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| The Biology and Evolution of Trematodes |
| An Essay on the Biology, Morphology,
Life Cycles, Transmissions, and Evolution of Digenetic Trematodes |
by
Kirill V. Galaktionov
Zoological Institute of the Russian Academy of Sciences, St.
Petersburg, Russia
Andrej A. Dobrovolskij
St. Petersburg State University, Russia
The
trematodes are parasitic flatworms of great medical and veterinary
importance. An understanding of the evolution of trematodes depends
on an interpretation of their complex and diverse life cycles. It
is the life cycles in general and the stages that comprise these
cycles that are the focus of the detailed analysis presented herein.
The book contains a broad scope of modern information on digenetic
trematodes, from descriptions of their morphology and development
to their behaviour and the structure of their populational groups.
The book provides information on all characteristics of trematode
organization and biology from an evolutionary standpoint. Possible
scenarios of early stages of life cycle formation are discussed
as well as a consideration of further evolution in different taxa
and ecological groups of trematodes. An original approach to the
elaboration of a natural system of these parasites is proposed.
The book is addressed to zoologists
and parasitologists, as well as to researchers from a wide range
of disciplines interested in understanding the evolution of life
cycles and host-parasite interactions. It will be useful as a textbook
for undergraduate and postgraduate students.
Kluwer Academic Publishers, Dordrecht
Hardbound, ISBN 1-4020-1634-4
November 2003, 620 pp.
EUR 210.00 / USD 231.00 / GBP 145.00
To order: http://www.wkap.nl/prod/b/1-4020-1634-4
RUSSIAN EDITION (1998)
The Origin and Evolution of Trematode
Life Cycle.
K.V. Galaktionov
and A.A. Dobrovolskij
Sankt-Peterburg,
"NAUKA", 1998, p.404, ISBN 5-02-026089-4 [in Russian].
CONTENTS
SUMMARY
Based on analysis of the original and literature data, the book
gives a review of some specific features of morpho-functional organization
of trematodes at all stages of their life cycle. The special attention
is paid to the parthenogenetic generations (mother sporocysts and
their larvae — miracidia, daughter sporocysts and rediae) which
drop out the field of vision of modem zoologists and parasitologists
investigating evolution of trematodes. It is shown that the main
trend of morphological evolution of miracidia is their simplification
and miniaturization which is accompanied by the passage from active
infection of the first intermediate host with free-swimming larva
to the passive one whereby miracidium hatching takes place in the
alimentary tract of molluscan host having swallowed the egg. The
evolution of parasitic stage of the mother sporocyst development
shows apparent trend to the increase of generative function. In
the primitive families (Fasciolidae, Paramphistomatidae, Echinostomatidae
and others) reproduction of mother sporocysts is practically ending
during miracidium development. The increase of mother sporocyst
reproduction in the «strigeidid» branch of higher trematodes usually
is not accompanied by any essential morphological transformations
of the parasitic stages. At the same time, the main direction in
the mother sporocyst morphological evolution in the «plagiorchiid»
branch is morpho-functional disintegration. As a result, individual
parts of the parthenite act as independent organisms. In the most
specialized specimens (Microphallidae, Lecithodendriidae, some Plagiorchidae
and others) generative units (germinal masses, germ cells, embryos
etc.) parasitize independently the host. The morphological simplification
is also obvious in the evolution of the daughter parthenogenetic
generation and may be demonstrated by reolacing of rediae by daughter
sporocysts in the more advanced trematode.
Heterochronies
resulting most often in juvenilization and miniaturization of the
organism are revealed in the evolution of different stages of hermaphroditic
generation, especially in cercariae of the higher orders Strigeidida
and Plagiorchiida. The simplification of the definitive organization
in the latters permits to purpose energetic sources for the development
of rather advanced adaptations (highly-differentiated gland complex,
tail with striated musculature, stylet in Xiphidiocercariae, host-finding
behavior, etc.) directed to the solution of main task of the larva
— infection of the second intermediate host. Formation of these
heterochronies is only possible under rather advanced interactions
between parasite and the second intermediate host. The effective
use of energetic sources of the latter has permitted to carry out
development of the definitive structure and preparation to infection
of the final host at the stage of metacercaria.
