Origin and evolution of alternative developmental strategies in amphibious sarcopterygian parasites (Platyhelminthes, Monogenea, Polystomatidae)

Most integrative studies involving phylogenetic, developmental and ecological trends showed that the diversity of developmental modifications among the Platyhelminthes was linked to transmission opportunity pressures. For parasitic flatworms with complex life cycles it was suggested that the evolutionary forces that constrained or enhanced developmental strategies implied heterochronic patterns. Similar patterns were also reported from the Monogenea with direct life cycles, especially for Polystomatidae, which infest amphibious Sarcopterygians. Polystoma, whose members are recovered almost exclusively from anuran hosts of the Neobatrachia, is capable of following two alternative developmental strategies depending on the physiological stage of its host. Processes by which parasites reach maturity are strikingly different, and lead to discrete adult phenotypes within the same parasite species. In the present study, we investigate the origin and evolution of developmental patterns of polystomatids in a phylogenetic framework, using an integrative approach of heterochrony and evolutionary ecology. The results suggest that both phenotypes have coexisted during the early stages of polystome evolution, and that neither of them can be considered as the ancestral one. The two developmental pathways, each associated with one life cycle, may have arisen independently prior to polystome diversification, when strictly aquatic sarcopterygians attempted colonization of temporary freshwater environments. The occurrence of these two patterns within species of the genus Polystoma is suggested to reflect the ancestral condition, and to have allowed both developmental strategies to be successful depending on shifts in transmission opportunities. Thus, host evolutionary ecology may be the main factor in shaping developmental strategies within polystomatids.     

Historical biogeography of amphibian parasites, genus Polystoma (Monogenea: Polystomatidae)

Aim  The present-day geographical distribution of parasites with a direct biological life cycle is guided mostly by the past dispersal and vicariance events that have affected their hosts. The Amphibia– Polystoma association (which satisfies these criteria) also exhibits original traits, such as host specificity and world-wide distribution. This biological model was thus chosen to investigate the common historical biogeography of its widespread representatives.
Location  North and South America, Eurasia and Africa.
Methods  We investigated the phylogeny of 12 species of neobatrachian parasites sampled from North and South America, Eurasia and Africa. Hosts belonged mostly to hyloids and ranoids of families Bufonidae, Hylidae, Leptodactylidae, Ranidae and Hyperoliidae. Phylogenetic reconstructions were inferred from maximum likelihood and maximum parsimony analyses from complete ITS1 sequences.
Results  The group of American species appeared paraphyletic with one species at the base of a Eurafrican clade, within which two lineages were seen: one composed of only Eurasian species, and the other of European and African species, with the two European species basal to an African clade.
Main conclusions  The route of Polystoma evolution is deduced from the phylogenetic tree and discussed in the light of host evolution. We conclude that Polystoma originated in South America on hyloids, after the separation of South America from Africa. The genus must have colonized North America in Palaeocene times and Eurasia by the mid-Cainozoic, taking advantage of the dispersal of either ancestral bufonids or hylids. Africa, however, appears to have been colonized more recently, during the Messinian period.     

Alternative development in Polystoma gallieni (Platyhelminthes,Monogenea) and life cycle evolution

Considering the addition of intermediate transmission steps during life cycle evolution, developmental plasticity, canalization forces and inherited parental effect must be invoked to explain new host colonization. Unfortunately, there is a lack of experimental procedures and relevant models to explore the adaptive value of alternative developmental phenotypes during life cycle evolution. However, within the monogeneans that are characterized by a direct life cycle, an extension of the transmission strategy of amphibian parasites has been reported within species of Polystoma and Metapolystoma (Polyopisthocotylea; Polystomatidae). In this study, we tested whether the infection success of Polystoma gallieni within tadpoles of its specific host, the Stripeless Tree Frog Hyla meridionalis, differs depending on the parental origin of the oncomiracidium. An increase in the infection success of the parasitic larvae when exposed to the same experimental conditions as their parents was expected as an adaptive pattern of non-genetic inherited information. Twice as many parasites were actually recorded from tadpoles infected with oncomiracidia hatching from eggs of the bladder parental phenotype (1.63 ± 0.82 parasites per host) than from tadpoles infected with oncomiracidia hatching from eggs of the branchial parental phenotype (0.83 ± 0.64 parasites per host). Because in natural environments the alternation of the two phenotypes is likely to occur due to the ecology of its host, the differential infection success within young tadpoles could have an adaptive value that favors the parasite transmission over time.     

