Molecular and Chromosomal Markers for Evolutionary Considerations in Torpediniformes (Chondrichthyes, Batoidea)

Due to their basal position in the vertebrate phylogenetic tree, the study on elasmobranch genetics and cytogenetics can provide remarkable information on the mechanisms underlying the evolution of all vertebrates. In recent years, different molecular approaches have been used to study the relationships between the different taxonomic groups of cartilaginous fish, among them are the physical mapping of specific nucleotide sequences on chromosomes. However, these are controversial, particularly in Torpediniformes in which the species have different karyological parameters. The purpose of this paper is to gather the molecular markers so far present in literature 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, Torpedo torpedo, T. marmorata and T. nobiliana. The 5S and 18S ribosomal DNA, the HpaI and Alu SINE, the telomeric (TTAGGG)n and the spermatogenesis-related SPATA 16, SPATA 18, and UTY sequences were particularly useful. These last genomic segments were also able to differentiate between the male and the female karyotypes. Moreover, the torpedoes showed a particular genomic organization, especially Torpedo torpedo, in which large quantities of highly repeated DNA and a characteristic distribution of heterochromatin, which is never centromeric, were observed. 1. Introduction The Chondrichthyes (cartilaginous fish) are one of the three extant major clades of jawed vertebrates and comprise two sister groups, the elasmobranchs (rays, sharks, and skates) and the holocephalans (chimeras and ratfish). They occupy an important phylogenetic position as the sister group to all other jawed vertebrates and as one of the first lineages to diverge in vertebrate evolution [1]. Despite this, there have been relatively few studies about chondrichthyan genetics and cytogenetics. Batoids are one of the biggest systematic groups that are included in elasmobranch fish [2]. 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