By Viviane Callier
Beetle horns are tremendously varied structures, both within and between species: some beetles have them, some don’t. The ones that do have them have evolved many special shapes and sizes. This diversity arises from the reuse of embryonic patterning genes during adulthood, according to two new studies appearing in Journal of Experimental Zoology and Proceedings of the Royal Society B.
For years, evolutionary developmental biologists have been investigating how evolutionary novelties arise. After all, it’s relatively easy to understand how natural selection might modify an existing structure, but not obvious how a completely new structure arises to begin with.
“Up until now, studies on evolutionary novelties have mostly dealt with patterning after the novelties began to develop,” said Yuichiro Suzuki, an evolutionary developmental biologist at Wellesley College who was not involved in the studies. “But probably the more important question is, how do you place a novel structure in a new location in the first place?”
To investigate this issue, Armin Mozcek and Eduardo Zattara, evolutionary developmental biologists at Indiana University in Bloomington, chose to study a genus of beetles called Onthophagus, which includes species with many different kinds of horns. To understand how adult horns develop from the larval head, they carefully killed selected cells in the larval head and observed how this affected horn development in the adult beetles. In this way, they discovered which larval cells correspond to the adult horn structures. They also mapped the larval head patterning boundaries by which the animal determines the position of its adult horn structures.
Next, they wanted to know whether two of the main players for patterning the embryonic head, the genes orthodenticle1 (otd1) and six3, played any role in the adult. To test this idea, they used a method called RNA interference to disable those genes, and then watched how that affected the adult head.
To understand the possibly novel role of these embryonic genes in the adult Onthophagus beetles, they first needed to know what the genes were doing in an evolutionarily primitive beetle. Tribolium, the flour beetle that is considered to represent the ancestral (evolutionarily primitive) state, does not have horns. In this species, otd1 is expressed in the adult head, but when the researchers disabled the gene, nothing happened, suggesting that the gene doesn’t have an ancestral role.
However, in Onthophagus taurus, disabling otd1 caused the existing male horns to become smaller, and also caused a new pair of horns to appear. And in Onthophagus sagittarius, loss of otd1 not only eliminated the normal adult horns but also caused a second pair of eyes to develop on the back of the head — and I don’t mean eye marks, I mean real eyes! They are complete compound eyes, and they connect to a crude optic nerve. The researchers have plans to investigate the degree to which those extra eyes are responsive to light.
“Our interpretation [of the extra pair of eyes] is that otd1 has a fundamental role in informing cells located along the midline where they are, and thus inhibits those cells from developing non‐midline structures including eyes,” explained Zattara.
“These studies provide interesting insights into where horns come from, both at the cellular level, and at the molecular level,” explained Suzuki. “The induction of horns as a result of otd1 knockdown is particularly interesting as it implies that the head has hidden abilities to produce horns.”
“It is striking that the same experiments produce different results in different beetle species,” added Kristen Panfilio, evolutionary developmental biologist at the Institute for Developmental Biology in Cologne, Germany, who was not involved in the study. “It will be fascinating to uncover precisely how species-specific horn patterns arise in future comparative studies, deepening our understanding of evolutionary diversity.”
The fact that the embryonic patterning genes show “leftover” expression in the adult, and that the gene otd1 wasn’t doing anything in the adult ancestrally, may have provided the foundation for the evolution of novel horn shapes and sizes, the researchers say.
“Our work raises the possibility that an important stepping stone or constraint in innovation may be availability of patterning genes that are already expressed in a given physical domain but at a different life stage,” said Mozcek. “Such genes are already in the right place, have interacting partners they ‘know’ how to work with, and now only need to somehow find a way to re-exert parts of their already existing functional repertoire during a ‘novel’ developmental stage.”
“When you have a gene that is expressed without any function, that means you can innovate without screwing up other things that you still need,” added Zattara.
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Viviane Callier is trained as an insect physiologist and is now a freelance science writer in Washington, DC. Find her on Twitter at @vcallier or on her website at https://vivianecallier.wordpress.com.