Ultrastructure of the protonephridial system of Polystomoides malayi Rohde and P. renschi Rohde (Monogenea: Polystomatidae)

Rohde K. 1973. Ultrastructure of the protonephridial system of Polystomoides malayi Rohde and P. renschi Rohde (Monogenea : Polystomatidae). International Journal for Parasitology3: 329–333. Polystomoides malayi and P. renschi have three types of protonephridial flames. The first type is a typical flame cell with internal and external ribs connected by a weir membrane without nephrostomes, and with internal and external leptotriches. The second type is a flame cell complex consisting of at least two flames reaching into a common cavity. The third type is a non-terminal (= lateral) flame in the protonephridial ducts, consisting of loosely arranged cilia many of which have lateral tube-like extensions and whose tips have irregularly arranged filaments gradually decreasing in number. The number of cilia in all types of flames varies. The smallest capillaries are strongly convoluted and have a smooth or slightly reticulated surface, the larger ducts have strongly reticulated walls and single cilia may be found in the cavities of the reticulum.     

Hox Genes from the Tapeworm Taenia asiatica (Platyhelminthes: Cestoda)

Hox genes are important in forming the anterior-posterior body axis pattern in the early developmental stage of animals. The conserved nature of the genomic organization of Hox genes is well known in diverse metazoans. To understand the Hox gene architecture in human-infecting Taenia tapeworms, we conducted a genomic survey of the Hox gene using degenerative polymerase chain reaction primers in Taenia asiatica. Six Hox gene orthologs from 276 clones were identified. Comparative analysis revealed that T. asiatica has six Hox orthologs, including two lab/Hox1, two Hox3, one Dfd/Hox4, and one Lox2/Lox4. The results suggest that Taenia Hox genes may have undergone independent gene duplication in two Hox paralogs. The failure to detect Post1/2 orthologs in T. asiatica may suggest that sequence divergence or the secondary loss of the posterior genes has occurred in the lineage leading to the cestode and trematode.     

Morphology and geographical variation of Pseudokuhnia minor n.g., n.comb. (Monogenea: Polyopisthocotylea)

A new genus, Pseudokuhnia, is established for Kuhnia minor, family Mazocraeidae. It differs from Kuhnia in the presence of two dorso-lateral vaginae, supported by surface ridges; differently shaped large genital hooks, large hamuli and ovary; an opisthaptor which is ventral to and well separated from the body proper; and a rudimentary antero-supplementarium of the clamps. The Atlantic population of P. minor differs from the Pacific population mainly in the larger size of the hamuli and the smaller size of the large and small genital hooks.     

Hox genes in sea spiders (Pycnogonida) and the homology of arthropod head segments

