*Image above is from http://ppcorn.com/us/2015/10/17/top-10-weirdest-looking-insects/
Related to the cicada, the ‘treehopper’ is a common name for a group of small Hemipteran insects belonging to the Membracidae family (Prud’homme, et al., 2011). Pictured in figure 1, each species of treehopper has evolved distinctive ‘helmet’ like structures protruding dorsally over the length of the treehopper’s body that can resemble elements in the insect’s environment (Prud’homme, et al., 2011; Heffer & Pick, 2013). The helmet is exclusive to treehopper species, and displays an extensive variety of shapes, sizes, colours and textures (Prud’homme, et al., 2011). Prior to a study conducted by Prud’homme et al (2011) scientific opinion considered the helmet to be an enlarged expansion of the pronotum on the thorax (Deitz, 1975; Funkhouser, 1950). Prud’homme et al, (2011) conducted research on the Membracidae helmet and hypothesised the helmet was a wing serial homologue appendage on the treehoppers first (T1) thoracic segment. Since these findings have been published, articles have contradicted the conclusions, prompting scientific debate (Yoshizawa, 2012; Mikó, et al., 2012). This essay will explain that the helmet provides the treehopper with adaptive potential by mimicking environmental objects and reducing risk of predation. It will outline the helmet is a novel feature for insects, due to it being a change to the insect body plan and a serial homologue on the T1 segment, an evolutionary change not seen on T1 segments in other insects. The essay will discuss key morphological and genetic similarities between the treehopper helmet and other insect wings, suggesting shared ancestry with other insects, similar genetic networks and natural selection has played a part in helmet evolution. Finally, the essay will summarise the genetic evidence suggesting that genetic changes downstream of the Scr gene have removed the ancestral repression of Hox genes, allowing for the expression of old genes, and the development and variation of the treehopper helmet.
Adaptation is defined by Olson (2012) as “the process by which form comes to reflect function as the result of the action of natural selection; also an organismal ‘part’ that reflects this process”. The helmet provides adaptive potential by assisting with environmental adaptation (Prud’homme, et al., 2011). Some treehopper helmets have evolved to mimic certain environmental objects such as insects, droppings, branches, thorns, bark, fungi and lichens (Prud’homme, et al., 2011; Olson, 2012). Similarities with environmental objects suggest natural selection has influenced the evolution of the helmet to mimic objects that will assist the treehopper with adaptation and camouflage from predation (Prud’homme, et al., 2011; Pick & Heffer, 2012).
The treehopper helmet can be considered a novelty amongst insects for a number of reasons. Evolutionary and developmental biologists study the helmet as it represents a key variation in the body plan of an insect, providing an example of a morphological novelty as a result of natural selection (Heffer & Pick, 2013). Prud’homme et al, (2011) hypothesised the Membracidae helmet was a dorsal appendage on the T1 segment, with a flexible attachment to the insect body. If this hypothesis is correct, the helmet would be the only wing-like structure attached to the T1 segment by jointed articulations in extant insects, as wings are only known to occur on T2 and T3 segments (Prud’homme, et al., 2011). To support the hypothesis, Prud’homme et al (2011) used scanning and electron microscopy to compare the helmet with wings, and identified genes and transcription factors responsible for wing development in helmets and wings. Further contributing to the helmets novelty, due to selection pressure the evolution of wings and legs in insects is usually only seen with the loss or reduction of these appendages, and not with the rare addition of new features (Maynard Smith, et al., 1985; Raff, 1996; Riedl, 1977). The fact that the helmet is a feature shared exclusively by treehoppers, coupled with the hypothesis that it is a rare wing serial homologue on the T1 insect segment categorises the helmet as a novelty feature amongst insects.
Encyclopædia Britannica (2015) defines Homology as “similarity of the structure, physiology, or development of different species of organisms based upon their descent from a common evolutionary ancestor”. Recent work by Prud’homme et al, (2011) suggests that the treehopper helmet could be considered homologous to other insect wings. Electron microscopy identified morphological homologies between helmet and wing structures; one of these homologies being a flexible “jointed articulation” observed in the Publilia modesta species of treehopper, allowing for movement (Prud’homme, et al., 2011). Other morphologic homologies with the wings of other insects included a non-sclerotized flexible cuticle; two layers of epithelial cells connected by cuticula columns that unfold when developing; a vein network throughout the helmet; and bilateral primordia fused together down the dorsal (Prud’homme, et al., 2011). The fused bilateral primordia is a key finding, as thoracic dorsal appendages are only found on the T2 and T3 segments in the wings of other insects (Prud’homme, et al., 2011). Transcription factors involved in Drosophila melonogaster wing formation were also found in similar patterns in developing treehopper helmets (Prud’homme, et al., 2011). The implications of these findings suggest similar genetic networks involved in the formation of treehopper helmets and insect wings (Heffer & Pick, 2013). It also suggests the treehopper shares a common ancestor with other insects and has undergone a genetic and morphological change due to strong selective pressures (Prud’homme, et al., 2011).
Building on comparison of morphological homologies, Prud’homme et al, (2011) found further evidence to suggest that the genetic changes involved in the evolution of the treehopper helmet, was not the result of new genes but the expression of old genes. Transcription factors such as nubbin (nub), play a part in the development of D. melonogaster (fruit fly) wings but is not found in the T1 segment on the fruit fly (Prud’homme, et al., 2011). Prud’homme et al (2011) found nub was present in developing P. modesta helmets, similar to that in developing wings. Two other genes; Distal-less (Dll) and homothorax (hth), involved in “proximo-distal axis specification of appendages” were also found in the developing helmet. Following on from this, the authors found that expression of Homeobox-containing genes (Hox genes), particularly Sex combs reduced (Scr) in treehoppers was similar to that in other insects (Prud’homme, et al., 2011). However the key discovery was that transcription factors such as nub were no longer responsive to repression from Scr, potentially due to changes downstream of Scr (Heffer & Pick, 2013; Prud’homme, et al., 2011). The findings suggest that genetic changes downstream from Scr, have stopped the ancestral repression of old genes, resulting in the evolution and diversification of the treehopper helmet (Prud’homme, et al., 2011).
In summary, treehopper helmets have provided the treehopper with adaptive potential by evolving a change in its body plan, mimicking environmental objects and assisting with environmental adaptation. As only treehopper species have evolved the helmet, and it is thought to be a T1 wing serial homologue, the helmet is regarded as a novel feature amongst insects. Observed morphological homologies such as jointed articulations and bilateral primordia, coupled with similar genetic networks to that found in wings, suggests the treehopper helmet is homologous to wings on other insects. The genetic evidence proposes that the wing development program of the treehopper no longer responds to ancestral repression by genes downstream of Scr, allowing for expression of old genes and the rapid diversification of the treehopper helmet over 40 million years (Prud’homme, et al., 2011).
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