Keywords: Cestoda; Onchobothriidae; Elasmobranch; Co-evolution; Host specificity. 1. Introduction group of vertebrates, dating back some million years. (Schaeffer Higher elasmobranch phylogeny and biostratigraphy. Zool. J. Linn. A Brief History of Elasmobranch Higher Systematics. Despite years elasmobranchs. dates back to descriptions of families of sharks and rays by Muller and Henle Higher elasmobranch phylogeny and biostratigraphy. Zoological. A Brief History of Elasmobranch Higher Systematics. Despite years elasmobranchs dates back to descriptions of families of sharks and rays by Muller and Henle Higher elasmobranch phylogeny and biostratigraphy. Zoological.
Introduction Carcharhiniformes ground sharks are the most speciose and widespread clade of extant elasmobranchs, containing about species arranged into eight families. Their evolutionary history extends back some Ma into the Middle Jurassic. Scyliorhinidae seemingly is the most plesiomorphic clade within this order Underwood and Ward, ; Cappetta, Most extinct and extant carcharhiniforms are small, but some extant members such as the tiger and bull sharks are amongst the largest marine predators.
Today, carcharhiniforms are distributed worldwide, from tropical to cold-temperate and even arctic waters Compagno et al. Some species have restricted geographic ranges, whereas others are effective long-distance swimmers and highly migratory Musick et al.
Carcharhiniformes represent the sister group to Lamniformes Musick et al. The family Scyliorhinidae catsharks is by far the largest family, with at least species in 17 genera Ebert et al. Triakidae houndsharks and Carcharhinidae requiem sharks are among the most diverse carcharhiniforms occurring in warm to temperate seas. Triakids and carcharhinids are known since the Early Cretaceous Cappetta, ; Maisey, ; Guinot et al. In Antarctica, chondrichthyan remains are very common and occur in the early Eocene to?
Most batoids have a strongly depressed disc-like body derived from a rhinobatoid-like as in rajids or from a plathyrhinid-like as in higher myliobatiformes ancestor [ 3 ].
Many different views have been proposed on batoid interrelationships. In this paper, Batoidea higher classification by McEachran and Aschliman [ 3 ] and Nelson [ 2 ] has been given. According to these authors, Torpediniformes are to be considered as a sister group to the remaining batoids.
Torpediniformes, commonly named electric rays, are known for being capable of producing an electric discharge up to volts depending on speciesused to stun prey and for defense. Electric rays are found from shallow coastal waters down to at least 1, metres depth. Three species of Torpediniformes, belonging to family of Torpedinidae, are frequent in the Mediterranean Sea, the common torpedo Torpedo torpedo Linnaeusthe marbled electric ray T.
Higher elasmobranch phylogeny and biostratigraphy - Dimensions
The first is characterized by yellow-reddish stains on the back with five blue spots surrounded by a black halo; the second is the most common and exhibits a yellowish-brown body with dark spots. The third, purplish-brown back and whitish on the belly, which reaches one meter in length, is much less present. Although monophyly of the batoids electric rays, sawfishes, guitarfishes, skates, and stingrays is widely accepted and well corroborated, the interrelationships within batoids remain controversial.
In particular, the most contentious issues concern the phylogenetic position of the Torpediniformes. The aim of this paper was to report the already known molecular markers that were used to reconstruct the phylogenetic position of Torpediniformes with respect to the other Batoidea and to discriminate between the various chromosome pairs in the endemic species in the Mediterranean Sea.Paleontology - The Best Evidence for Evolution?
These last genomic markers were also able to differentiate between the male and the female karyotypes. The cytogenetic assays collected the classic reconstruction of species karyotype and the fluorescence in situ hybridization FISH technique using the genomic fragments mentioned above. Phylogenetic Relationships and Karyological Properties The superorder taxonomy in cartilaginous fish is undergoing major revisions; currently there is no agreement as regards to the higher systematic level in these species.
A large number of different interpretations exists in scientific literature based on morphological characters [ 3 — 11 ], on molecular studies [ 12 — 14 ], and on conventional cytogenetics [ 1415 ].
Many scientists consider that batoids are monophyletic, even if the time of their divergence from the other elasmobranch fish remains controversial. Several more recent morphological lines of evidence seem to support the derivation of batoids from sharks [ 716 ], but immunological and molecular studies corroborate the hypothesis of batoids as separate from sharks [ 131718 ]. However, molecular phylogenetic evidence published in the last few years has principally focused on relationships among the elasmobranch superorders.
In fact, very few molecular studies addressing interrelationships within Batoidea have been published. One of them concerns only the phylogenetic relationships among the major lineages of myliobatoids, including one species for each of the four remaining batoid orders [ 19 ]. This paper represents the first attempt to introduce a species of Torpediniformes used as an outgroup in a phylogenetic reconstruction based on molecular markers bp of mtDNA: The karyological approach could provide an informative tool to contribute to the corroboration of systematic relationships in Elasmobranchii, and particularly in Torpediniformes.