The pycnogonids (or sea spiders) are an enigmatic group of arthropods, classified in recent phylogenies as a sister-group of either euchelicerates (horseshoe crabs and arachnids), or all other extant arthropods. Because of their bizarre morpho-anatomy, homologies with other arthropod taxa have been difficult to assess. We review the main morphology-based hypotheses of correspondence between anterior segments of pycnogonids, arachnids and mandibulates. In an attempt to provide new relevant data to these controversial issues, we performed a PCR survey of Hox genes in two pycnogonid species, Endeis spinosa and Nymphon gracile, from which we could recover nine and six Hox genes, respectively. Phylogenetic analyses allowed to identify their orthology relationships. The Deformed gene from E. spinosa and the abdominal-A gene from N. gracile exhibit unusual sequence divergence in their homeodomains, which, in the latter case, may be correlated with the extreme reduction of the posterior region in pycnogonids. Expression patterns of two Hox genes (labial and Deformed) in the E. spinosa protonymphon larva are discussed. The anterior boundaries of their expression domains favour homology between sea spider chelifores, euchelicerates chelicerae and mandibulate (first) antennae, in contradistinction with previously proposed alternative schemes such as the protocerebral identity of sea spider chelifores or the absence of a deutocerebrum in chelicerates. In addition, while anatomical and embryological evidences suggest the possibility that the ovigers of sea spiders could be a duplicated pair of pedipalps, the Hox data support them as modified anterior walking legs, consistent with the classical views.Supplementary material is available for this article at and is accessible for authorized users.Guest editors Jean Deutsch and Gerhard Scholtz     

The Hox Paradox: More complex(es) than imagined

An understanding of the origin of different body plans requires knowledge of how the genes and genetic pathways that control embryonic development have evolved. The Hox genes provide an appealing starting point for such studies because they play a well-understood causal role in the regionalization of the body plan of all bilaterally symmetric animals. Vertebrate evolution has been characterized by gene, and possibly genome, duplication events, which are believed to have provided raw genetic material for selection to act upon. It has recently been established that the Hox gene organization of ray-finned fishes, such as the zebrafish, differs dramatically from that of their lobe-finned relatives, a group that includes humans and all the other widely used vertebrate model systems. This unusual Hox gene organization of zebrafish is the result of a duplication event within the ray-finned fish lineage. Thus, teleosts, such as zebrafish, have more Hox genes arrayed over more clusters (or "complexes") than do tetrapod vertebrates. Here, I review our understanding of Hox cluster architecture in different vertebrates and consider the implications of gene duplication for Hox gene regulation and function and the evolution of different body plans.     

Hox genes define distinct progenitor sub-domains within the second heart field

Much of the heart, including the atria, right ventricle and outflow tract (OFT) is derived from a progenitor cell population termed the second heart field (SHF) that contributes progressively to the embryonic heart during cardiac looping. Several studies have revealed anterior-posterior patterning of the SHF, since the anterior region (anterior heart field) contributes to right ventricular and OFT myocardium whereas the posterior region gives rise to the atria. We have previously shown that Retinoic Acid (RA) signal participates to this patterning. We now show that Hoxb1, Hoxa1, and Hoxa3, as downstream RA targets, are expressed in distinct sub-domains within the SHF. Our genetic lineage tracing analysis revealed that Hoxb1, Hoxa1 and Hoxa3-expressing cardiac progenitor cells contribute to both atria and the inferior wall of the OFT, which subsequently gives rise to myocardium at the base of pulmonary trunk. By contrast to Hoxb1^Cre, the contribution of Hoxa1-enhIII-Cre and Hoxa3^Cre-labeled cells is restricted to the distal regions of the OFT suggesting that proximo-distal patterning of the OFT is related to SHF sub-domains characterized by combinatorial Hox genes expression. Manipulation of RA signaling pathways showed that RA is required for the correct deployment of Hox-expressing SHF cells. This report provides new insights into the regulatory gene network in SHF cells contributing to the atria and sub-pulmonary myocardium.     

Complex selection associated with Hox genes in a natural population of lizards

Hox genes are recognized for their explanatory power of bilateral development. However, relatively little is known about natural variation in, and the evolutionary dynamics of, Hox genes within wild populations. Utilizing a natural population of sand lizards (Lacerta agilis), we screened HoxA13 for genetic variation and an association with incidence of offspring malformations. We found significant effects of parental genetic similarity and offspring sex, and their interaction, on risk of hatching malformed as an offspring. We also found within population genetic variation in HoxA13, and identified a significant effect of a three-way interaction among Hox genotype, parental genetic similarity, and offspring sex on the risk of hatching malformation. Since malformed offspring in this population do not survive to maturity, this study reveals complex and ongoing selection associated with Hox genes in a wild reptile population. Importantly, this demonstrates the utility of natural populations in unveiling microevolutionary processes shaping variation in highly conserved genes.     

