Evolutionary mechanisms yield biological diversity


An international team of scientists has discovered how changes in both gene expression and gene sequence led to the diversity of visual systems in African cichlid fish.

In research published in a the latest issue of the journal PLoS Biology, Assistant Professor Karen Carleton, together with others describe how over 60 species of cichlid fish from Lake Malawi and Lake Victoria have adapted their visual sensitivity in response to specific ecological factors, including what they eat and the clarity of the water in which they swim.

Evolutionary biologists seek to understand the mechanisms behind genetic changes that have led to the vast diversity of life on Earth. There are two important molecular mechanisms that contribute to organismal diversity - changes to the sequence of genes, and changes in the way genes are expressed, including when, where, and how much of a gene is made.

First study

This study was one of the first to look at how both gene sequence and gene expression can contribute to the same trait, and showed that they contribute in complementary ways.

“African cichlid fishes are some of the most diverse animals on the planet. Their visual systems differ dramatically in their sensitivity and represent some of the largest differences known in vertebrates,” explains Hofmann in a University of Maryland press release. “Yet there has been little understanding as to why such diversity exists. Our findings have important implications for understanding both the factors and the mechanisms responsible for generating biodiversity.”

Different genes

Cichlids have several different cone opsin genes that enable them to detect light across the visible and ultraviolet regions of the spectrum. Different species express different subsets of these opsins to create alternate visual systems.

The method of foraging for food was a key factor influencing fish vision. Fish whose diets consist primarily of zooplankton were more likely to have UV sensitivity, which enables them to detect the presence of these small transparent aquatic organisms that absorb ultraviolet light. In contrast, cichlids in the murky waters of Lake Victoria expressed longer wavelength combination of opsin genes, regardless of what they ate.

This long wavelength combination matches the light that is best transmitted through the murky water. A few Lake Victoria fish at clearer sites turned on shorter wavelength genes, suggesting that opsin expression matches the light environment. Therefore opsin gene expression in both lakes is adaptively determined based on important ecological variables.