Sticky Toes Provide Clues to Evolution

Newswise — Like their more famous cousins, the geckos, anole lizards have evolved adhesive scales on their toes that allow them to climb and stick to even the smoothest leaves. When these “sticky toes” first evolved in anoles millions of years ago, it gave them such a survival advantage that it triggered an evolutionary event called an “adaptive radiation” during which a multitude of species rapidly appeared.

Yet, how this key innovation evolved remains a mystery locked within the leathery shell of a lizard egg. Now, Dr. Thomas Sanger at Loyola University in Chicago has developed new techniques to understand more about the process of evolutionary diversification by observing development in real time.

Sanger’s research begins in the Caribbean Islands where the results of the adaptive radiation event are apparent. Over 200 species of anoles reside here, and although all the species he studies have adhesive toes, the size of their toepads and number of scales located on the bottom of their feet differ. These differences in toepads determine whether a certain species can be found up in the tree canopy, on twigs, or scampering around on the ground. In order to determine how this diversity arose, Sanger collected species from different environments and brought them back to raise in his lab. “The goal is to understand the nitty gritty details of how you generate diversity” explains Sanger.

When a sticky toepad first develops, little ridges emerge before turning into the specialized adhesive scales covered in tiny, sticky hairs. These little ridges are not present in anoles relatives that lack sticky toes as adults. Yet, Sanger has found that the early ridges are present in all anole species that have toepads, regardless of their eventual shape and size.

“Across this range of ten to 12 species, which probably represent 30-40 million years of evolution, you still have the same exact events occurring early on in development” Sanger explains.

This means that whether or not a lizard will someday have sticky toes is determined in the earliest stages of development by a process that creates the early toepads (the ridges). But, a different mechanism that occurs later in development is responsible for creating the variation found in toepad size and scale number. Because the anoles develop hidden within eggs, researchers have historically been limited to only a snapshot view of embryonic development. This means they were not able to track a living, developing anole and observe changes that occur of a short time scale. Sanger solved this problem by developing a way to remove a portion of the fibrous outer layer of the eggshell without harm to the embryo, and then keep it safe as it continues to develop.

Now, if you enter Sanger’s lab during one of his studies, you will find incubators filled with growing anole embryos that have a portion of their eggshell removed. If you carefully remove the developing baby anole from the incubator, place it under a microscope, and don’t have anywhere to go for a few weeks, you can watch all the stages of development occur before you until a hatchling lizard appears. Having direct access to the embryo for the first time opens up a world of possibilities for conducting studies that will identify the genes and mechanisms involved in creating the diversity found in toepads. By doing so, Sanger’s work will unlock the secrets of sticky toes to determine just how these species diversified millions of years ago.

Dr. Sanger presented his research at the Society for Integrative and Comparative Biology meeting in New Orleans, LA.