G. sinensis Chen & Wang,
2015, respectively. Probably it results from the
Stocchino et al.
mixing up of “….a secretion of some sort ….”
(Ball, 1971, p. 15) and sperm that is also present
in the seminal vesicle (Chen et al., 2015).
4.2
Karyology
Our karyologycal analysis revealed a constant
diploid chromosome complement (2n = 16; n =
8). The basic haploid complement of 8 chromosomes is in agreement with previous studies (cf.
Benazzi & Benazzi-Lentati, 1976; Kawakatsu et
al., 1981; Ribas et al., 1989; Benazzi, 1993). While
for sexual populations only diploid karyotypes
have been reported, fissiparous populations
show diploid, triploid or mixoploid (diploid
and triploid) karyotypes (Dahm, 1958; Benazzi
& Benazzi-Lentati, 1976; Kawakatsu et al., 1981;
Ribas et al., 1989; Benazzi, 1993).
That all chromosomes in our specimens
examined are metacentric is in agreement
with previous data on European populations
(Ribas et al., 1989 and references therein). In
contrast, for several non-European populations karyotypes were reported that consisted
of 7 pairs of metacentric chromosomes and
one pair of submetacentric chromosomes
(the 6th chromosome set): (1) a sexual population from Canada (Puccinelli & Deri, 1991); (2)
sexual and asexual populations from South
Brazil (Kawakatsu et al., 1981); (3) asexual populations from Japan (Kawakatsu et al., 1985,
1993; Tamura et al., 1985).
4.3
Intraspecific morphological
variability
Our results highlight the marked intraspecific
morphological variability of several features
of G. tigrina, such as: (a) pharynx pigmentation, being present or absent; (b) position of
the mouth opening, which may be located at
the hind end of the pharyngeal pocket or may
be shifted anteriad (located at about 1/6 or 1/4
of the distance between the posterior end
of the pharyngeal pouch and the root of the
pharynx); (c) length of the ejaculatory duct,
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EX-FISSIPAROUS ITALIAN FLATWORM GIRARDIA TIGRINA
which may be long or very short; (d) number
of the seminal vesicles, which may be single
or double; (e) course of the vasa deferentia,
which, before penetrating the antero-dorsal
wall of the penis bulb, in some cases simply
curve dorsad, while in other cases they recurve
considerably in caudal direction. These features will be discussed below.
Although a pigmented pharynx was considered a diagnostic character for G. tigrina
(cf. Kenk, 1972; Ball & Reynoldson, 1981), further studies have also reported on specimens
with unpigmented pharynx (Ribas et al.,
1989; Benazzi, 1993). Nevertheless, Sluys (2001)
considered the pigmented pharynx to be an
apomorphic character for the genus Girar
dia, in spite of the fact that a few species are
polymorphic, in that individuals may have
either pigmented or unpigmented pharynges,
and very few have an unpigmented pharynx.
Our finding of specimens with unpigmented
pharynx thus confirms the variability of this
character.
The position of the mouth opening was
considered a feature that warranted further
consideration, since the mouth was found to
be at different positions in the pharyngeal
pocket of animals from different geographic
localities (Sluys et al., 2005). However, this
variability appears to be much less due to a
geographic gradient than to intraspecific variation, as in our three animals from Italy the
location of the mouth turned out to be highly
variable, even within a single population.
Presence of two seminal vesicles in one of
our individuals and a single intrabulbar vesicle
in the other two individuals confirms previous reports on the variability of this character
(cf. Kenk, 1972; Ribas et al., 1989). According to
Kenk (1972), a single seminal vesicle would be
a transitory condition, but we interpret it as
being the result of intraspecific variation.
The seminal vesicle(s) may continue either
as a long ejaculatory duct or as a very short
251
duct, both conditions having been reported in
the present paper and also in earlier studies
(e.g. Ball, 1971, figs 5, 7; Kenk, 1972; Kawakatsu
& Mitchell, 1981).
Another character showing marked differences among populations concerns the arrangement of muscles around the bursal canal.
Sluys et al. (2005) suggested that the Nearctic
and Neotropical forms of G. tigrina concern
sibling species, in view of the fact that South
American populations have a bursal canal
musculature consisting of a well developed
coat of intermingled circular and longitudinal muscle fibres, whereas North American
forms are characterised by a simple coat of
muscle around the bursal canal made up by
a thin, subepithelial layer of circular muscle,
followed by an equally thin layer of longitudinal muscle. The latter condition was found
also in introduced populations, such as those
from Spain, France, southern Italy (Ribas
et al., 1989; Vila et al., 2004; Sluys et al., 2005),
and our population from Liguria, northern
Italy. This may be an indication that all of the
above-mentioned introduced European populations originate from the Nearctic Region.