In fact, many authors agree about an intimate correlation between the development of a new species and the simultaneous modification of its karyotype, confirming the close evolutionary links between chromosome number and morphology, speciation, and morphological diversification [ 20 ].
The Torpedinidae is the most investigated family of the order as concerns karyological morphology 5 species have been examined to date. Only two species have been studied in the Narcinidae and Narkidae families, while no data are available for the Hypnidae family. The species belonging to the order of the Torpediniforms exhibit different morphology in their karyotypes. In fact, considering the various species, it can be seen how their karyological parameters are different. Moreover, their karyotypes do not show sex chromosomes [ 15 ].
Karyological parameters for Torpediniformes species studied so far. The genome of many cartilaginous fish underwent a DNA increase during its evolutionary history followed by chromosomal rearrangements mainly in the form of fusions and translocations [ 15 ].
Especially within the superorder Batoidea karyologic evolution took place from the least evolved species to the most specialized. The progressive reduction of the diploid number, the increase of the number of chromosomes with two arms and the disappearance of microchromosomes occurred, probably as a consequence of polyploidy followed by diploidization and Robertsonian rearrangements Figure 1 [ 14152325 — 27 ].
Schematic representation of karyologic evolution within Batoidea superorder that led to a progressive reduction of the diploid number. This chromosomal rearrangement took place from the less-evolved species to the most specialized through Robertsonian fusions, that is, centric fusions between two acrocentric elements to form a metacentric or a submetacentric one.
This karyological pattern is clearly evident in the Torpedinidae family.
In fact, in T. In a previous work, the first assessment of relationships among different batoid species by using both ribosomal 16S mtDNA and 18S nuclear sequences was reported. These molecular data were further discussed with karyological evidence coming from the literature [ 14 ].
The phylogenetic results obtained by the analysis of the two separate gene sequences were similar to those coming from the combined data set. In general, 18S sequences proved better than 16S at resolving higher-order relationships. However, the opposite holds true for the study of lower-order relationships: The molecular relationships based on 16S and 18S sequences placed the Torpediniformes distant from the other batoid species in all the phylogenetic trees coming from this analysis.
The result was consistent with the earliest hypotheses by Compagno [ 42829 ] and Maisey [ 30 ], but also with a more recent interpretation by McEachran et al. Cladograms illustrating the relationships of extant Batoidea obtained from molecular [ 14 ] a and morphological data by Compagno [ 28 ] b and by McEachran and Aschliman, [ 3 ] c. Nodes show bootstrap values for neighbor joining NJmaximum parsimony MP and posterior probability for Bayesian analyses. Some studies are currently in progress to implement the information on the evolutionary relationships of other species of electric rays.
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Research is also being carried out on additional DNA fragments that could be used as molecular markers. Characterization of Repeated Sequences Only in the last few years different molecular approaches, first of all physical mapping on metaphase chromosomes, have been used to investigate the cytotaxonomic relationship among the living cartilaginous fish [ 27 ].
Comparative cytogenetics, as a powerful tool to study karyotypic variation, is based on accurate chromosome identification. Physical mapping involves in situ hybridization of specific segments of genomic DNA to their physical location on chromosomes. It is extremely useful in terms of gaining an insight into structural arrangements within the genome. The use of these molecular cytogenetic techniques have often either suggested new taxonomic implications or confirmed the existing phylogenetic relationships among the different fish species analyzed [ 273132 ].
Such molecular cytogenetic techniques were particularly helpful in elasmobranch karyology because in these fish genome compartmentalization is not present, as has also been demonstrated in the majority of teleostean species [ 3334 ]. FISH allowed the detection of fluorescent signals on the telomeres of both uni- and biarmed elements.
This interstitial FISH pattern might represent further evidence supporting the hypothesis that karyotype evolution in Torpediniformes occurred by a progressive reduction of chromosome number due to centric fusions [ 2636 ]. In fact, species that underwent karyotype rearrangements as a result of Robertsonian fusions also show nontelomeric sites of the sequences in addition to the telomeric ones [ 3738 ].
In situ hybridization of digoxigenin-labeled probe onto T.
Note the interstitial labeling on bi-armed elements arrows. Furthermore, the presence of additional interstitial sites of the sequences in T. In fact, the interstitial sites containing sequences are often involved in recombination events [ 37 ]. Retroposons represent a significant portion of repetitive DNA in Eukaryotes. They are mobile genetic elements that are amplified by a reverse transcription of an intermediate RNA [ 39 ].
Two large families of retroposons, first identified as interspersed repeated sequences, belong to this genomic component. The LINE long interspersed nuclear elements family is made up of long sequences, while the SINE family has short sequences, both irreversibly inserted in the genome [ 40 ]. Such a result is similar to that evidenced by Perez et al. The location of these specific Alu-like sequences on the chromosomes of T. Probably, through a similar mechanism, these Alu-like sequences retrotransposed within the T.
The hybridization patterns shown were practically identical in the two rays and in the two Torpedo species Figures 5 and 6. The results fixed the relationships present among the species attributed to the different superorders examined [ 29 ].