By Katharina Moser, Windhoek
Do sharks exist? Of course, every reader will answer. But is that really true, as we think? Possibly not, according to a genome study that examined numerous shark species and their genomes.
The genome study of dozens of shark species and their close relatives suggests that the ocean's apex predators may not form a natural biological group, contrary to what studies with more limited genetic data have suggested. This is according to a report in the journal Nature.
The analysis, published last month on the preprint server bioRxiv, concludes that when examining some “ultra-conserved” parts of the genome, a special family of sharks called Hexanchiformes may be part of an evolutionary lineage that differs from the group to which all other sharks and rays belong.
The results, which have not yet been peer-reviewed, suggest, according to the Nature report, that most animals that humans refer to as sharks are more closely related to rays and skates than to Hexanchiformes shark species. Biologists refer to such groups as paraphyletic.
Whether or not a group of animals is paraphyletic is the big question for many scientists. But accurate family trees or phylogenies—including one for sharks—help researchers trace the evolution of important characteristics. “An accurate phylogeny is a way to understand the processes that have shaped life,” says Gavin Naylor, an evolutionary biologist at the Florida Museum of Natural History in Gainesville, according to Nature.
Parts of the genes studied confirmed the existing “monophyletic” family trees of sharks, with rays and skates classified as separate from all sharks. However, other sequences suggested that sharks were paraphyletic: an unusual group called Hexanchiformes formed a lineage that differed from the one from which all other sharks and rays emerged.
One implication of this phylogenetic tree is that flat-bodied animals, including manta rays, evolved from shark-like ancestors. “This result implies that rays are just another type of shark and that the shark body plan came first,” Brownstein says in Nature.
However, he and Near prefer the hypothesis that sharks are paraphyletic, partly because their analysis supported this phylogenetic tree more strongly than a monophyletic one. To determine which of the two phylogenetic trees is correct, it may be necessary to sequence additional shark species and examine other types of genetic markers.v