At
the post-cercarial stages of the ontogenesis specimens of hermaphroditic
generation in Plagiorchiida and Strigeidida demonstrate different
ways of their morphological evolution. In the former family the
trend to juvenilization and miniaturization covers adults, too,
whereas in the latter one «imaginization» (i.e. complication of
definitive structure) takes place and, as a result, among trematodes
strigeidid adults have the most complex structure. The mentioned
evolutionary trends demonstrates two probable ways of the increase
of individual fecundity of adults: (1) due to elongation of longevity
of adults producing a few large eggs enriched with yolk (Strigeidida)
and (2) production of numerous small eggs containing full-formed
miracidia during the short term of the adult existence in the final
host (Plagiorchiida). The both strategic; provide a successful completion
of the parasite life cycles in ecosystems of different sort. Other
trematodes demonstrate transitional versions of these strategies.
The diversity of the versions reflects a real diversity of ecosystems
where the trematode life cycles are completed.
A
significant part of the book is devoted to the analysis of adaptations
in trematode life cycles (both cycle as a whole and individual stages).
The analysis is carried out both at the level of specimen and population.
The description of the specific feature in spatial and temporal
structure of groups of miracidia infecting actively and passively
their hosts, generations of mother and daughter parthenites, cercariae,
metacercariz (including adolescariae and mesocercariae) and adults
is given separately. The analysis has permitted to propose a scheme
of successive stages in biological radiation of trematodes. It was
shown that inhabiting of ocean pelagic areas was connected with
complication of circulation ways and including one more host providing
development of mesocercaria between the first intermediate host
and the second one. Just opposite trend take place in the course
of trematode penetration into terrestrial and littoi ecosystems.
Here secondary dixenic life cycles devoid of free-living larvae
have been formed.
The
analysis of trematode evolution itself is based on the principle
of the posse sing equal rights study of all stages of the life cycles,
i.e. according to the opinion that trematode evolution is evolution
of their life cycles. From this point of view all cum notions on
this topic including widely-distributed D. R. Brooks and his co-workers'
ideas (Brooks et al., 1985, 1989) have been critically examined.
Searching for I ancestrial («sister») group among modern trematodes
is proved to be ineffective even specimens from the so-called «primitive»
families (Fasciolidae, Paramphiston tidae, Notocotylidae, Azygiidae
etc.) have both archaic and advanced features. Evolutionary development
of trematodes is considered as result of complex evolution, transformations
of their life cycles, the individual stages and generations composing
them are characteristic of different directions of the adaptations.
The
authors have attempted to typologization the life cycles of modern
trematodes The original scheme of probable ways of their transformations
in the course of taxon evolution and its penetration into ecosystems
of different sorts is given. An obligatory dixenic life cycle with
stable heterogony is considered as an initial one. The infection
of the final host was carried out per os by free-swimming adult-like
larvae devoid any adaptations to prolongation of longevity in the
environment. In modern trematodes such cycles are probably absent.
The
further evolutionary transformations of dixenic life cycle were
probably connected only with changes in biology of dispersive stages.
The better probability of successful infection of the final host
is got by two ways. The larvae keeping arc organisation gained ability
to encystment in the environment, and the adolescaria stage was
added to the onthogenesis of hermaphroditic individuals. Among the
modern trematodes such a life cycle may be found in families Fasciolidae,
Paramphistomat (and closely-related groups), Notocotylidae, Pronocephalidae.
The other trematode group chose cercaria specialization, and their
longevity in the environment increased due to improvement of locomotion
and appearance of movement of discrete type. In this case final
hosts became infected after swallowing of free-swimming larvae.
Archaic cycles of this type have also disappeared. Probably, Azygiidae
and Bivesiculidae which cercariae infect their final hosts per os
are their direct descendants.
The
next stage is appearance of trixenic cycles; in most cases it was
connected with replacing of free-living adolescaria by parasitic
metacercariae. Facultative trixeny appeared independently in Notocotylidae
and Echinostomatidae but only in the latter the initial «lodgerment»
is replaced by true metacercarial parasitism. Life cycles of this
type are characteristic of the most modern trematodes both keeping
some archaic characters (Heterophyidae, for example) and the most
advanced (Strigeidida, Palgiorchiida). But trixeny arised, probably,
several times and by different ways as in all above-mentioned groups
(with the exception of Strigeidida, possibly) the second intermediate
host is the latest one whereas in trixenic azygiids the latest host
is the modern final host. In this case cycle becomes longer due
to a «superstructure» of a sort. The initial final host becomes
the second intermediate one.
