Odatria_14_NOV13 - Victorian Herpetological Society

Roughly half of all frog species have a life cycle that startswith eggs laid in water, which hatch into aquatic tadpoles,and then go through metamorphosis and become adultfrogs. The other half, according to the authors, “includesan incredible diversity of life cycles, including species inwhich eggs are placed on leaves, in nests made of foam,and even in the throat, stomach, or back of the female frog.There are also hundreds of species with no tadpole stageat all, a reproductive mode called direct development.While the structure of the human heart has been knownfor a long time, the evolutionary origin of our conductionsystem has nevertheless remained a mystery. Researchershave finally succeeded in showing that the spongy tissuein reptile hearts is the forerunner of the complex hearts ofboth birds and mammals. The new knowledge provides adeeper understanding of the complex conductive tissue ofthe human heart, which is of key importance in many heartconditions.FORERUNNER OF CONDUCTIVE TISSUE“The heart of a bird or a mammal -- for example a human-- pumps frequently and rapidly. This is only possiblebecause it has electrically conductive tissue that controlsthe heart. Until now, however, we haven’t been able tofind conductive tissue in our common reptilian ancestors,which means we haven’t been able to understand howthis enormously important system emerged,” says BjarkeJensen, Department of Bioscience, Aarhus University.Along with Danish colleagues and colleagues from theUniversity of Amsterdam, he can now reveal that the geneticbuilding blocks for highly developed conductive tissue areactually hidden behind the thin wall in the spongy hearts ofreptiles. The new results have just been published in thejournal PLoS ONE.Microhylid frog from New GuineaFor decades, it has been assumed that the typical mode(with eggs and tadpoles placed in water) gave rise to directdevelopment through a series of gradual intermediatesteps involving eggs laid in various places outside water.“However, the results show that in many cases, specieswith eggs and tadpoles placed in water seem to give risedirectly to species with direct development, without goingthrough the many seemingly intermediate steps that werepreviously thought to be necessary,” Dr. Wiens said.“The results also suggests that there many potentialbenefits for species that have retained aquatic eggs andtadpoles, such as allowing females to have more offspringand to colonize regions with cooler and drier climates.These advantages may explain why the typical frog lifecycle has been maintained for more than 220 million yearsamong thousands of species,” said Professor Wiens.Science DailySeptember 2012Researchers Find Our Inner ReptileHeartsThe genetic building blocks behind the human heart’ssubtle control system have finally been identified.An elaborate system of leads spreads across our hearts.These leads -- the heart’s electrical system -- control ourpulse and coordinate contraction of the heart chambers.25The reptilian heart has a thin wall surrounding a spongy inner part.In many ways, this resembles the embryonic state in birds, humansand other mammals. The anatomy of their hearts is subsequentlycompletely different from reptiles, but studies of the genetic buildingblocks now show that all the hearts have a common molecularstructure. The reptilian heart can thus provide us with insight intohow the heart works in a human.DIFFERENT ANATOMY CONCEALS SIMILARITY“We studied the hearts of cold-blooded animals likelizards, frogs and zebrafish, and we investigated the genethat determines which parts of the heart are responsiblefor conducting the activating current. By comparing adulthearts from reptiles with embryonic hearts from birds andmammals, we discovered a common molecular structurethat’s hidden by the anatomical differences,” explainsDr Jensen. Since the early 1900s, scientists have beenwondering how birds and mammals could have developedalmost identical conduction systems independently of eachother when their common ancestor was a cold-bloodedreptile with a sponge-like inner heart that has virtually noconduction bundles.