4.4
Spotted vs. striped morphotypes
With respect to its external features, G. tigrina
is a polymorphic species that varies from
spotted to striped, both types of individuals
sometimes occurring in the same population
in its native area (Kenk, 1972). Several of these
conditions are expressed also in European
populations: 1) all sexual populations are of
the striped type (Ribas et al., 1989; Benazzi,
1993; Gee et al., 1998; Vila et al., 2004); 2) the
majority of fissiparous populations are spotted without exhibiting sexualization processes (i.e. producing ex- fissiparous animals) (cf.
Ribas et al., 1989 and references therein); 3) a
single case of a fissiparous striped population
and with development of ex-fissiparous specimens (present paper); 4) some fissiparous
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252
Stocchino et al.
spotted populations exhibiting sexualization
processes (Grasso, 1972, 1974; Ribas et al., 1989).
Ribas et al. (1989) related the external pigmentation pattern of several Spanish populations with the pharynx pigmentation, thus
distinguishing three morphological classes:
A, fissiparous “spotted” with pigmented
pharynx; B, fissiparous “spotted” with unpigmented pharynx; C, sexual, “striped” with
unpigmented pharynx. Class A and the group
made by classes B and C together were considered as natural groups or races, also on the
basis of biochemical data. To these classes we
can now add a fourth class, comprising the
Ligurian fissiparous, “striped” population with
unpigmented pharynx.
The presence in Europe of several morphological groups may be explained as the result
of multiple independent introductions of G.
tigrina to this continent, very likely from the
Nearctic Region (see above), as already suggested by Ribas et al. (1989) for the Spanish
populations. Future molecular analyses might
form appropriate tests of this hypothesis.
chroa (Ball & Reynoldson, 1981), and S. medi
terranea (M. Pala & G.A. Stocchino, pers. obs.)
as this has been observed under laboratory
conditions, and thus may imply strong competition with the native planarian fauna.
Another possible feature facilitating its
spread over the world may be related to our
observation that the animals are extremely
sticky, which greatly enhances their accidental passive dispersal on, for example, aquatic
plants. This may explain their occurrence in
the botanical garden in Genoa.
Adhesiveness in flatworms is known to be
facilitated by the secretion of adhesive glands.
It is not known whether the observed stickiness of G. tigrina is due to an overproduction
of mucus or to particular characteristics of
the mucus released by the adhesive glands of
this species. To our knowledge there are no
comparative studies on this subject, which
deserves further investigation.
4.5
This research was supported by the following
grants to G.A. Stocchino: a grant in memory
of Prof. N.G. Lepori (Università di Sassari), a
Temminck Fellowship from Naturalis Biodiversity Center (Leiden, The Netherlands),
and by a grant from Prof. R. Pronzato (Dipartimento di Scienze della Terra, dell’Ambiente e
della Vita, Università di Genova). We acknowledge financial support from Fondazione di
Sardegna and Regione Autonoma Sardegna
(RAS2012-LR7/2007-CRP-60215” Conservazione e valorizzazione delle grotte sarde: biodiversità e ruolo socio-economico-culturale”).
Prof. M. Pala is thanked for her kind support. We are grateful to Dr. P. Ramoino and
Prof. G. Barberis (Dipartimento di Scienze
della Terra, dell’Ambiente e della Vita, Università di Genova) and Dr. E. Mora (Botanical
Garden of Genoa) for their kind help in collecting planarians and
identification of plants.
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Dispersal and establishment of
populations
Many underlying causes have been suggested
for the remarkable dispersal and establishment capability of G. tigrina, such as (a) fissiparous reproduction, enabling it to rapidly
colonize new water bodies (Wright, 1987),
(b) extreme tolerance to suboptimal and demanding environmental conditions, allowing
it to spread along water bodies which are denied to native species (Wright, 1987), (c) ability to exploit a wide variety of food resources
with overlap in the diets of G. tigrina and
native triclads, thus suggesting a potentially
strong inter-specific competition for food (oligochaetes, isopods, chironomids, snails, caddisflies, mayflies, amphipods, and cladocerans) (Pickavance, 1971; Gee & Yang, 1993), and
(d) perhaps also cannibalism on other species
of planarians such as Polycelis spp. and S. poly
Acknowledgements
via free access
EX-FISSIPAROUS ITALIAN FLATWORM GIRARDIA TIGRINA
G.A. Stocchino, A.H. Harrath and L. Mansour
extend their appreciation to the Deanship of
Scientific Research at King Saud University for
funding the work through the research group
RG-164. Finally we want to thank Marta Riutort and two anonymous reviewers for their
constructive comments.
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RECEIVED: 29 NOVEMBER 2018 | REVISED AND
ACCEPTED: 25 APRIL 2019
EDITOR: R. VONK
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