The
further transformations of the life cycles were connected with two
just opposite trends. The common phenomenon is decreasing in host
number — passage to dixenic cycles took place independently in different
groups. Sometimes it was not connected with abridgement of the trematode
life cycle itself, which maintained all stages. In this case either
mollusk may act as both the first and the second intermediate I
hosts (Heronimus mollis — Heronimidae, some Echinostomatidae, Microphallidae,
etc.), or the final host acts as first as the second intermediate
host (genus Opisthioglyphe — Plagiorchiida and others). Dixenic
cycles of the other type also results from progenetic development
of parasites and are characteristic of disappearance of the final
host as the metacercaria gets reproductive maturity. In the case
the reduction of circulation ways is accompanied by hypomorphosis
combined with disappearance of the adult. The good example is Paralepoderma
brumpti (Plagiorchiidae) life cycle. The authors consider that disappearance
of marita stage has been resulted in the development of numerous
group of blood parasites (Schistosomatidae, Spirorchidae, Sanguinicolidae)
and ectoparasites (Transversotrematidae).
Monoxenic
life cycles are of different origin. In plagiorchiids and hemiurids
they resulted from the progressive decrease in number of the hosts
in the typical trixenic cycles or dixenic ones where molluscs combined
functions of two hosts. Monoxenic cycles in Azygiids possessing
initially the archaic dixenic cycle is of the other origin.
Just
opposite trend (increase in number of the animals used in the life
cycle) is also revealed in trematode evolution. Sometimes up to
four hosts may be recorded (tetraxenic life cycles). This is accompanied
by appearance of one more developmental stage — mesocercaria. The
origin of tetraxenic cycles is rather unclear. In hemiurids and
dydimozoids this phenomenon probably resulted from «superstructure»,
as trixenic cycle in azygiids did. In Strigea and Alaria it resulted
from including of one more step in the chain of carnivores circulated
(infected) by the parasites.
The
given data demonstrate that evolution of trematode life cycles as
any other evolutionary processes are accompanied by numerous appearances
of parallelisms. The each case needs a special analysis for adequate
evaluation of importance and nature of either phenomenon observed.
Creation of the «natural» trematode system reflecting the real phyletic
interactions between certain groups requires a thorough accounting
of the specific features in structure and biology of all stages
of the life cycles. Only this will permit proper identification
of plesiomorphic states and adequate estimation of apomorphies and
homoplasies.
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CONTENTS
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Introduction
Chapter 1. Specific features in the structure of specimens
from parthenogenetic and hermaphroditic trematode generations.
1.1. Parthenogenetic generations
1.1.1. The first (mother) parthenogenetic generation
1.1.1.1. Miracidium
1.1.1.2. Parasitic stage in mother sporocyst development.
1.1.2. The second (daughter) parthenogenetic generation.
1.2. Hermaphroditic generation.
1.2.1. Cercaria.
1.2.2. Metacercaria.
1.2.3. Adult (marita).
Chapter 2. Trematode life cycle as a system of adaptations.
2.1. Mother sporocyst.
2.1.1. Miracidium adaptations.
2.1.1.1. Actively-infecting miracidia.
2.1.1.2. Passively-infecting miracidia.
2.1.2. Parasitic stage in mother sporocyst development.
2.2. Daughter generation of parthenites.
2.3. Hermaphroditic generation.
2.3.1. Cercaria adaptations.
2.3.2. Metacercaria adaptations.
2.3.3. Adult (marita) adaptations.
Chapter3. Typologization of trematode life cycles.
3.1. Trixenic life cycles.
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3.2. Trixenic life cycles with two endogenous
agglomerations
3.3. Dixenic life cycles.
3.4. Homoxenic life cycles.
3.5. Tetraxenic life cycles.
Chapter 4. Structure specific features in trematode population.
4.1. On the nature of parasite populations.
4.2. Host-parasite interactions and their manifestations at
population level.
4.3. Phase analysis of trematode populations.
4.3.1. Hemipopulations of mother sporocyst larvae.
4.3.2. Microhemipopulations of parthenogenetic generations
4.3.3. Cercaria hemipopulations.
4.3.4. Metacercaria hemipopulations.
4.3.5. Adult (marita) hemipopulations.
4.3.6. General notes.
Chapter 5. Main principles and trends in trematode evolution.
5.1. Main trends in morphological evolution of parthenogenetic
and hermaphroditic generations.
5.1.1. Parthenogenetic generation.
5.1.2. Hermaphroditic generation.
5.2. Ways of trematodes expansion in ecosystems of different
types.
5.3. Evolution of trematode life cycles.
Conclusion.
Summary.
References.
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