Evolutionary Origin Of Fins, Limbs Discovered By UF Scientists

[Guest guest]

Evolutionary Origin Of Fins, Limbs Discovered By UF Scientists

http://www.medicalnewstoday.com/medicalnews.php?newsid=48160

Performance on the dance floor may not always show it, but people are

rarely born with two left feet. We have genes that instruct our arms

and legs to grow in the right places and point in the right

directions. They also provide for the spaces between our fingers and

toes and every other formative detail of our limbs.

Evolutionarily speaking, the genetic instructions used to construct

and position our limbs were being perfected more than half a billion

years ago in fishes, not along the sides of the body where the fins

that preceded human arms and legs sprouted, but at the midline that

runs along the backbone and belly.

This midline -- think of the dorsal, tail and anal fins of a fish -

is where the genetic template to produce fins originated, about 100

million years before paired fins evolved and about 200 million years

before paired fins evolved into limbs, according to University of

Florida genetics researchers. The findings, published online today in

the journal Nature, also provide insight into the evolutionary

history of genes involved in human birth defects.

" Given that paired fins made their evolutionary debut at a particular

location on the sides of the body, intuitively one would think the

genetic tools for fin development would be brought together in that

place, " said developmental biologist Cohn, Ph.D., an associate

professor with the UF departments of zoology and anatomy and cell

biology and a member of the UF Genetics Institute. " We've discovered

that the genetic circuitry for building limbs first appeared in an

entirely different place - the midline of the animal. "

The appearance of paired fins on the sides of early vertebrates was a

major evolutionary innovation toward fin - and eventually limb -

locomotion, Cohn said. The earliest fishes lacked paired fins,

similar to the modern-day lamprey - a species of jawless fish with a

dorsal fin and tail but no side fins - considered by biologists to

share many features with the ancestor of all vertebrates.

" The emergence of paired appendages was a critical event in the

evolution of vertebrates, " Cohn said. " The fossil record provides

clear evidence that the first fins evolved along the midline. The

sequence of evolutionary events leading to the origin of limbs has

been known for some time, but only now are we deciphering how these

events occurred at a molecular genetic level. "

Researchers isolated genes from the spotted catshark, a type of slow-

moving shark from the eastern Atlantic Ocean. By studying the

activity of a dozen genes in shark embryos, they determined shark

median fin development is associated with the presence of genes such

as HoxD, Fgf8 and Tbx18, which are vital in the development of human

limbs.

They also used molecular markers for different cell types to

determine which cells give rise to the median fins, finding that they

arise from the same cells that form the vertebrae. These same genes

dictate the emergence of symmetrical pairs of fins on the animal

sides, showing a shared developmental mechanism in completely

different locations, according to Renata Freitas and GuangJun Zhang,

co-authors of the paper and graduate students in UF's zoology

department.

Extending their genetic analysis to the lamprey - a living relic from

the time before fish had paired fins - researchers found the same

genetic cues in place.

" That we see these same mechanisms operating in lamprey fins tells us

they must have been assembled in the median fins first, and later in

evolution this entire genetic program was simply reutilized in a new

position to build the first paired fins, " Cohn said. " It tells us our

own arms and legs have their evolutionary roots in the dorsal, caudal

and anal fins of our fishy ancestors. "

Many of these genetic mechanisms are involved in human birth defects,

which provide insight into the evolutionary history of genes and

their functions.

" Knowing that many of these genes are responsible for limb defects in

humans is intriguing, " Cohn said. " What we've done is identify where

those developmental pathways originated during our evolutionary past

and how they became involved in limb development. "

Learning the mechanics of development enriches our understanding of

evolution, according to Ann Burke, Ph.D., an associate

professor of biology at Wesleyan University who was not connected

with the study.

" Using modern molecular techniques, this confirms in a lovely way an

idea that's been around for over 100 years about how paired fins may

have evolved in the first place, " Burke said. " To translate a 19th

century observation about fin development into modern molecular data

is a great thing for science. It has become increasingly important to

understand developmental processes in our attempts to understand

evolution. "