Re: Sharks, Humans Share Genetic Kinship

In message <1179842765.480078.324100@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>, Peter Pan <peterpan55555@xxxxxxxxx> writes
http://dsc.discovery.com/news/2007/05/21/sharkancestry_ani.html?category
=animals&guid=20070521103030&dcitc=w19-502-ak-0000

"Four hundred and fifty million years ago, sharks and humans shared a
common ancestor, making sharks our distant cousins. According to
recent research, this kinship is evident in our DNA, since at least
one shark species possesses several genes that are nearly identical to
those in humans.

The elephant shark's genome is so similar to ours that we wind up
having more in common with it - genetically speaking - than with other
species, such as teleost (bony skeleton) fishes, which are nearer to
us on the evolutionary tree.

"This was a surprising finding, since teleost fish and humans are more
closely related than the elephant shark is to humans," lead author
Byrappa Venkatesh told Discovery News.

The difference in time between the last common ancestor of mammals and cartilaginous fish and the last common ancestor of mammals and teleost fish is relatively short compared to the distance in time between the latter and the present day. Consequently, as the rate of change of DNA sequences varies between lineages and loci, we do not have a strong expectation for the relative similarity of mammals and cartilaginous fishes versus that of mammals and teleost fishes.

Note that the comparison in the paper is between specific species of mammals, cartilaginous fishes and teleost fishes, rather than between the groups as whole (which can't be performed until we have larger samples). The selected species may not be typical; in particular Fugu and Tetraodon were sequenced because of their small genome size.

Teleost fish are inferred to have undergone an additional round of genome duplication compared to the great majority of cartilaginous fish and tetrapod species, including those under consideration here. Consequently, an enhanced rate of loss of genes (as some are duplicated and redundant) is expected in teleosts (for example zebrafish and pufferfish have lost different sets of Hox genes), and therefore, if everything else was equal, we would expect a greater degree of conservation of synteny between man and elephant shark than between man and pufferfish or man and zebrafish. This is what the paper reports.

The paper also cites other work indicating that cartilaginous fish are the slowest evolving group of the three, and teleost fish the fastest. Given this a greater degree of sequence conservation between elephant sharks and humans than between either and teleosts would not be surprising.

Venkatesh, principal investigator at the Institute of Molecular and
Cell Biology in Singapore, and his team determined that sets of genes
on chromosomes, as well as actual genetic sequences, are "highly
similar in the elephant shark and human genomes."

The researchers not only analyzed the elephant shark genome, but also
the genes for other animals including pufferfishes, chickens, mice and
dogs.

Their findings were recently published in the journal PloS Biology.

The citation is Venkatesh et al, Survey Sequencing and Comparative Analysis of the Elephant Shark (Callorhinchus milii) Genome, PLoS Biology 5(5): 932-944 (2007).

The researchers identified 154 genes in humans that have comparable
matches in mice, dogs and elephant sharks. The similarities between
people, mice and dogs were not unexpected, given that they are all
mammals. Sharks, however, are cartilaginous fish that seem to bear
little physical resemblance to mammals.

Upon closer examination, sharks and humans do share certain
physiological and biochemical processes.

http://dsc.discovery.com/news/2007/05/21/sharkancestry_ani_02.html?categ
ory=animals&guid=20070521103030

"The researchers also found that shark and human immune systems are
very similar, since sharks have all four types white blood cells found
in mammals.

Sean Van Sommeran, executive director of the Pelagic Shark Research
Foundation in Santa Cruz, Calif., told Discovery News that he was not
entirely surprised to learn about the shark-human links.

"The field of genetics is a Pandora's box," Van Sommeran said.

He added, "Sharks copulate like mammals and females give birth to live
young, so sharks do have features in common with mammals. It makes
sense that these would show up in the genome."

Venkatesh said future studies on the elephant shark genome, which is
relatively small and easy to study, could reveal information about
human genes, such as how the immune system develops. Since sharks are
the oldest living jawed creatures with a backbone, studies on them may
even uncover how humans and other mammals evolved.


--
alias Ernest Major

.

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