Science Literature

Water striders are found all over the world, moving effortlessly over the surfaces of ponds, lakes and rivers. Fossil Gerridae and Hydrometridae date back to the Upper Paleocene and fossil Veliidae and Mesoveliidae have been discovered in Lower Cretaceous rocks. This implies an origin in the Middle Cretaceous or earlier followed by relative stasis. The insects belong to the Infra order Gerromorpha and all have piercing and sucking mouth parts. Although there is remarkable diversity of leg lengths and shapes, the group is distinguished by the relative lengths of their mid and hind legs. "The mid-legs are disproportionately long and function as oars, whereas the hind-legs are shorter and function as rudders."

A water strider showing its long mid-legs (source here)

The researchers were aware that the "Hox gene Ultrabithorax (Ubx) is known to play multiple roles in defining specific morphological differences among the segments along the anteriorposterior body axis in arthropods, including appendage size, shape, and function". Consequently, testing the role of Ubx became an objective of their research: does this gene affect the leg dimensions of these insects?

Of the three pairs of legs, the front legs are designated L1, the mid-legs L2 and the hind-legs are L3. In the presumed ancestor, the ground plan is L3>L2>L1. The Gerromorpha have L2>L3>L1. The changes are associated with adaptation: locomotion on water surfaces. The research involved mapping early stage development and identifying events where Ubx is expressed.

"Our results show that Ubx establishes the appendage ground plan in water striders through elongating L2, and through multiple functions that establish the identity of the third thoracic segment, including the shortening of L3. In other insects that present the common L3>L2>L1 appendage ground plan such as O. fasciatus and Acheta domesticus, Ubx is expressed in L3 only and functions to elongate its size. Therefore in the novel L2>L3>L1 ground plan of water striders, Ubx has evolved a new expression domain but maintained its ancestral elongating function in L2, whereas in L3, Ubx has maintained its ancestral expression domain but evolved a new shortening function."

In an accompanying press release, Professor Locke Rowe expresses surprise that the one gene had two opposite functions. At the present stage of the research, there is no insight yet into the way this gene operates and why it is different from other insect ground plans.

"To our surprise, we discovered that Ultrabithorax performs opposite functions in different limbs. It lengthens the mid-legs but shortens the hind-legs to establish this unusual body plan that allows water striders to glide on the water surface."

So far, so good. This is all interesting stuff. But then the focus shifts from development (involving empirical studies) to evolution (which goes into the realm of history). The press release continues:

"Determining how these major evolutionary changes happen is a central goal of evolutionary biology, explained Rowe. "Many have marveled at the ability of water striders to walk on water, and we are excited to have discovered the gene that has affected this evolutionary change." "

This research can be understood in terms of the evo-devo approach: small genetic changes affecting early development can achieve significant evolutionary transformations in adult organisms. Let us suppose that the authors are right in thinking that the "evolution of a novel appendage ground plan in water striders is driven by changes in the Hox gene Ultrabithorax". The novelty is in the relative lengths of the mid and hind legs. This is not a new organ and there appears to be no change in the complexity of the specified information.

We need to be realistic in our assessment of what has been achieved. In a blog, Cornelius Hunter writes about this research: "Incredibly, evolutionists were quick to add their gratuitous, scientifically meaningless, interpretation of the findings. [. . .] How cogent. This would be like entering an automobile manufacturing plant, finding the robot that installs the doors, and claiming to have discovered how the doors evolved." There are likely to be many other characteristics of water striders that need to be in place before there is a functioning organism. Perhaps these need to be analysed in some detail before getting too excited by the Ubx story.

Evolution of a Novel Appendage Ground Plan in Water Striders Is Driven by Changes in the Hox Gene Ultrabithorax
Abderrahman Khila, Ehab Abouheif, Locke Rowe
PLoS Genetics, 5(7): e1000583, 2009 | DOI: 10.1371/journal.pgen.1000583

Abstract: Water striders, a group of semi-aquatic bugs adapted to life on the water surface, have evolved mid-legs (L2) that are long relative to their hind-legs (L3). This novel appendage ground plan is a derived feature among insects, where L2 function as oars and L3 as rudders. The Hox gene Ultrabithorax (Ubx) is known to increase appendage size in a variety of insects. Using gene expression and RNAi analysis, we discovered that Ubx is expressed in both L2 and L3, but Ubx functions to elongate L2 and to shorten L3 in the water strider Gerris buenoi. Therefore, within hemimetabolous insects, Ubx has evolved a new expression domain but maintained its ancestral elongating function in L2, whereas Ubx has maintained its ancestral expression domain but evolved a new shortening function in L3. These changes in Ubx expression and function may have been a key event in the evolution of the distinct appendage ground plan in water striders.

See also:

Luke, K. U of T scientists identify gene that enables water striders to glide across water, University of Texas Press Release (August 13, 2009)