Pseudodactylogyrus kamegaii sp. n. (Monogenea: Pseudodactylogyridae) from wild Japanese eel,Anguilla japonica

The monogenean Pseudodactylogyrus kamegaii sp. n. is described, based on specimens collected from the gills of wild Japanese eel Anguilla japonica caught in Chiba Prefecture, Japan. This species is the most similar to P. anguillae (Yin and Sproston, 1948), but different in the shape and measurements of the male copulatory organ, vagina and marginal hook. This new species was collected from the eel in brackish waters, while P. anguillae and P. bini, the other known pseudodactylogyrids of Japanese eel, have been recorded only in fresh waters.     

Closing the mitochondrial circle on paraphyly of the Monogenea (Platyhelminthes) infers evolution in the diet of parasitic flatworms

Relationships between the three classes of Neodermata (parasitic Platyhelminthes) are much debated and restrict our understanding of the evolution of parasitism and contingent adaptations. The historic view of a sister relationship between Cestoda and Monogenea (Cercomeromorphae; larvae bearing posterior hooks) has been dismissed and the weight of evidence against monogenean monophyly has mounted. We present the nucleotide sequence of the complete mitochondrial (mt) genome of Benedenia seriolae (Monogenea: Monopisthocotylea: Capsalidae), the first complete non-gyrodactylid monopisthocotylean mt genome to be reported. We also include nucleotide sequence data for some mt protein coding genes for a second capsalid, Neobenedenia sp. Analyses of the new mt genomes with all available platyhelminth mt genomes provide new phylogenetic hypotheses, which strongly influence perspectives on the evolution of diet in the Neodermata. Our analyses do not support monogenean monophyly but confirm that the Digenea and Cestoda are each monophyletic and sister groups. Epithelial feeding monopisthocotyleans on fish hosts are basal in the Neodermata and represent the first shift to parasitism from free-living ancestors. The next evolutionary step in parasitism was a dietary change from epithelium to blood. The common ancestor of Digenea + Cestoda was monogenean-like and most likely sanguinivorous. From this ancestral condition, adult digeneans and cestodes independently evolved dietary specialisations to suit their diverse microhabitats in their final vertebrate hosts. These improved perspectives on relationships fundamentally enhance our understanding of the evolution of parasitism in the Neodermata and in particular, the evolution of diet.     

Synchronous and early activation of planarian Hox genes and the re-specification of body axes during regeneration in Dugesia (G.) tigrina (Turbellaria; Tricladida)

Seven Hox cluster-related genes (Dthox-A to -G) have been isolated from the freshwater triclad Dugesia (G.) tigrina, their sequence compared to other Hox genes and their expression in intact and regenerating organisms analyzed by whole mount in situ hybridization. Sequence comparison analyses show high similarities of D. tigrina Hox genes to anterior and medial groups of coelomate Hox genes. Expression analyses show very early, synchronous, and overlapping expression of Dthox -A, -E, -G and -F in anterior, posterior and lateral regenerative tissues. At one hour of regeneration all Dthox genes studied showed a neat, clear expression at the wound boundary. Later, as the blastema grows, the expression area expands to more proximal regions covering the blastema and the distal postblastema regions. Blastemas formed by intercalary regeneration also show a synchronous expression of the same Hox genes though the onset of activation is much delayed. The finding that the same set of Hox genes is synchronously activated in anterior, posterior, intercalary and lateral regeneration is in sharp contrast to its well established role in specifying antero-posterior pattern during embryonic development. The implications of these results as regards ancestral versus co-opted roles of Hox genes in development and regeneration are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.