Changing the environment one lives in may require significant changes in animal’s morphology and behaviour. This is especially the case if we are talking about such drastic change as a switch from living on land to living in or on water. Some 200 million years ago ancestral species of a group of semi-aquatic bugs, known as Gerromorpha, had made such transition and over time, evolved traits allowing them to live on the surface of water. One of many new challenges facing animals living on water is being exposed to predators not only from above but also from below. Insects within the infraorder Gerromorpha have adapted their leg morphology to both be able to glide on the surface of water and escape the water-dwelling predators. The more ancestral-like species of Gerromorpha have midlegs which are shorter than hindlegs and move in a similar pattern like the closely related terrestrial species, whereas the more derived species, e.g. water striders, have longer midlegs and move with rowing-like leg motions. It is quite difficult to identify the exact genetic factors which determine development of adaptive such traits because these traits are often a result of both genetic and epigenetic modifications as well as are usually not a single gene determined. However, a recent study by a research group from The Institute of Functional Genomics of Lyon has been able to find a link between the morphology of water strider’s legs, an anti-predator response and the genes involved.
Water striders detect predators via changes in vibrations of water caused by an approaching attacker. To escape an attack the strider bends its mid- and hindlegs, pushes off the surface of water and induces a vertical jump up and out of reach of a predator (see video 1 and figure 1). It takes around 29ms for the strider to leave the surface of water once the threat has been perceived and only the mid- and hindlegs are involved in generating the jump. Interestingly, though an adult strider can itself fly, the anti-predator response is jump-action based because to initiate a flight takes much longer than a jump and by that time a poor strider would be long gone (see time comparison in video 1).
Video 1. Water strider’s predator escape response and reaction time comparison between initiation of jump and flight movements.
It is the specific leg morphology (longer midlegs and shorter hindlegs) of water striders which enables an efficient jumping response. To understand which genes may be involved in development of this specific trait the research group compared the expression profiles of genes expressed in midlegs and hindlegs during the development of a water strider. Notably, a gene called gilt had a profile that matched the difference in leg morphology: in long midlegs it was abundantly expressed and in shorter hindlegs its expression was absent (figure 2, blue stain indicates gilt expression sites). The function of gilt is unknown and it has never before been linked to development but a similar gene has been previously described to be involved in immune responses in flies and mammals. When the expression of gilt was assessed in three Gerromorpha species that have different locomotion types and don’t jump in the presence of predators (i.e. the species that are more closely related to a ground dwelling ancestor) no gilt expression was found in their midlegs, which again suggested the link between gilt and acquisition of specific leg morphology in water striders (note the absence of blue staining in legs of more ancestral species in figure 2).
A gene called Ubx has been previously described to be involved in modulating leg morphology in water striders. Notably, Ubx has an inverse expression profile compared to gilt, meaning it is minimally expressed in midlegs and highly expressed in hindlegs. Based on this observation scientists decided to silence the expression of Ubx in developing embryos of the striders and observed that, in the absence of Ubx, gilt can now be expresses both in mid- and hindlegs. These observations suggest that Ubx is modulating (either directly or indirectly) the expression of gilt.
Having identified the potential genetic culprits that can affect leg morphology the next step was to show the functional link between anti-predator response and the noted genes. The researchers used adult striders in which expression of gilt was inhibited and showed that giltless insects are unable to jump as high as those striders who can express the gilt gene (compare the jump height in video 2). These results suggest, that gilt gene has facilitated adaptation to living in aquatic environments by contributing to development of longer midlegs in water striders. Longer legs allow the insect to jump higher and more likely out of reach for a predator in response to an attack from below. The study was unable to test how Ubx gene affects the behavioural response because in the absence of Ubx the striders’ embryos don’t survive.
Video 2. Comparison between the height of an anti-predator jump acheaved by gilt-less gilt RNAi and wild type (untreated) water striders.
Overall, the study has showed how changes is gene expression during evolution of Gerromorpha lineage have enabled these insects to adapt to new challenges posed by a different environment. However, as the authors themselves note, this single identified change in gene expression is only one of many changes that slowly accumulated during the course of evolution and led to insects which we see today and which appear to be so well suited for the niche they live in.
Armisén D, Nagui Refki P, Crumière AJ, Viala S, Toubiana W, & Khila A (2015). Predator strike shapes antipredator phenotype through new genetic interactions in water striders. Nature communications, 6 PMID: 26323602